Regenerative Radio

How about a radio that uses a dozen parts, including one transistor and a battery? I built one last night, it's called a regenerative radio, meaning it uses radio frequency feedback to build up signal levels. This technique goes back 100 years when radio tubes were new, and circuits were simple, as a way to get the most out of one tube.

Regeneration works sort of like an open microphone in an auditorium, when you hear echo and squealing, and the PA operator has to adjust the mic level to stop the feedback. In a radio circuit, the feedback is almost instantaneous, and before the circuit breaks into outright oscillation, it can amplify a radio signal 100,000 times or more.

A regenerative radio needs a tuned circuit to select a radio frequency, an amplifier, a way to feed the amplifier's output back to the input in a controlled manner, and a way to demodulate the signal into sound that you can hear. I found the base circuit I wanted to use in a ham radio publication, looking for something simple, and without any audio transformers or hard-to-find germanium detection diodes needed.

modern-regen-schematic.gif

In my case a loopstick antenna coil from a pocket radio was used, and since I want to listen to the AM broadcast band, 530 to 1700 khz., it's just right. For the tickler I used the output coil that's already wound on the loopstick. Main tuning and regeneration capacitors are the kind used in pocket AM radio tuners, with two sections, 140 and 60 picofarads. The FET is an MPF 102, a very common j-fet for radio circuits. The RFC choke coil is measured at 4 millihenries, and off of a circuit board from an old TV. Many regenerative radio designs call for a 2.5 millihenry choke, and some use a resistor instead of the choke. For audio output, I just fed the source lead of the FET into the + side of a 10 uf electrolytic capacitor, and to some RCA jacks, feeding the line-in on a boombox as an amplifier. Many circuits feed a transistor or IC audio amp and speaker or headphones. Power is from a 9 volt battery.

regenerative-radio-fet.jpg
Boomer's regenerative tuner, click for a bigger picture

It worked right away, I had sound as soon as it was set up and the battery was connected, and I could hear local AM stations! On the strongest stations I'd estimate the audio output to be around 100 millivolts. Fidelity was unlike anything you hear out of mass produced AM radios today, with lots of upper end frequency response, not the usual telephone quality audio. In fact, the highs are too stong, and that's because stations use pre-emphasis on the treble range, similar to the boost put on vinyl records to reduce the level of surface noise. It works because the receiver pulls the treble range back down again to normal levels, reducing any noise that happens between the transmitter and receiver. I'd like to try an audio stage that has the complimentary de-emphasis built into it, along with amplification for weaker signals.

Tuning only went up to somewhere in the 900 khz. area, so I'll need to remove the one of the capacitor sections, the 60 pf side. Regeneration is too strong as well, close to oscillation even at the lowest capacitance. That must be due to the loopstick's output coil being too close the tuning coil, so I'll make a tickler that can be slid along the coil and moved farther away. With refinements, I think this could make a nice hi-fi AM broadcast tuner for local stations.

References:

https://en.wikipedia.org/wiki/Regenerative_circuit

http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf

Boomer