I have received several e-mail messages from people looking for a design for a very simple receiver. As most people reading this page, have access to a computer, I decided to use a computer sound card as the A.F. amplifier for a simple TRF receiver.
This single transistor regenerative TRF (tuned radio frequency) receiver can receive SSB, CW, and AM. The transistor is a VHF dual gate mosfet. I used a 3SK45 but a 40673, 3N201, 3SK88, BF961, BF981, or any similar device should be suitable. For L1 I use 30 turns of insulated wire, wound on a half inch diameter plastic tube. The aerial coupling coil L2 is 5 turns wound over L1. With this inductor, I can tune from 3.5 MHz to about 7MHz. Use more turns for lower frequencies, less turns for higher frequencies. For frequencies above 15MHz, use 220pF capacitors in place of the 560pF capacitors. The A.F. output from the receiver is connected to the Mic. input of the computer. This increases the total number of transistors used, from one to several million :-) If you don't have a computer, you can use the audio amplifier at: 80M_.html
To receive AM, set the fine Regen (Reaction) control to about half way, set the coarse Regen control to slightly below the point of oscillation. Tune in the AM station with the main tuning control. To receive SSB or CW, set the coarse Regen control to just above the point where oscillation begins. Tune in the SSB/CW station with the main tuning control. There is a lot of interaction between the three controls.
Very strong SSB signals cause some frequency variation on voice peaks. CW and AM reception is very good. The selectivity is surprisingly good, for a receiver with only one tuned circuit.
I use a 9 Volt battery (6F22 or PP3) as the power supply. If you use a mains supply, make sure that is well smoothed and regulated. DO NOT connect a high current supply to your computer mic. input.
When receiving SSB or CW, some of the oscillator signal will be radiated by the antenna. In the valve (tube) era, regenerative receivers were notorious for causing interference. This is not likely to be a problem with this low power circuit. The radiated power should be less than 1 mW.
SSB. ssb.wav SSB signal received on 80M
CW. cw1.wav CW signal received on 80M
AM. am.wav AM broadcast station from USA.