Hello all -
Mal and others
have been asking for details of the high power amplifier I described
earlier. At the moment only the QRP 650W version has been tested and the
2-3kW design concept was a best otline estimate based on my tests of that
breadboard.
As this sort of
design is not for the faint hearted I am not just going to give
out circuits that anyone could try building from (at this stage
anyway). Just think of the damage 5000uF of capacitors charged to 340V
would do if shorted. That is nearly 300 joules of energy, equivalent to
dropping a 5kg sledge hammer from the 6m second floor of a
building. I blew 4 IRF840s on the breadboard due to various (now
obvious) reasons, and each time they went short with an almighty band that set
ears singing and me into a state of shock ! This design has 5 times that
stored energy.
The outline
concept given is enough to get experienced constructors going.
If you need more info than that, then you're not ready to be building such a
beast !!!!!
Here's the outline
again :
Drive stages, 2.Fc
input, divide by two, BPSK phase modulator or other on-off gating, HP4426/7
driver chip. NO RAMPING OF DRIVE TO TRY TO AVOID CLICKS, don't even attempt
it. Could try using the invert/non invert HP4428 driving the drive
transformer in push-pull (double the primary turns) - gives more power to
drive 4 FET devices together.
4 x IRF450 or
IRF460 in full - bridge driven as per the Decca transmitter output stages with
BYV29-500 catch diodes. Current spike soak, 1-2uH CT choke on each pair
of devices, damped with 20 ohms Look at hte LF experimenters
manual for the Decca Tx design.
Tank circuit Ql =
6, caps rated at 3kV, use series parallel to take the current / voltage.
Inductor wound with Litz wire - use Rayners formula to estimate the turns
needed and measure from there.
Impedances and
transformer taps calculated as per standard techniques
Vcc = Vpeak.
Vsine peak = Vsq peak * 4/pi. Pout = Vcc^2 /
2.Rl
PSU Mains -
20A bridge rectifier, 5000uF capacitor(s) output 340V DC,
(obviously with soft start and surge limiting).
Plenty of input
filtering, lots of PSU rail decoupling at wide frequency band - all the usual
etcs. of PA design
One of my aims
over the Winter Solstice enforced holiday season is to build up a website
where circuits and software can be placed. However, don't hold your
breath, building more interesting RF projects and software will probably
get in the way, so probably won't happen
:-(
Andy
G4JNT
