Alan,
May I timidly ask if anyone here can fathom the purpose of the article
on page 14 of the current ARRL QEX magazine?
Unfortunately I cannot, because for well over a year the magazines from
ARRL (both QST and QEX) aren't arriving here. Either they aren't sending
them, or someone in the mail service is stealing them. But the renewal
reminders sure arrive, all of them! My QEX subscription is lapsing now,
and ARRL membership is running out next month. I see no point in
renewing, if the magazines just don't arrive!
When I complained over a year ago, I got one issue of each, then
delivery stopped again. Very suspicious.
It describes an amplifier
using a high voltage IGBT which is operated for 600 cycles of 20MHz RF
once every 50ms. The magazine uses the by-line "A Forum for
Communications Experimenters" but I can't see what use this amplifier is
for communications. Do I just lack imagination?
Well... with all the freebanding and piracy going on, maybe
comunications is indeed moving to using high power, in brief pulses,
with some sort of pulse code modulation, on "nice round" frequencies!
Just tune through the HF spectrum, watch all those sweepers, pulsers and
other noises, plus the out of band transmissions by all kinds of people,
and you will soon notice that only a small minority of HF users these
days pay any attention to rules and regulations.
Of course, if I was interested in building an over the horizon radar
then this article might seem more relevant.
They aren't using pulses nowadays. Mostly they use sweeped carriers.
Most use sawtooth FM, but some use more complex frequency sweeping, with
logarhythmic single sweeps, or sweeps with modulation on them, or
something else.
When I see any amplifier rated for pulsed service only, my immediate
thought is that there was someone at work who knows RF design, but has
no idea about thermal management, so his amplifiers burn out as soon as
he tries to transmit continuously! :-)
Manfred
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