Many preppers and other radio communications enthusiasts want to be well equipped for receiving and transmitting under adverse conditions, but most modern hams, shortwave listeners (SWLs), preppers and observers are not familiar with the evolution of receiver and transmitter designs and how that progression can give us a useful advantage in gearing up for different scenarios. Old tech is, after all, appropriate tech when the going gets rough-the rougher, the older in many instances.
I want to start with receivers because a transmitter without a receiver is useful only for broadcasting, and in our endeavors broadcasting is generally our last goal. One concept I want to make clear is that in the design of radio communications technology, ‘Ontogeny recapitulates phylogeny’. The most simple and easiest to implement systems are usually those that in fact evolved early on, and they are often the most resistant to failure. But they also tend to have limitations which were why they were superseded: however, in extreme circumstances knowing how to implement them and having a few key items on hand could mean the difference between success and failure.
The primary steps or changes in the design of radio receiver construction are, the “crystal set” or Passive Tuned-Radio-Frequency (TRF) receiver, the Active TRF, or ‘straight’, receiver, the Regenerative (‘regen’ or ‘howler’) receiver and the superheterodyne (superhet, or mixing) receiver. There are other designs such as the ‘superregenerative’, ‘direct conversion’, and ‘homodyne’, but they are not significant for our purposes and are used today, if at all, for specific niche purposes. The four main categories each may have a use but only one, the last, will generally prove practical for a modern, all purpose radio and constitutes 99% of all commercially manufactured sets. All are worth knowing about and may have some specific application depending on the resources you have and what you are trying to accomplish.
The Crystal Set
The first radio receiver to be available in any quantity, and the first radio construction project for three generations of schoolboys and hobbyists, was the “crystal radio”. It had no vacuum tubes or other active devices, and consisted of an antenna, an earth ground, a tuned circuit to select a particular ‘wavelength’ or frequency to pick up, a ‘crystal’ detector to turn the modulated signal into an audio voltage, and some type of transducer to enable the listener to hear that small voltage. There were no batteries or any power source, other than that provided by the signal itself. There were no active devices to power either. The ‘crystal’ was not, as in modern electronics of all types, a small piece of resonant cut quartz used to provide a frequency reference but rather a semiconductor junction made up of a chunk of certain kinds of rock or mineral and a small needle or ‘whisker’ of dissimilar metal. Galena was the preferred detector, usually mounted in a holder with a convenient gizmo to make putting the whisker on a certain part of the little rock where it would work best, but many materials would work and a rusty razor blade and a stick pin or wire often saw service as well. Modern crystal radio builders usually just use a regular diode, preferably salvaged from a junk television, computer or what have you: Most any diode or transistor will work but germanium or Schottky diodes are preferred for best sensitivity.
Almost any old boy’s electrical or science book will have diagrams of crystal radios and some instructions on building various types. There are many different circuits but the keys are always the same: a good antenna, a good ground, a sensitive transducer, and patience.
The transducer, or speaker, bears some comment. A modern loudspeaker will not reproduce anything, usually, with a crystal set. Even modern headphones are useless. The little crystal ear-bud they used to include with the old transistor radios will provide some results, but best is the old, high impedance, “watch case” headphones. They are very sensitive and have many uses for electronic servicing and antique radio use besides crystal sets. They were made up until the 1960s or early 1970s for the military, and can be found at hamfests or online. I have found that piano and organ stores often have a set or two lying around because for some peculiar reason Wurlitzer used them, with a matching transformer, with their home organs, most of which have little value today and will be parted out by organ servicemen.
The upside of the crystal set is that it is eminently suited to do-it-yourself construction, needs no batteries, and will enable users to hear local AM stations during the day and a number of clear channel stations at night. In rural areas, with enough antenna and a really good ground, many AM broadcast band stations and, with the right tuning coils, even an occasional shortwave broadcast station can be heard at night.
There was a good reason it fell into disuse except as a novelty or boys’ project after the mid-1920s: it couldn’t be used to listen to CW (Morse code) or SSB signals, it only got strong stations in the day (and when it did, often more than one at the same time), and you had to listen carefully with headphones. In other words, poor sensitivity, poor selectivity, and low audio output. Also, it was infeasible to build a crystal set useful above roughly 40 meters, unless you were monitoring the transmitter next door.
Under ideal conditions, though, it could provide superb fidelity and that’s why crystal sets were manufactured commercially again in the 1950s, as a tuner for high fidelity buffs who wanted good AM reception. When FM became popular, this ended. Another later use for crystal sets was on ships as an emergency receiver, if for some reason all else failed. Since they were light, small, inexpensive and consumed no power, they were usually built into the ship’s radio equipment.
My recommendation for more information on crystal sets is to obtain, if possible, the first three volumes of Alfred P. Morgan’s Boys Books of Radio and Electronics. These are somewhat scarce, in contrast to his “The Boy Electrician” whose early editions are now public domain and therefore reprinted widely. Modern publications include those sold by Lindsay Publications, such as “The Impoverished Radio Experimenter” and the books put out by The Xtal Set Society . Ed Romney’s ‘How To Fix Up Nice Old Radios’ has useful material on these and later sets as well.
The TRF Receiver
People wanted better selectivity and sensitivity and above all they wanted to be able to listen to the radio at a normal volume, without headphones. Amplifying the signal, tuning it carefully, and feeding it to a loudspeaker solved those problems and an invention called by the British a “Thermionic Valve” made that possible. We on this side of the Atlantic know it more commonly as a vacuum tube.
The valve, or tube, started out as a light bulb that had been fitted with a metal plate across from the filament. If the light bulb was running and a second voltage was applied from the filament, which became a cathode, the plate became an anode and current would flow in one direction but not the other. Thus it could rectify alternating current and act as a detector for modulation put on a transmitted carrier. It could not amplify a weak signal, though, until it was found that if a “fence” or grid was put between the filament and plate, a third voltage would in effect open or close the gate and cause current to flow, or not to flow. Moreover, it could make the current flow a lot or a little, like the throttle on an engine’s carburetor. (Remember those?) A small change in voltage could cause a large change in current, so in effect it could amplify weak signals. And it could do it at DC or as high as many megacycles, meaning it could amplify both audio and radio signals.
There you have the TRF receiver. It consisted of one or more stages of radio frequency amplifiers, each with a tube, and with separate tuned circuits in between so the desired radio frequency would be received at the exclusion of others: then, a detector that just as with the crystal set changed the modulated RF signal into an audio signal: and then one or more untuned audio amplifier stages that made the signal, louder than before but still no match for a loudspeaker’s needs, big enough to drive a speaker that everyone in a room could hear. Of course you could still use a headset, but only hams and “night hawks” or “DXers” did that. The TRF set meant that radio was now a family affair, and Dad tuned the set so everyone could listen. The TRF set was expensive and delicate enough that in most families, the kids (and even Mom) were not allowed to fool with it. Radio had changed a lot.
Although most houses had electricity, except on farms, most TRF radios ran on batteries, because no simple and cheap method existed to turn AC into smooth and quiet enough DC to run a radio set. Two, and sometimes three different kinds of batteries were needed: a low voltage high current supply, usually 6 volts for the filaments (called the A battery) and a high voltage low current supply made up of a lot of small dry cells in most cases. The A battery was usually borrowed from the family car if they had one. (More did than not, except in New York.) The B battery was bought new and discarded and this made for a considerable expense. So did the tubes, which at first only lasted a few dozen hours if that.
The TRF became obsolete within a few years, around the same time AC powered tubes and usable capacitors made batteries obsolete and tubes began to last longer and cost less. A few were built later on, again occasionally for “hi-fi nuts”, and in Britain especially, they were used for surveillance by MI5 and MI6 and so forth because they had no local oscillator to give their presence and location away. Spies, real or imagined, would listen to what were then “regular” radios and the counterintelligence service would monitor their local oscillator emissions to catch them. American military and intelligence services had a different solution, which didn’t occur to the Brits. Then again, they put the plumbing on the outside of the building so they can get to it easier when it freezes up.
I can’t think of any circumstance where you’d want one today, but the TRF does exist and did work.
The Regenerative Receiver
The regenerative receiver, often called a ‘regen’ or a ‘howler’, is based on another property of the vacuum tube, and amplifiers generally. If you feed some of the output of an amplifier back into the input, in phase, it will start swinging back and forth electrically, or “oscillating”. A radio transmitter, in its most basic form, is an oscillator. Its frequency is controlled by a tuned circuit or by some other kind of filter.
If you set up a vacuum tube as an oscillator, and fed in a radio frequency signal, and adjusted it so it was just barely ready to oscillate, it would make a really sensitive detector. The regen was cheap to build, it had a single tuned circuit, and it was sensitive and it became a really popular homebrew project. Never popular as a broadcast receiver, the regen made listening to short waves possible for anyone who could scrape up a tube, a headset, and enough wire to wind a coil or two plus some batteries. Since there was a Depression on, and since people were (even so) throwing out stuff with wire and tubes you could salvage, building a regen was the stuff of schoolboys and impecunious hobbyists.
The one tube regen wasn’t a great radio. It would pick up a lot of stations, often more than one at once. The sound quality was poor, because the detector distorted. The antenna was part of the tuned circuit so any wind or movement near it made the frequency wander. And since it was still an oscillator, it would transmit as easily as it received, causing interference and neighborhood fights. (Today it still might, and with guns rather than fists in a real disaster.) And, it would sometimes oscillate at audio frequencies as well as radio ones, causing the listener to be blasted with a sine wave so loud he’d rip the headset off and throw it.That’s why they were called howlers.
Later regens, commercial or homebrew, improved somewhat on these problems. An RF amplifier and tuned stage, as on the TRF set, was added, both for more sensitivity and to keep the regenerative detector’s RF inside the set where they belonged. A stage of audio amplification was also added after the detector, to reduce the load on it, help stop howling, and give more volume. And the set was shielded and a precise tuning and regeneration controls added. Probably the best regen ever built was the National SW-3, which would still be a nice thing to have today, even with its limitations, for a serious prepper. But compared to a modern superhet communications receiver, it has poor selectivity, poor audio quality and good sensitivity only up to about 10 megacycles. (They call them megaHertz, MHz, now.)
But building a regen is good experience and could be a lifesaver in a situation where no other radio is around. You can use transistors instead of tubes, as well, should you have any that still work. (And if you correctly stash away a few dollars’ worth of them at surplus prices now you will have hundreds of them that work). They quit building regens commercially just as soon as the superhet became understood and the patents didn’t stop them, except for hobbyists and a few kits in the late 1950s for kids. The exception: Marine suppliers made and sold regens as late as the 1960s for 500 kHz marine service on ships. Even though the Germans sank ships with them by listening for their characteristic emissions, homing in on them and torpedoing them in WWII.
The Superheterodyne
Major Edwin H. Armstrong invented a new kind of receiver and patented it in 1918. (He had also invented the regenerative and the superregenerative receiver and would go on to invent frequency modulation later. ) It solved the problems of making a good radio that could be tuned to different frequencies by having a variable local oscillator-in other words, a small generator or transmitter built into the radio itself-that could be tuned easily and that would convert the received signal into a second, intermediate frequency, that could be filtered, amplified and detected. This meant that with a single knob, a mass produced, inexpensive set could be as sensitive and selective as the most complicated and fussy precision TRF set, and it could detect the signal cleanly, with great fidelity.
Superhet receivers became utterly dominant by 1930 and still are. They could be built with as few as two or as many as 40+ tubes and when transistors came out they could be built cheaper with those. Modern superhet receivers may consist of just one integrated circuit or ‘chip’ with a few, tiny, inexpensive capacitors and coils around them.
One of the most popular early types of superhet radio you should be aware of is the famous “All American 5”. It can run on AC or DC, has no power transformer, and was called that (it’s sometimes referred to as an AA5 in print) because it had five tubes, in a particular layout. There was a converter tube that acted as an RF amplifier and a local oscillator in one, an IF amplifier, a detector and first audio amplifier tube, a power output tube and a rectifier tube. They were often offered as a kit for hobbyists or for training in vo-tech schools from about 1940 to as late as 1975 or even 1980. (Yes, tubes were obsolete, but government funds weren’t.) They were also sold by the tens-maybe hundreds- of millions in every country with 100-130 volt power. Most of them were just for the AM broadcast band, and they gave good local station performance, but a few AM and shortwave versions and even a few VHF aeronautical band versions exist. These radios have one dangerous characteristic: One side of the chassis is hooked to the AC power line, and if it’s the hot side the radio will work just fine, but if you come in contact with any metal parts connected to the chassis you will get a severe shock. The radio must be repaired or junked if the case or knobs break or the chassis is exposed. If it is a metal case radio then it should only be used with an isolation transformer no matter its condition.
All of the communications receivers and transceivers you will use are superhets and with that exception, most have a power transformer and are isolated from the AC line, or in the case of solid state radios they may run from 12 volt DC. Building superhet radios from scratch requires intermediate frequency transformers and other specialized parts, as well as test equipment to align them, and will not be something you’ll do on a field expedient basis. Even experienced hams quit building their own superhets because they could buy them commercially made cheaper than the parts cost and because most were too cheap to buy good test equipment: by 1960, kids had other interests and ham radio was mostly middle aged guys whose ham shacks were a diversion from nagging XYLs and an excuse to buy expensive stuff to impress their ham friends, just as it is now.
What you will want to do is to buy the good radios now while you can, learn to use and maintain them, and to be able to select the good ones from the not so good. You will definitely want to buy some tools and test equipment while they are cheap now, because few want them. You will want to stash away those parts you can get cheaply, and acquire at least the skills to know what you don’t know and where to go to change that. (More to follow in Part 2.)
Her books, DVDs, and flash cards all have a well-deserved positive reputation.