2007-01-13
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.
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".