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The tools in the picture above contains, from left to proper: meccano hand-cranked gramophone with acoustic recorder (out of frame) to chop 78 rpm disc data on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio beneath it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my major transistor receiver. Small multimeter in entrance of S-C radio. National Panasonic 4-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones linked to it. The curly wire above the transistor radio was our communal non-public telephone line to my associates David and Bruce Bowden, who lived across the corner in Pleasant Road. HMV portable wind-up gramophone kind C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the phone line. Various broadcast-band and shortwave DX QSL playing cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from memory!). In my left hand, I held the oldest disc record then in my collection, London, circa 1903, of a brass band taking part in the 'Tancredi' overture. On the proper, spare elements for our personal neighbourhood line, and an previous electrodynamic speaker. This was the only picture of my 'radio shack' taken in the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, five miles East of Melbourne's GPO (Australia). The house was demolished in 1997. Three residence units now stand there.

Author's observe, 28 April 2005: As this web page offers with my very own modulated gentle experiments I hope readers will tolerate these reminiscences earlier than I reproduce my 1979 article on the subject from 'Amateur Radio' magazine:

WHY MODULATE Light?

Today's young Australians can barely think about the frustration of teenagers in the so-called 'radical' 1960s where digital communication was involved. By trendy standards, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on inflexible publish-colonial British public service strains. Something as innocuous as an intercom line run along your again fence to adjacent mates was illegal - 'a financial threat to the monopoly of the public telecommunications network' within the eyes of the PMG. I discovered this once i arrange a neighbourhood telephone community to other native kids' places early in 1967! Music transmission by way of novice radio had been banned in Australia since 1939, in order that the material legally conveyed on any 'wireless' system was limited, notably for music-mad teenagers. CB radios, largely of the only channel hand-held selection, were imported in restricted quantities and bought freely - however paradoxically they couldn't be legally licensed or used. You needed to be fifteen years old to sit down for the newbie radio license, then involving a trifecta of exams in full idea, laws, and Morse at 12 words per minute. For the theory, discursive essay solutions had been required, and there was no degree of idea exam below the one most normal. From a younger individual's perspective, you needed to drag your self over a subject of damaged glass to attain a authorized entry point. Even then, you had to attend to your sixteenth birthday to function ham radio legally in Australia.

If, like me, you have been 13 years outdated in 1967 and had electronic experimenter associates nearby, your aspirations to speak faced years of seemingly endless frustration. There were good reasons to investigate 'various' communication technologies - together with modulated mild.

Together with that we had the Vietnam War; the imminent threat of the compulsory army draft; a conservative government in energy since our start; and British-styled school uniforms unsuited to our hot summers. Australian teenagers of the 1960s had many legitimate reasons for discontent. The protest movement had its roots in lots of features of Australian society, not simply the political issues for which any avenue march was a convenient excuse...

During the following decade Australians saw the appearance of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor government coming to energy (1972); the first Sunbury rock festival, Australia's 'Woodstock' (1972); novice novice radio licensing (circa 1973); group broadcasting (1974); ethnic, multicultural broadcasting (1975); and the local legalisation of CB radio (1977). Finally, in the 1980s, personal carriers in the telecommunication marketplace had been allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies had been broken and the legal restraints on private electronic communication eased markedly. With the advent of the Internet, how might it's in any other case at the moment?

I would argue that Australia was fairly not like America within the 1960s, in that conservatism reigned via a lot of the 1960s right here, particularly in our communication laws. Australia's 'liberation' - if you may call it that - largely got here after 1970...

The advent OF Educational Solid-STATE KITSETS

So, how did your common kid with greater than common curiosity get into electronics, and more notably into optical communication, forty years ago?

In the mid-1960s, when the cost of germanium transistors had fallen to an inexpensive stage, electronic construction kits have been made out there to stimulate the minds of future radio hams and experimenters. These kits were in contrast to the one-venture meeting outfits then available from Heathkit and Lafayette. Their accent was on training, providing a collection of electronic components to be organized in numerous configurations on an insulating 'breadboard' with spring clips and connecting wires. With the help of a guidebook explaining the operation of every arrangement, more than twenty different circuits might be constructed, including Morse code oscillators, easy radio receivers, high-gain audio amplifiers and low-power radio transmitters. Right now, we additionally had the bonus of a neighborhood Melbourne journal for young experimenters, 'Transistor Kits' printed by Colin Witchell - extra recently of 'Talking Electronics' magazine fame - from a tiny store in Church Street, Brighton. For individuals who were fascinated, the mysteries of electronics held the important thing to a brand new world of technical prospects - and lots of Colin's tasks concerned mild detectors in some creative method...

An previous friend from my main faculty days, Rowland Legg, acquired a Philips twenty-in-one kit for the Christmas of 1965, and I was given an analogous Japanese 'Eleco' package on the identical day. My dad and mom had apparently noted my early aptitude for building crystal radio receivers, so they'd decided to 'push the envelope'. The natural outcome was that over the subsequent 5 years Rowland and i spent many afternoons together, unravelling the mysteries of electronic amplification. Whatever time wasn't spent indoors with the kits or on different electronic initiatives, we spent up on our rooves erecting lengthy wire radio antennas, or trying to get them up on progressively increased timber or supporting masts.

During these antenna-elevating sessions, Rowland Legg and I found that we may see each other's houses from vantage factors on our rooves. In an try to set up a signalling system, we spent many evenings up ladders with kerosene lanterns, utilizing a black card to chop off the light and send messages to one another in Morse code. The usual Morse alphabet seemed rather advanced, so Row devised one among his personal, following a logical mathematical development:

This Also proved to be too complicated to memorise and, with light flashes, too troublesome to relate to any typed list! We searched for a more facile technique of communication, with enough security not to attract consideration from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I grew to become conscious of the potential for transmitting speech over modulated mild beams after i bought a copy of an ancient, leather-based-certain ebook 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early affect was a book published in 1921, 'The Boy Electrician', which gave constructional particulars of selenium mild sensitive cells, Tesla coils, audio transmission by multi-turn induction loops (which I constructed) and even the small print of a small X-ray machine (which, thank God, I didn't construct).

Like many teenagers of the late 1960s, I experimented with modulated mild communication utilizing amplifier-pushed torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A page from my bench notebook dated 25 May 1968 - a number of weeks after my 14th birthday - shows my earliest plans for an optical system:

On the next weekend, 1st June 1968, a highschool good friend named Howard McCallum and that i set up the deliberate modulated light system with an incandescent torch bulb for transmitting and a CdS LDR in collection with a 9 Volt battery and headphones for receiving, collimating a light beam between the two with magnifying lenses. The results over a distance of 4 metres have been loud, however very distorted with frequency doubling effects - a results of our preliminary attempts to transmit with out DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, revealed the details of his simple 'photophone' in the 'A Reader Built It' web page of March 1969's 'Electronics Australia' (pps. 91-93). The challenge's simplicity made it a beautiful proposition for younger folks, and many Australian experimenters of my technology will remember it:

The geographical setting for our own reconstruction of Murrell's device was lower than salubrious. In fact, it was one of the eccentric radio 'shacks' that I've ever encountered. At the underside of my good friend Rowland Legg's back yard, a big wood packing crate originally used for delivery a Volkswagen car to Australia was set up by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we referred to as it. To maintain out the drafts, its partitions were papered with out-dated promoting posters for Melbourne's weekly scandal-rag, 'The reality', obtained from our kindly native newsagent across the nook in Tooronga Road. It was the sort of newspaper that no person would admit to buying, although the newsagent assured us of its excellent local circulation. From each angle in Rowland's radio shed, headlines in an infinite typeface assaulted the attention: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance among these posters was tactfully hidden behind a cupboard door, its wording being one thing like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm nonetheless wondering concerning the implications of that!

In these inglorious surroundings, and in the backroom of my dwelling proven within the photo at the highest of this internet web page, we spent many weekends constructing electronics projects of every conceivable type. Together, we quickly had a version of Murrell's photophone transmitter built into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was eventually mounted (with yards of PVC tape!) on a sizzling water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's dwelling.

However, my own initial optical comms checks with the Murrell tools were completed round March 1970 from the the backyard radio shack then occupied by Hughie Paton, VK3ZEP, at 49 Havelock Road, throughout that house's again backyard, down its again driveway and throughout Torring Road to the glassed-in entrance veranda of our family home (to the left of the front door within the photograph under) - a distance of about 50 metres. The system supplied a hyperlink in one path only. These tests had been logged on open reel audio tape, so that I could check my audio quality without the necessity for an assistant. My mother occasionally got here onto our veranda to listen to the end result and talk her response by waving by way of the windows. The tape still indicates the fairly good audio quality that one could get with a restricted depth of modulation. It additionally demonstrates the very excessive hiss stage of the OCP71, the precise device used on that day being seen in the photograph on the right.

I used to be out within the open air at the back of the Havelock Road property, across Torring Road from my home, shown above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a 3 volt 300 mA torch globe at its focus. I later found that the excessive frequency response of the system was inversely proportional to the thermal inertia of the filament. The perfect high frequency response was obtained with lamps of decrease rated maximum present, a thin filament with thick lead-out wires to dissipate the heat rapidly.

The transmitter gear, consisting of a microphone pre-amp, 3 watt transistorised audio power amp with transformer output, and two size D torch cell battery provides (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. At the receiving finish, a 7.5 cm diameter magnifying lens focussed the image of the Aldis lamp onto an OCP71. I can not remember whether the OCP71 was straight related to the microphone enter of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether or not it was working into the road enter through the standard two-transistor pre-amp (photoconductive association). I seem to remember that both were tried at different instances, with a minimal difference of results. An occasional transient buzz may be heard when widespread Australian blowflies happened to fly through the beam, modulating the sunshine with their wing beats. In the latter a part of the extract, a mild shower of rain occurred, the droplets falling by way of the beam producing a sort of comfortable 'plip-plap' sound. The effect of waving one's hand by the beam was additionally demonstrated. Whenever the beam was interrupted the hiss degree rose considerably - an impact for which, even now, I don't have any explanation, except that it obviously had something to do with the impedance of the phototransistor supply rising as the light input fell.

The audio tape of the check was made on a warm, pretty overcast Saturday afternoon. In the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, might be heard washing dishes after lunch and planting hop bushes within the backyard. The first World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a new ingredient for some dwelling brew! The son of the household, Brian Harris (1936 - 1992), was briefly VK3ZFH within the late 1950s. Brian confirmed the author easy methods to tune up a transmitter and browse an oscilloscope on the age of 4, in 1958. This set Chris on the lifelong 'downward' path into technical pursuits... One of Brian's final jobs concerned the design of a number of the communications gear for the Hubble space telescope. Suburban life may very well be attention-grabbing with neighbours just like the Harris family at forty nine Havelock Road!

On this ten-minute mp3 extract from the original 45-minute tape, the creator at the age of fifty one in 2005 introduces his squeaky-voiced 16-12 months-previous self in 1970. Computer nerds didn't exist then - WE had been RADIO nerds! Anyone interested can download the audio log file of this March 1970 take a look at as an mp3 file right here:

(Currently broken - coming quickly) March1970.mp3

One curious facet of the germanium phototransistor OCP71 was its excessive infra-purple sensitivity. It was doable to transmit audio, as one can hear on the tape above, with the torch bulb operating at a voltage so low that it had no perceptible visual output. The germanium detector may resolve the modulated HEAT from the lamp filament. Its work operate as a photodetector was very low, which meant that it was inherently topic to massive quantities of thermal noise, much more than silicon and really a lot more than photomultipliers:

On 1 September 1970 we prolonged the vary of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my dwelling at 6 Torring Road, East Hawthorn, where I stood atop a ladder subsequent to our yard bungalow with an an optical unit fabricated from tin cans, like Murrell's within the photo above. The audio link coated about 600 metres, and i could clearly see the sunshine focussed on the OCP71 within the receiver tube. Actually, I could aim the receiver by having the focussed spot disappear behind the square delicate materials inside the phototransistor. The hiss level from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was insufficient for the vary and the trebles were restricted by the thermal inertia of the torch bulb, but this was our first actual 'gentle beam DX'. On the age of 16, the excitement of listening to these outcomes caused my fingers to shake, so that my optical receiver's intention was erratic, but I managed to log the entire contact on an audio tape in two 'bursts', with a pause to telephone Rowland to tell him that every one was being obtained, midway. Initially, Rowland and his pal Neil Florence were simply relaying the 7:30 pm news broadcast from the Melbourne broadcast station 3AK with its items on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission progressively rose out of the noise - although not by much! Later, typical teenage music of the day will be heard - The Crystals singing 'And then He Kissed Me' (with Rowland attempting to interpolate a 1970-vintage type of 'karaoke') and the report of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... that is being transmitted on the first day of Spring, the first of September 1970 - and if you can't hear this in any case this hassle I'll need to kill you!', I may easily have fallen off the ladder in amazement. We'd lastly devised a means of circumventing the unlawful usage of a radio transmitter - although to be practical, with this appalling signal-to-noise ratio the success was only marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've hardly ever experienced more thrill from experimental work than I did on that night 35 years ago, and that i need hardly add that instantly afterwards, adrenalin-charged, I ran non-stop to Rowland's to report our success! The three of us posed for a photo round that time, with me holding a pair of 1920s-vintage headphones which we steadily used. The image might be titled 'hear evil, SEE evil, DO EVIL' - however mostly, our backgrounds were just too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, different Australian experimenters had been attempting the potential of atmospheric optical communication extra severely. In Adelaide throughout 1968, my future collaborator Mike Groth (presently VK7MJ, then VK5ZMG) and a lab assistant buddy, Stewart Powell, constructed a pair of optical communication items in the suburb of Hammersmith utilizing torch globes and OAP12 germanium photodiodes. They'd a most range of about 1 km, and, as Mike places it, "terrible fidelity". Mike's spare time for optical exams was limited on the time by the demands of doing the third yr of a BSc - with a brand new wife.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of the course initiatives in that 12 months involved modulated mild. He defined the subsequent occasions to me in a letter dated 28 February 1988:

"The undertaking concerned the evaluation of modulated mild as a technique of transmitting geomagnetic knowledge over brief distances to avoid wire links to the remote sensors, which tended to introduce hum loops if not fastidiously balanced. This was a golden opportunity to be taught the speculation behind optical hyperlinks and search back by way of the literature on the University Library. It became apparent that it can be no downside to transmit knowledge over a couple of hundred metres on a clear night time using the brand new infra-purple diodes as sources, but the reliability of the hyperlink was unknown, particularly because it was for use at a area station in the Adelaide hills, the place fogs and mists were widespread. I constructed an infra-crimson link at 930 nm which measured the trail loss over the winter and spring of 1969, and my estimates of the trail losses [within the 1987 'Amateur Radio' article 'Photophones Revisited'] were primarily based on this knowledge.

Optical communications fell into the background for the next 16 years, as I spent 1970 in New Guinea educating and moved to New Zealand in 1971 to do postgraduate work on the University of Otago in Dunedin. I returned to Australia at the top of 1979, but was slightly inactive in experimentation until 1985, once i decided to write down my experiences as a overview of the possibilities of optical hyperlinks. The effort took almost 18 months..."

Mike's resultant article published in 1987 with revisions from 2005 could also be discovered at:

Probably the most powerful novice atmospheric optical communication checks in Australia around 1970 had been carried out by an Adelaide workforce and reported in an extraordinary Tasmanian magazine. The Hobart-based 'Electronics Exchange Bulletin' was revealed round this time by the Tasmanian crew of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary journal inspired articles by native experimenters on every conceivable matter associated to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two college students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour fuel discharge lamps, reaching atmospheric ranges in excess of 3.5 miles (about 5 km), simply exterior Adelaide in valleys shielded from town lights.

Though their tools was massively bulky by fashionable requirements, Burlinson and Averay pursued a novel line by driving their fuel discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation effectivity problems. In this fashion, the efficiency was similar to that of the celebrated (at the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I was not aware of the reviews of those tests until the beginning of 1976, in any other case I may have skipped the next few steps in my own exams...

OUR Switch TO Gas DISCHARGE LAMPS

A seek for higher modulated gentle sources than incandescents drew my consideration to neon lamps. Gas ionisation is a much faster course of than the incandescent heating of a filament. In those days, neons were cheaply accessible in any dimension from a pea lamp to a full dimension "beehive" bulb from one marvellous supply. Melbourne experimenters energetic in the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the top of a slender stairway main down from the pavement was a rather grubby Aladdin's cave of tables laden with cable, conflict surplus junk (every conflict except Vietnam), containers of valves and khaki-painted gadgets of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates full of technical cast-offs of each description. I acquired a number of bins of neons there, along with varied different fuel discharge units to try. A few years later, I discovered that a few of these contained traces of radioactive materials to assist ionisation. Let the buyer beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, sort B406. The orangey-pink glow across the cathode on these neons is fairly intense, although it doesn't even method the intensity of fashionable LEDs or lasers. After establishing the neon modulator I was rewarded with close to excellent audio from my OCP71 for the first time. Fortunately, a parental veto on constructing mains-fed energy provides ended on my fifteenth birthday (8 March 1969), so the required 300 volt rail was no downside.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the idea for a easy tv system in 1970, by including a couple of Nipkow scanning discs to the outfit. This diverted me from the modulated mild communication checks for a couple of years as I delved into all the pre-war Baird television texts to bring the images to an appropriate normal. This brought me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, name sign VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' internet site. They had been the 'ringleaders' of a larrikin group of amateurs working residence built AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' amateur radio pursuits have been unique and individualistic. The majority of conservative hf (shortwave) operators had a slender and obsessive emphasis on 'communication quality' modulation, often 300 Hz - 3 KHz, clipped, non-linear and cruddy. Dan and Tony have been each hello-fi fanatics. Their transmitters had been relatively broadbanded and immeasurably low in distortion, exceeding the audio specifications of many broadcasters. AKG or Western Electric microphones and broadcast-quality audio peak limiters with twin time constants have been concerned. The design and building of their modulation transformers and amplitude modulators was nothing wanting an art. Their 160 metre receivers, also, employed biased, low distortion envelope detectors working through rigorously designed audio amps into monumental speaker systems with vented enclosures. The content and audio high quality of their in-depth conversations on communications technology made listening to their transmissions an absolute pleasure. Even their commonplace of audio compression served to convey the listener into the acoustic surroundings of their homes - an underestimated side of making a realistic auditory illusion. Their exercise ruffled quite a number of amateur operators' feathers on the time... which solely elevated my youthful admiration for both of them.

I truly met Dan (3UI) - dare I admit it - on the air on thirty first December 1969, as the results of a quick dalliance I had with pirate radio transmissions on 1.8 MHz. He lived in Hawthorn solely a mile from my home, near the corner of Glenferrie and Riversdale Roads. Naturally he was among the first to listen to my feeble and unstable transmissions, and he inspired me to experiment additional to realize the information to get the amateur 'ticket'. We had been both given a 'cease it or else' ultimatum by an over-zealous radio inspector known to the locals as 'Uncle Ugh' (many will nonetheless know who I mean), but Dan and i turned great pals as a result. Dan was about two years older than I, and was highly influential on the quick future course of my life. As the year 1970 progressed, I found myself spending extra time with beginner radio mates and less with Rowland and the old fashioned mob. Rowland finally carved out a very successful profession within the Victorian Police Force (ironical, isn't it?), and i believe he nonetheless does, however his early interest in electronics declined...

In subsequent a long time, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for varied frequency segments. In consequence - and I'll specific an opinion here - they've systematically eliminated the legal foundation for the kind of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' are now operators of economic 'black field' transceivers which might elegantly present single channel phone high quality (or worse) on each accessible band for $4000+, but which regularly cannot be correctly tailored to every other mode or form of experiment. From my perspective, the result's that novice radio has had progressively much less attraction as a passion, and I'm positive I'm not alone in expressing this opinion. For my money, should you legislate towards experiment you kill the one attraction that novice radio ever had. So lengthy as the emissions do not unfold past the beginner band edges, what is the issue? Anyone who pushes the hoary outdated argument that "bandspace is at a premium, so transmissions should be of the minimal possible bandwidth" should be deaf and blind to the steadily declining stage of novice band utilization over the past fifteen years. Who is to say what beginner radio should entail, so lengthy as it supplies training and encourages experiment? Many individuals wonder why I've never bothered to pursue an beginner radio license. I hope that I've justified my place. Thank God for alternatives with extra freedom of content material and bandwidth, like mild beam communication!

Dan (3UI) and i ultimately organized check transmissions of narrow band tv using mechanical disc scanners over his 160 metre transmitter early in 1972. The subsequent couple of years gave all of us a very good grounding in the principles of gentle detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others in the United Kingdom, we formed the Narrow Band Tv Association, nonetheless in active operation and now represented on a web site:

http://www.nbtv.org

One particularly form donation to this mechanical Tv scanner challenge was provided by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev worked in telecine at Melbourne's authorities Tv station, ABV channel 2 in Elsternwick, operating an archaic monochrome Marconi 35mm film scanner. This was originally mentioned to have been designed for the 405 line British service and used at BBC Tv's authentic studio on the Alexandra Palace in London. By 1972 it was only being used for half-hour a day, at about 4:00 pm, to broadcast 35 mm movie episodes of the youngsters's serial "The Cisco Kid" - the only common program material they had which nonetheless used that gauge of movie. The EMI 6097 photomultiplers on this Marconi telecine were written off as soon as they developed spots on their photocathodes - Kev called them 'dynode spots' - but they had been still quite serviceable for gentle detection. In 1972 Kevin saved some of these from the dustbin for us.

Dan (3UI) and i tailored the EMI 6097 photomultiplier to be used in our experimental digicam and for the modulated light receivers. Its sensitivity was such an unlimited quantum leap from the OCP71 that it opened an entire new world of technical prospects to us. Further photomultipliers were acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for sluggish scan tv scanners in those days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

During the Autumn of 1975, I built a modulated light communication link that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first flooring degree, dealing with one another throughout suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about seven-hundred metres. The modulator from my Baird mechanical television receiver by then used a 6L6 beam pentode in collection with the neon, which was re-mounted at the focus of a tough 30 cm moulded glass parabolic reflector provided by Tony (3AML). The reflector had initially been utilized in a visitors signal.

This optical transmitter was placed on Paul's balcony in Glenhuntly Road, on the nook of Clarke Avenue. The receiver was placed within the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a thirteen cm diameter magnifying lens focussing onto a 0.5mm focal airplane aperture, with a 931A photomultiplier catching the transmitted light behind it.

Using Dave's 160 metre newbie transmitter because the return link - the last word in break up frequency operation - Paul managed to carry on a crossband contact by means of the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue sensitive photomultiplier. The neon was pushed past its current rankings, in order that its bulb was rapidly blackened by cathode sputtering, however a minimum of it could possibly be fully modulated. I was later in a position to measure the bandwidth and was astounded to find that the neon could be modulated to round 500 KHz, which was a terrific improvement on the torch bulbs. Distortion was severe. We have been pushing the modulation fairly arduous, owing to the poor sig/noise ratio. A sample of the audio log of the contact is currently available on Dave's net site. The outcomes have been sufficiently good for us to realise that we have been heading in the right direction for further improvement. The contact was logged on tape and an extract may be heard on VK3ASE's web site:

[ Actually, Dave seems to have eliminated this one for now. (Tony, VK3AML) ]

The primary problem to be overcome was the low characteristic depth of the neon discharge. We also wanted a supply with significant blue output, to match the spectral response of the photomultipliers, which may solely detect violet, blue or inexperienced gentle. Development was accelerated by the involvement of another member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the top of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my dwelling, I was lucky in having an elevated position near the highest of a hill in East Hawthorn, South of Camberwell Junction and fairly near the high copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs round to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong as well as components of Malvern, Richmond, Abbotsford and Northcote in an unbroken a hundred and twenty degree arc. Just on my facet of the Yarra, on the highest of a rise in clear view, was VK3ZGJ, with his shack dealing with me at the third ground rear of an old Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was splendid for optical communication exams, precisely two miles (about 3.5 km) on an East-West path.

In the early 1970s, earlier than I met him, John (VK3ZGJ) constructed a collection modulator for fluorescent lamps, consisting of several 807 output valves in parallel, with the fluoro in the anode return. He additionally built a portable gentle dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias into a load of 10 megohms or extra, and with its low noise preamplifier it had much better sensitivity and spectral match to a fluorescent lamp's output than my previous OCP71. You needed to arrange for the image of the fluoro to focus precisely onto the hole between the conductive combs on the LDR's sensitive surface. This involved peering at the LDR by means of a 'spy-gap' within the optical mounting while deftly manipulating the receiver's alignment.

LDRs have a very sluggish response, rolling off at least 6dB per octave above 50 Hz. Treble enhance might solely partly right the issue, as John found. My photomultipliers offered the answer to John's receiver problems, just as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled assets over the subsequent 5 months.

Late in December 1975, John and that i obtained the communication system working between our houses, using a vertically mounted fluorescent lamp on the roof which "broadcast" light in all directions. The modulator consisted of several (four, I believe) sort 807 beam pentodes feeding the 40 watt fluoro of their anode circuit. A rail voltage of about 600 volts DC was utilized to the lamp. We did not use a lamp starter or a ballast choke to operate our fluoros. Instead, we had a novel starting association consisting of a band of aluminium foil wrapped across the glass near the cathode finish of the tube, which was connected to the secondary of an automotive spark coil. To start out the discharge, you'd apply the 600 volt rail, then energise the ring across the cathode by making use of a battery briefly to the spark coil major. The high-voltage spikes utilized to the glass close to the cathode began a barely perceptible glow discharge inside that finish of the tube, which would instantly unfold the entire length of the tube underneath the affect of the 600 volts DC rail. Standing current was different by altering the worth of the cathode resistor on the collection 807's. The filaments at both finish of the tube were by no means heated up with this arrangement, which appeared to increase the working life of the tube.

Our first 40 watt 'fluoro' test transmission came just after the Christmas of 1975. It was in a single direction only, with John transmitting and me receiving. 3ZGJ managed to arrange some quite elaborate music applications interrupted by bulletins, one among which I recorded on tape. In the early 1970s a British group, calling themselves "Radio Love" (hey man, actual groovy) had proposed a system of local light beam broadcasting. This seems to have been an try to avoid the heavy hand of British officialdom, which at that time had pressured a number of unbiased broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its initial plans, including the diagram under which neatly summarised their concepts:

John 3ZGJ, in imitation of this 'Radio Love', jokingly announced his applications as being transmitted from "Radio Hush". The identify was a bit less poofy, yet still retained the essential environment of the authorized fringe-dweller! Later we duplicated the system to supply full duplex communication (simultaneous transmit and receive) in each instructions. A typical extract from these fluorescent gentle transmissions in the final week of December 1975 might be heard on this tape, recorded from the photomultiplier output at my finish of the hyperlink, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers solely managed to achieve 15 dB sig/noise over the 3.5 km range with this association. Fluorescent lamps had more than their fair proportion of problems as a modulated source. Their phosphor coating had time lag, the persistence limiting the upper modulated frequency to about 5 KHz. The time lag was not fixed with the wavelength of the emitted light. The purple phosphor components had very long persistence, while the blue phosphor was a lot sooner. The system's higher audio frequency restrict due to this fact assorted with the spectral response of the detector, however with a blue-sensitive photomultiplier it was more than satisfactory for audio.

The fluorescent lamp discharge would wander and 'snake' contained in the tube, particularly at swap-on, interfering with the modulation. For some purpose which we might by no means explain, the output on the cathode end of the fluoro, and only on the cathode finish, was modulated by an erratic whine at about four hundred Hz, probably caused by the discharge hopping about from one part of the coiled cathode filament to another. This all the time set a definite limit to the sig/noise achievable, even the place there was plenty of mild signal to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an effect undoubtedly brought on by acoustic resonances of the modulated mercury plasma column within its tubular enclosure. One could actually hear a faint acoustic radiation from the tube while it was in operation, as these acoustic waves actually penetrated the glass walls. Certain modulation frequencies, obviously related to 'organ pipe' resonances within the fluoro tube, would cause the mercury discharge to extinguish, or to break up into a sequence of spaced glow discharges along the tube. I later discovered that this drawback had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" within the guide 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps thirteen - 14):

"Enclosed arc lamps operated on a.c. power in the audio-frequency vary, or on d.c. and modulated by a.c. currents might cause sound vibrations to be produced throughout the arc chamber. They are brought on by thermally induced variations in gas pressure that consequence from changes in present density within the arc. At certain essential frequencies, resonance of appreciable depth is constructed up by reflection from the bulb walls. The scale and form of the bulb, sort of gasoline or vapour filling, temperature and operating circumstances determine the frequency of the plasma oscillations which might be much like standing sound waves within the discharge. Ordinarily this phenomenon shouldn't be noticed as a result of lamps are operated on d.c. or low frequency a.c. with sufficient ballast to ensure stable operation. In long tubes the discharges assume a constricted, snakelike look on the crucial frequencies and are caused by sound vitality reflected from the ends of the bulb. In a spherical bulb the sound waves spread to the bulb partitions and are then focussed again upon the arc to supply instability on the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.Three cm lengthy with three Amp d.c. current utilized with a 2 amp a.c. modulation applied. He noted:

"[...] the arc confirmed violent distortions at 2250 Hz however was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and three A a.c. modulation at 2500 Hz, the discharge again confirmed pronounced disturbances, but was stable at 2300 and 2700 Hz. The instability could begin at either electrode, whereupon the discharge constricts into a skinny luminous ribbon with sinusoidal shape, and the voltage will increase because of elevated arc length. [Instability at] harmonics of the fundamental frequency may also be noticed[...]

"[...] In a spherical bulb with electrodes at the centre [like a Xenon arc] acoustical resonance occurs when the bulb diameter is equal to one-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will actually 'blow itself out' by its personal sound waves if any of the strong resonance frequencies are utilized to the lamp for an appreciable time [...]

[...] In a low strain discharge lamp [eg fluorescent], assuming a mean fuel temperature of 250 levels Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's formulation. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence on the electrodes happens at a half wavelength from the nodes which are on the ends and centre of the lamp."

Owing to the low intensity and extended source area of the fluorescent lamp, the radiated flux could never be correctly collimated. These limitations, and the frequency response irregularities clearly indicated that our light transmitter wanted a change of method.

At this level, Rodney Reynolds VK3AAR drew our attention to EEB's publication of the sooner work of Burlinson and Averay, whose modulator circuit and mercury arc source was an entirely novel approach. Their optics, nevertheless, have been fairly crude, not nearly directive enough for our work in suburban Melbourne, the place avenue lights and illuminated promoting signs proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We wanted a higher intensity supply, no fluorescent coating, and an output rich in blue light to match the photomultipliers. From the electrical supplier Arthur J Veall in Bridge Road, Richmond, John and i obtained some Philips excessive pressure mercury arc lamps in January 1976, of the kind used for manufacturing facility lighting. Most of these had inconvenient fluorescent coatings, apart from the smaller lamps below 100 watts output, which sadly had frosted glass envelopes. We bought a few HP80 mercury lamps rated at eighty watts. Driving these was an actual problem. The fluorescent lamps that we beforehand used have been low current, high voltage gadgets, modulated simply with commonplace output valves. By comparison, the high strain mercury arcs ran at medium voltages (30 to a hundred volts) but with high present (0.5 to 3 amp standing current), they usually exhibited a severe destructive resistance characteristic - their voltage drop decreased sharply with rising present stream.

Other issues arose owing to the instability of the mercury arc. Its hanging voltage diverse extensively with ambient temperature, and its standing present diversified with time as heat brought about the mercury to vaporise and the arc strain elevated. At its peak, the quartz arc tube had to withstand internal pressures of round 30 atmospheres, so it was a device that you simply needed to deal with with nice respect!

The slightest overmodulation peak would extinguish the discharge, and the entire system then had to cool before it could possibly be re-struck at a fairly low voltage. The lamp polarity additionally needed to be reversed at frequent intervals with a DPDT switch to avoid premature cathode failure by ionic bombardment. The lamps had been designed for AC operation, where the effects of cathode heating were shared 50 times per second by each electrodes. In spite of utilizing DC bias on the tubes, for which they were not designed, we managed to get a couple of hundred hours out of them, which was Ok when the lamps only cost $7 apiece. Today they cost about ten times that.

The modulator had to steer the current passing to the arc. Quiescent class-A amplifier conditions demanded that not less than half of the supply rail ought to be dropped by the sequence modulator, the remainder being dropped by the mercury arc. The unfavorable resistance of the arc was our stumbling block. As the current by way of the arc increased its voltage drop decreased. This dragged the voltage utilized to the collection management machine up in the direction of the rail voltage as current reached a maximum. With 200 volts-on the rail and a peak current approaching 5 Amps, no transistor commonly available in 1975 may handle the job. They couldn't take the excessive peak currents concurrently with the excessive emitter-collector voltage that this load with its damaging resistance would current to the output transistor at school A. We had some expensive pyrotechnic shows of 2N3055 and BUX80 transistor failure owing to this secondary breakdown point being exceeded. Like many foolhardly experimenters, we rarely used quick-blow fuses in our gear. Youthful arrogance, I guess!

The elegant solution, suggested a few years later by Rod Reynolds (VK3AAR), was to run the control transistor in parallel with the arc, with a single series resistor as much as the supply rail from each. In that configuration, the transistor passed maximum voltage at minimum present, and vice versa, so that a a lot smaller transistor could be used than within the series modulator configuration, the tradeoff being solely a decreased total energy effectivity. However, at the age of 21 in 1975 - and John was 20 - neither of us considered that...

Instead, we used a category-A vacuum tube amplifier of elephantine proportions in sequence with the arc. The facility supply was capable of 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt score! For the modulator, four hefty 6080 or 6AS7 excessive present, low gm triode regulators were positioned in parallel with low-worth balancing resistors in their cathode returns. The anodes had a nasty habit of glowing a dull red when the arc was initiated, because it took a few minutes for the mercury discharge to run as much as its typical a hundred Volts drop. During that point, the major part of the provision rail was utilized to the valves, which had been sent beyond their dissipation limit. Under these conditions, one may see alarming high resistance 'spots' scintillating with a brilliant and sparkling yellow gentle on the massive oxidised cathode surfaces of the 6080s. I all the time anticipated a 'bang' but in some way I used to be always fortunate...

Each 6080 filament consumed 2.5 Amps at 6.Three Volts. The valve heaters alone consumed a total of 63 watts! It was all brute drive, ignorance, bulk energy and heat! Fan cooling was obligatory. We used a home fan of 1928 vintage with a bum sleeve bearing, which rattled constantly throughout QSO's. Nevertheless the system worked remarkably properly, and due to Melbourne's many disposals stores, it could possibly be built for a number of dollars. Not so now! Electronic disposals stores of that kind at the moment are largely a thing of the previous.

This mercury arc transmitter was fairly unstable in operation, liable to permitting the arc to drift into a gradual thermal runaway. To appropriate this tendency, one had to trace the current of the arc by manipulating the grid bias of the output tubes - hence the explanation for the metering of arc current and voltage within the modulator circuit, above. The temperature of the quartz bulb was a very powerful variable, as this and the arc's standing present were closely related. The bulb temperature controlled the mercury vapour strain in the quartz phial, and therefore the electrical resistance of the arc. With insufficient common current move, the heat of the arc could be too low to take care of the mercury vapour strain, inflicting the voltage across the arc to slowly fall, in turn leading to an extra fall of present. If the standing current was too high, the arc would turn out to be so sizzling that it not solely turned harmful, however was impossible to modulate fully. One couldn't leave the system unattended for more than about five minutes. Arc current would differ, significantly in the time just after 'change on', as the log record under clearly signifies. Warm-up would take round 15 to half-hour earlier than the system lastly settled into static current situations, with the heat pumped into the arc electrically equalling the heat misplaced to the surrounding environment. The typical standing current assorted with the audio program content and with the symmetry of the audio waveform utilized.

A typical instance is offered by the next log that I took of the arc transmitter's electrical standing, measured by the evening of Saturday thirty first January 1976. Operation on that night was finest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a unfavourable peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

eighty V @ 0.42 Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only earlier than nightfall. Arc has been on for 45 mins of heat-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright points!). Ambient temp. 350 C. VK3ZGJ not listening yet. Power and heater transformers are both hot to contact, however Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used on this system were additionally designed around elements obtainable from disposals sources. Photomultipliers are superbly delicate, and not nearly so fragile or difficult to set up as some so-known as specialists counsel. They require a easy one thousand volt supply at about 5 or 10 mA - simply arranged with an outdated valve radio transformer working right into a voltage doubler. The hand-held photomultiplier receiver was a relatively hazardous device whenever you had been manipulating it at evening from the top of an earthed metal ladder! Particularly so when the a thousand volt supply had eight µF of oil-filled block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It became much more harmful when rain was falling! In my youth I had scant appreciation of my very own mortality. Few teenagers ever do!

The old disposals photomultipliers typified by the 931A had just about no response to crimson light, so they could not be used with LEDs or HeNe lasers. More moderen photomultipliers with gallium arsenide photocathodes have good pink sensitivity, however at prices exceeding $one thousand they're out of my league.

Overall results with this cumbersome hyperlink system were remarkably good. Considering the potential theoretical problems of thermal lag and ionisation time, the mercury lamp's limit of 50% modulation at 10 KHz was fairly usable, completely acceptable for direct amplitude modulation at audio frequencies. Piles of sign have been obtainable. The output was terrific. At night, with one's eyes darkish-tailored, you wanted to protect your eyes with darkish glasses or welding goggles. But data and slender band video modulation revealed wild section shifts above 5KHz. I believe that the mercury plasma, when thermally modulated at an audio rate, was making an attempt to broaden and shrink against its quartz tube housing with each modu1ation cycle, interacting with the arc cavity in a posh series of acoustic resonances. Indeed, one might truly hear the modulation coming from the arc, significantly at high audio frequencies, by placing your ear down close to the quartz bulb. One undesirable facet impact was that certain modulation frequencies close to the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds were especially risky. In reply to a question in regards to the modulator I remember saying over the link "it is working perfectly!" At the opposite end, all John heard was "it's working perf - Click". The arc extinguished itself, right on cue!

Over the four months as much as April 1976 the system was in operation up to six nights a week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all but the heaviest fogs and rain showers. On highly regarded nights, low frequency noise and fast flutter fade had been evident, but hardly ever obtrusive. The effect was due to scorching air cells rising from the warm ground into the cooler evening sky. This triggered a heat shimmer, with a consequent scintillation of the acquired mild. We considered the use of FM subcarrier modulation to clip off the scintillation on the acquired mild modulation, however the mercury arc lamp hadn't a sufficiently quick rise time to support this. Subsequently I found that the usage of longer wavelengths - crimson or infra-purple gentle - enormously scale back scintillation. FM approach could also be desirable for hyperlinks of over five miles vary, however for shorter distances amplitude modulation's greater accessible base bandwidth most likely makes it the popular mode.

The previous mercury arc/ photomultiplier system might have had appreciable long vary potential but we by no means had the opportunity to test it. The heavy energy demands of the terminal gear demanded a mains supply at both ends of the hyperlink. We by no means successfully tried DX away from our respective houses and two miles (3.5 km) was the best distance tested. We may generally obtain 45 dB sig/noise over that distance.

Communication ceased in the course of the day owing to the scattered blue gentle of the sky. The photomultipliers have been saturated by it. Interference filters may have been used to extract a dominant spectral emission line of the mercury discharge from the final background radiation, however I was unable to acquire one until the 1980s. They're costly and suitable just for parallel rays of gentle. There are higher ways of achieving good sig/noise in the course of the day, using a monochromatic supply and a wavelength away from the peak output of the sun.

However, I'd power up the photomultiplier with a comparatively low voltage - about 500 volts - simply before sunset, to catch John's first distinct phrases because the ambient mild faded. On sizzling evenings, the signal was all the time lower than expected. I couldn't account for this, until one evening I climbed the ladder to regulate the receiver and seen an odd scent. The receiver's optical cavity was stuffed with smoke! I had forgotten that John was nearly directly West of me, and as the summer time solstice passed, the position at which the sun set on the horizon was transferring North on successive evenings. That night, it was setting directly behind John's house - so my 13 cm receiver lens was focussing the solar's picture on the again of the receiver housing, which was product of black-painted particle board. Awk! We have been so lucky to not have set fire to everything - but I stored the incident a secret from my people. Things that my dad and mom didn't know couldn't hurt them! With some embarrassment, I fitted the optics with a removable lens cover.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low energy lasers in all probability could never rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years in the past I acquired a pair of 1 metre diameter searchlight mirrors to try the idea. The mercury arc might be mounted in one, photomultiplier in the other, each tracking the identical cloud. Street lights as we speak use the same type of mercury discharge because the modulated source, so that one hundred Hz hum background plus harmonics would be unavoidable in an urban surroundings. I've yet to attempt the concept, as direct linking is undoubtedly extra reliable and environment friendly, and has therefore been my fundamental line of research.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as soon because the optical link between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, several instances each week. For all practical purposes, I was on an intercom in 3ZGJ's shack, and there was no regulation against that. Legally and technically, I was a supervised (ie, pull the plug out if he misbehaves) visitor who just happened to be present close to 3ZGJ's microphone!

Through the early a part of 1976, there have been many cross-band contacts on 160 metres with me "on lightbeam" which seemed to add technical novelty to the proceedings. The discussions have been absolutely open to any topic, offered that the due decorum of the medium was maintained. On some occasions, it wasn't! One night we someway drifted onto the subject of funerals, morticians, Egyptian mummification and the 'dear departed'. This contact, with many others from that 'gentle beam link' period, was logged on audio tape by Tony VK3AML and by other listeners, and extracts will be downloaded from Tony's 'bluehaze' internet site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content assorted from creditable technical depth to immature ratbag ramblings. I by no means stored monitor of all the log tapes, and that i never know when bits of it can re-emerge on the weekly replays which were maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our issues with the previous gasoline discharge lamp system have been more comical than theoretical, and most have been completely unforseen. Moths, as an example, had been irresistibly attracted to the extremely-violet output of the mercury arc. Bogong moths, big Emperor Gum moths, Christmas beetles, flying ants - the full field and dice! They swarmed the transmitter arc on scorching nights, attenuating the beam flux and suiciding against the hot lamp. Every couple of hours I'd should scrape their smouldering bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the light they reflected formed a suggestions path into the adjoining, repeatedly operating, duplex optical receiver. If I had my transmitter microphone working close to the monitor loudspeaker, the presence of an Emperor Gum moth (which, I ought to explain for non-locals, are the scale of a small dinner plate) was signalled by ear-splitting feedback warbling at their wing-beat frequency of about 15 Hz. This was often adopted by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'turn my bloody noise down'! As we incessantly operated between midnight and 3 a.m., I can hardly blame them. We known as the effect "moth-back" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would cease to perform. The lamp itself appeared perfectly Ok, even on close examination. We lastly found that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias set up an electroplating motion, dissolving the wires connecting the base with the bulb. The problem was fairly merely solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, backyard foliage and power strains made steady contact between the terminal stations the topic of persevering with concern - and some unusual QRM. The situation worsened in windy weather as there was a tall gum tree very close the the optical path, in a backyard about a half mile distant from 3ZGJ. In a Northerly wind this huge nuisance had branches that would sway again and forth into the beam path, slicing phrases and phrases out of our conversations. Eventually the rattling factor turned such a drag that we made half-severe plans about wearing balaclavas on an illicit midnight raid on its owner's backyard with ropes and a pruning noticed! Another suggestion involved borrowing a surgical reducing laser from Melbourne University's medical school, and trimming the foliage from a distance - the distance of 3ZGJ's shack veranda, in actual fact. Just a few years afterward, I took a have a look at the same path with binoculars, and found that the nuisance tree, by then with several others, had grown to completely block the optical path. L.O.S. paths can be impermanent! The growing of tall timber in suburban streets and gardens is now far more fashionable than it was thirty years in the past. It could also be unlucky for optical communication fanatics, nevertheless it gives an aesthetically pleasing means of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the shiny side of life" whereas I'm typing this)!

The color of the mercury lamp additionally supplied issues. It looked the same as another street mild in Melbourne. One night time, we drove up to 1 Tree Hill in the Dandenong Ranges exterior Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance report. At the top of the outdated lookout tower there (eliminated in 1981) we seemed down on the lights of Melbourne, which had been as numerous because the grains of sand on a beach. With John's optical receiver we searched the varied lights seen in the general path of Hawthorn for about half an hour, https://ticklingup.com/ vainly trying to hear audio modulation among the grunts and farts of mains-fed mercury road lights. We never did find our audio-modulated