This transverter is based on my 6M receive converter. A 32MHz TTL oscillator is used as the local oscillator for both transmitting and receiving. RX/TX switching is handled by three SPDT relays. The first relay is used for switching the 18MHz input/output. The second relay is used as the 50MHz antenna relay. The third relay is used to switch the DC power supply.
Details of the receive converter are here: 6M receive converter.
The 18MHz drive signal is converted to 50MHz by a dual gate mosfet mixer. The mixer is followed
by a band pass filter consisting of three parallel tuned circuits. The three inductors are six
turns wound on a Toko 10K former. The three 180 Ohm resistors are one watt carbon. Do NOT use wirewound.
The mixer needs about three watts of drive at 18MHz. The drive level can be adjusted by changing the
value of the 2K2 or 47 Ohm resistor.
The low-level signal from the transmit mixer is amplified by a two stage, class A amplifier. This increases the power to several hundred milliwatts. I had several QSO's with the few hundred milliwatts output from this stage. T1 is four turns, bifilar wound on a small ferrite toroid. RFC is a radio frequency choke. Anything around 1 microHenry should bo OK. Both transistors require a clip on heatsink.
The driver and PA transistors were pulled from an old VHF two-way radio. The driver is an SD1012, the PA is a Motorola SRF1585. Both stages are operated in class A. T2 is a 4:1 transmission line transformer. I used 4 turns, bifilar wound on a ferrite toroid. Both RFC's are about 1 microHenry. If you can't find any high current chokes, try using two or three turns of wire on a ferrite toroid. All resistors are low inductance carbon types. DO NOT use wirewound resistors. The 100pF capacitors are silvered mica. 100pF trimmer caps are hard to find. You can use 10 to 60pF trimmers with a fixed 47pF capacitor in parallel. You probably won't be able to find a supply of these very old transistors. You need a VHF driver transistor, capable of dissipating several watts. The PA transistor should be able to dissipate 25 Watts, or even more. When I find time, I will try to build a new PA, using more readily available devices. L1 is 3.5 turns of 1mm copper wire. L2 is 5 turns. L3 and L4 are 7 turns. L1 to L4 are all 11mm O.D. Both transistors are mounted on a large heatsink.
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