Homebrew QRP transceiver for 40 meters HM7
When I started as a HAM on HF, I built my first Rig myself. This homebrew QRP transceiver comes from “Solid State Design for the radio amateur” page 214…218. In the 10 years that I used this CW transceiver, I worked many European countries with a power of only 500 milliwatts in many normal QSO’s and many contest QSO’s on 7 MHz. I used this transceiver as a homestation and on holiday.
Homebrew, QRPp and CW (Morse code) is a fabulous combination.
The transceiver tunes a 40 kHz section of the 40 meter CW band. The power of the transimitter is adjusted to 500 mW. The selectivity of the receiver is provided by a homemade 3-pole crystal filter of the lower side band ladder type. The bandwidth is 400 Hz and the rejection of the undesired side band is about 60 dB. A good working T/R switch switches automatically from receive to transmit by pressing the key.
The receiver is shown in the upper part of the schematic. The front end is is a dual gate MOS-FET 40673 with a single tuned circuit as a preselector. The IF amplifier has a gain of 40 dB and has no AGC. The gain variation of 60 dB is provided by a manual gain control potentiometer. The product detector is balanced and has 4 diodes. The BFO injection is provided by the oscillator. (RX) The audio amplifier consists of 4 transistors.
The VFO on 2.6 MHz has an excellent stability because of the low frequency.
In the lower part of the schematic, is the transmitter part. The carrier oscillator is a bipolar transistor in the Collpitts configuration. To adjust the frequency to the center of the i-f band pass it was necessary to add inductance and capacitance to the circuit. I use a LM1496 as a mixer to make the signal on 7 MHz. The output of the mixer is applied to a two-pole bandpass filter on 7 MHz. The pre-amplifier is keyed. For the final amplifier I choose a SC1678 because of the high Vce of 65 V. Even with no antenna connected, the transistor survives time after time. hi The output network is a half-wave filter. I don’t use the side tone cicuit because there is an audio tone in the head set with the key down.
The crystal filter gives the receiver its excellent selectivity.