Initial Thoughts

ARNSW is running an 80 homebrew challenge this year. I am yet to clarify what "AM modulated" means, IMO that either means full sideband + carrier AM, or any non-FM/PM mode, i.e. CW, DSB, SSB, etc. I'd like to build a DSB rig, or an "SSB in place" one, as they are mandating crystal-locked on 3.6864 MHz. I'd also like a full TRX, not just a TX. I don't think it will really matter that much, but I intend to go along to the next meeting and clarify the requirements.

OK, not much so far, but I have a crystal oscillator (currently using a colour-burst crystal) with a class-A buffer and a class-C driver that delivers about 150 mW into 47 Ohms.

Local Oscillator, Buffer and Driver

The collector load for the buffer is about 800 ohms, matched into the base of the driver with a 4:1 transformer.

The collector load for the driver is about 200 Ohms, matched to the load using an L-network comprised of an 28 turn inductor on a T50-2 core (about 3.8 uH) and a 390 p capacitor. More power is available with good stability by increasing the gain of the buffer with emitter resistor bypassing (and/or tuning the L network more precisely, it needs a bit less inductance for a perfect match - do the math yourself and see) but I am happy with this level. +21 dBm is plenty to drive a mixer, in fact with one more stage (BD139 or 2N3053) it would make a respectable QRP CW station, just key the buffer.

Only another 21 dB to go!

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Parent article: "2007 80m Homebrew Challenge".