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80m DC rig

News: I wrote an article (in Swedish) about the rig, part of it got published in SSA's magazine QTC.

This is the first somewhat useful rig I ever built. It's simple, single-frequency (unless you consider switching 3 crystals to QSY an option - for now 3.579 MHz is enough) and has a maximum output power of about 1 W (with a 9 V battery you get a bit less, about 500 mW). It can run on between 8 and 15 volts, in receive mode it uses about 30mA of current. The power amplifier is basically a single 2N7000 MOSFET transistor, followed by a low-pass filter. A few percent of the energy is wasted in the low-pass filter - for a small power increase, you might want to try a different one (like 680pF/1200pF/680pF + 2.16/2.16uH). After some amplification the RF is mixed with the LO in a simple diode ring mixer, the AF is filtered and then amplified by an LM386 IC (configured to give 46dB of gain). Not shown on the schematic is the 4-pole, 3-position switch I use for RX/TX/OFF switching. In RX mode, it connects the antenna to the filter and feeds the 78L06 with the supply voltage. In TX mode the 78L06 gets no voltage, the antenna is disconnected from the input filter and the 10 pF capacitor in the XTAL oscillator is bypassed.

The total cost of the project is about USD 6 (excluding wiring, case, speaker/earphones and antenna - which shouldn't cost more than a few dollars together, anyway). I got most of the parts from Mouser and Futurlec.

I have tested the rig now, with SM0JZT. I used about 250 mW and JZT's K2 transceiver gave 100 mW, very strong signals in both ends over a distance of 3 km.

Improvements to be made

Parts list

Total cost: about USD 6.40 (but to get some of these components this cheap, you'll have to order a few (usually 25-100) of them

Schematics

Pictures