On 02/23/16 06:29, Peter Wieck so wittily quipped:
But, I suggest you look into the AMT3000 or 5000 transmitter (made by a neighbor of mine, Phil Bolyn) that even shipped down under is reasonably priced and with excellent performance. They are Part-15 compliant in the US, so likely might be legal down under as well. They can certainly be throttled as needed to cover either just your dwelling - or several handfuls of hundreds of meter/yards/cubits with the proper antenna and a bit of elevation.
http://www.sstran.com/pages/AMT5000/..._overview.html
looks like a nice transmitter. it'd probably work well for an amateur
AM radio station, driving a licensed power amplifier and antenna, so
long as freq instability doesn't cause it to cross into anyone else's bands.
I was thinking you could build one with some kind of balanced modulator,
with a detector on the output creating negative feedback to limit
distortion. you'd bias the modulator at 50% for zero signal level, and
then it should give you up to 100% with minimal THD. But that
transmitter also has a 5:1 compressor and other things you really need
for an AM transmitter. So yeah, probably can't build one cheap enough
(and have it be 'legal').
open-ended balanced modulators would stink for THD, but using negative
FB would make it work well, I bet. So yeah, just have a detector off of
the output, then use for NFB.
100mw into 50 ohms is ~2.2VRMS so NFB would be practical. 300 ohms
would be ~5.5VRMS (appropriate calculation for 100% modulation would
also give you 'that value' as the detected audio RMS voltage).
I suppose you could design an output transformer (air core even,
tunable) that would give you both impedences, then adjust RF gain
accordingly. A sharp cutoff pentode with an AGC-like circuit would do
nicely, such that the '50% modulation' output level on the appropriate
tap gives you 100mw.