Hi Friends. You know, there must be a big interest in solid state radios in my neighborhood. Last night I was sitting outside, in front of my radio, with the headphones on. Nearly everyone that drove by stared at my radio. It is nice to see so much interest here.
The idea for this radio came from a fellow radio builder and BCB dxer, Mike Tuggle. He built this radio for the 2007 1AD contest. The original design, as far as I can tell comes from an article by G.W. Short called the Homodyne published in the UK in March, 1972. I won't go into the theory of operation as the two links above explain it better than I could.
This radio uses one FET and one PNP transistor. The outstanding feature of the Homodyne is the limiter (with the two diodes back to back) and the feedback network (220k ohm resistor). The original circuit had an additional transistor, and this all would drive an optional audio amplifier. I built mine as Mike Tuggle did as I also am using sound powered (balanced armature) headphones which are very sensitive. If you want increased drive, you can build it as originally shown in the British magazine.
The radio requires 9 volts at just a few milliamps.
This radio is capable of it's internal oscillator (from the regeneration) locking on to the incoming signal carrier. This provides enhanced reception performance. Most regen radios, when the gain is cranked up will tend to lock on to the signal carrier, but this set does it better.
My Homodyne radio is built on a small chassis and front panel made from two pieces of garolite. the chassis is 7 by 6 inches wide.(17,75x 15,25 cm). The front panel is 5 inches (12,7 cm) tall. This layout is much like my superhet and Hikers' 1 radios.
The three curves on the front panel were drawn using a large pizza pan. (I told you I build these things in my kitchen.) This gives the front panel of my Homodyne that most sought after skyscraper look.
All the parts are mounted on top of the chassis. The small parts are mounted on a circuit board made with 0.1 inch hole perfboard. I used to make a lot of projects with this board. I was glad to see that I hadn't lost my touch.
My radio is mounted on an oak base. There is room behind the chassis where the loop antenna mounts. The loop is mounted on a 1 inch dowel. The length isn't important. I selected the length by seeing how long a piece would fit under my drill press.
The loop is a 12 inches in diameter (30 cm) and mounts on top of the dowel mast. I cut a 6 inch hole (15 cm) from the center to make the loop lighter and add some visual interest. Two thumb nut posts provide the connection of the loop. When transporting the radio, it is better to disconnect the loop and carry it separately.
Two wires are need to connect the loop to the rest of the radio. I used two pieces of small test lead wire because it is very flexible. The wires you use should be held together to keep the capacitance changes at a minimum while turning the loop. A piece of RG-174 mini coax would work well too.
A smaller loop could be used. I went to a larger size to increase the capture area. Here is a place where you can use your imagination and come up with something that is really cool. I used 330/46 litz wire to make this antenna. I wanted to use something fairly robust as signal separation is important. This helps keep the station locked, even if there is a brief fade. The tuning then is less likely to jump to a neighboring louder signal.
My version uses a Bogen T725 transformer. An impedance selector switch is incorporated. You may select the taps that best suit the headphones you have.
I used a 540 pf variable capacitor (dual gang 270) in my Homodyne. I also used a 4-30 pf trimmer capacitor so that I could set the tuning range closer. The problem with the loop adjustments is when you take off one turn, you remove a fair amount of inductance. Since half turns weren't practical, I compensated for this with a trimmer.
A smaller capacitor, say a 440 to 500 pf could be used, if it were a single gang capacitor that would have a low minimum capacitance. Otherwise it is better to stick with what I used or up to a total of 600 pf. In my case, the tuning range is 525 khz to 1720.
The capacitor has a 2:1 vernier and I added a 6:1 in front of that, giving a total of a smooth 12:1 or 6 full turns of the tuning dial. I made a 3-1/2 inch (9 cm) dial from 1/8 inch (3 mm) HDPE. I was out of styrene, but this worked the same. I made some vertical numbers, just as I had in my superhet radio with my Brother P-Touch labeler. The regen-lock pot on the front panel has a metal case. I soldered a wire on to this case and to ground. This was suggested in the original article as a way to improve stability.
There is not too much alignment with the Homodyne. Just try to cover the entire band, by carefully adjusting the coil. You can always test by romoving one turn, and let the wire lay loose. Then see how it tunes. If you are going in the right direction, then cut and tin the litz and try again.
There is a 500 ohm regenration pot on the board. It isn't necessary to panel mount this control. I found a single setting that would work across the entire band.
The Homodyne is quite easy to operate. It is just point the antenna and tune the radio, with an occasional movement of the regen-lock control. After a short time of operation, you will notice the rado will stay locked on to the carrier. It sure is a fun radio, and the design has been contest proven.
Happy building de N2DS!