The radios described here are a regenerative design utilizing a single JFET and cheap audio amplifier integrated circuit.
(JFET: “Junction Field Effect Transistor”, a small electronic device which can amplify radio signals and is used as the basis for these radios.
Regenerative: employing “regeneration” (feedback), a technique used in simple radio designs to increase the amplification of radio signals.)
Here in Taiwan, there don’t appear to be many people dabbling in radio as a hobby. My recent experiments with simple radios that students might be able to make, have been regarded somewhat like “black magic”. So it hasn’t been too difficult to get students interested in it and a few of them have made the radios described below with very good results.
If one is hoping to generate some interest in this subject it’s always best to start with crystal sets, of which there are many designs around the web. I chose to skip that step though, because of the difficulties of putting up an antenna in this “concrete jungle” and the very real danger of lightning strikes every summer. These radios therefore are designed to work well with no external antenna.
Another design problem was the tuning capacitors. They’re becoming harder to get as most radios are using digital methods nowadays. In the end I found a way to make them out of blank copper clad printed circuit board material. It ended up more educational anyway, since you can see exactly what it is, compared to the small plastic ones that were common in transistor radios. You can’t really see inside them so it tends to add to the mysteriousness of the subject.
This is the circuit diagram of the AM radio:
The antenna coil is 40 turns of thin, plastic covered hookup wire in a spiral construction used in radios of old. The reasons why I chose that were: 1) ferrite rods are also difficult to find in Taipei electronics shops, 2) low inter-winding capacitance gives it good band coverage – this one tunes 500 kHz – 2.0 MHz, and 3) it gave a convenient way to set up the feedback coil (two turns of the same wire) using a simple mechanical adjustment. This method of regeneration doesn’t seem to require much change from one end of the band to the other. In use, it’s seldom necessary to adjust it unless one is carefully trying to pull in far-off stations. I was surprised at how well it worked. There were many stations in Mainland China to be heard at night. Note that although it’s shown with headphones, it can drive a speaker very loudly.
The tuning capacitor was made from three 50mm x 100mm pieces of double-sided pc board. The shape was kind of guessed but it’s not terribly important. The single blade rotor was made from a 1/4″ bolt with some kind of threaded brass things I found at the DIY shop to hold it in place. I didn’t use cheap 1/4″ nuts as they didn’t seem to be machined very straight – they didn’t hold the rotor plate at 90 degrees to the shaft and it wouldn’t slide cleanly between the stator plates (the two fixed plates). Some stainless steel ones I got were, however quite OK, but they didn’t look quite as good as the brass. The rotor is earthed through a small piece of brass sheet which is curved to keep some tension against the nut. The fixed plates have two pieces of thin plastic A4 sheet protector stuck to the inner surfaces so they don’t short onto the rotor. They’re first held in place by hand then soldered to the small piece of blank pcb on the base.
The fine tuning capacitor was an afterthought. It was cut from a tin can, soldered to a brass fitting and screwed onto another 1/4″ bolt (also earthed). It’s handy to have when you’re running it at maximum regen, where the selectivity will be highest, but with a large tuning knob and with practice it’s not essential to have it.
The construction of the variable capacitors involved a lot of trial and error. Although I made a few with multiple rotor plates, a single rotor plate was by far the easiest to assemble. Here are some photos of the ones made so far by myself or my students. I hope it’s clear enough to see the basic construction method. The white material is a kind of nylon kitchen cutting board and is easy to work with. In future I think I’ll stick with that rather than wood.
One of the capacitors in the above shot has plates made from aluminum flashing. This became buckled and twisted too easily so I reverted back to copper clad board.
Here are some shots of one of my students making the AM radio:
Short wave radio with tuned loop antenna
The short wave radio looks more like a kite. Although I’ve made many radios over the years, I had never made one with an antenna quite like this. I was amazed at how much I could hear at night when conditions are best. One of the first things I heard was Radio Australia one night reporting on an earthquake in New Zealand. I’ve also heard stations in the U.S., Japan, The Philippines and many others in languages I couldn’t recognize at all. I’ve heard a lot of activity on the 40M ham radio band (7MHz), although the ones we’re making now have been adjusted for 8 – 12 MHz in order to cover two of the busiest short wave commercial broadcast bands. Of the few stations around in the daytime, most of the strong signals are from China, but at night, really loud stations come in from all around the world. They often fade up and down in strength. Some even develop distinct echoes as their signals arrive via two different paths, long and short. Interesting phenomena with some education potential.
I tried many methods of regen control. The one I found to be the smoothest was actually just an alligator clip attached near the earth end of the loop antenna (the first quarter of it is tinned copper wire, while the rest is plastic covered hookup wire). This was surprisingly constant over large sections of the band, requiring very little adjustment. To set it up, tune the radio around the middle, then slide the alligator clip further up the wire until it breaks into feedback, then back it off a little.
(Update 21 Aug 2012: I added a second regen control in the circuit below. The alligator clip works ok but when you’re trying to get absolute maximum sensitivity and selectivity, it becomes awkward to accomplish fine adjustment. The 10K regen pot in the diagram below gives you a fine control that is quite smooth to use. To set it up, tune the radio to the low end of the dial (capacitor plates fully meshed), set the regen pot to maximum (lowest resistance), then slide the alligator clip up until it just breaks into oscillation. Leave it at that point and back off the regen with the pot. (You will need to back it off further at the high end of the dial.)
Here is the circuit diagram:
The front panel was made from a nylon kitchen cutting board, but I plan to use aluminum in future ones, to reduce the effects of hand capacitance which can make it difficult to tune in stations. In use, it’s best to have it on a sturdy table so that your movements don’t shake the antenna around, as this will alter the tuning drastically. Also, avoid putting hands near the antenna, as that will alter the tuning too. In fact, before I added the fine tuning control I used to accomplish fine tuning by moving my hand near the antenna. (Difficult though, as your arm gets tired when you have to hold it up for long periods near the antenna.)
Here are some photos from different angles:
You may notice that the variable capacitor has only one fixed plate. Not so much capacitance was required to tune across the short wave bands.
Next I want to put some kind of tuning dial on the front panel so we know what part of the short wave band we’re listening to. That will probably have to wait for an aluminum front panel.