On 01/03/2018 06:43 AM, DK7FC wrote:
Am 03.01.2018 00:37, schrieb N1BUG:
I was forced to stop calling CQ on JT9 at 2320 UT when my PA again
became unstable.
So it is a thermal problem!?
Sure seems so, but nothing appears to warm up much. On MF the case
of the FET feels just slightly warm after a minute or two of
transmitting, but the cores, capacitors and heat sink don't feel
warm after 30 minutes of 40% duty cycle operation.
On LF the situation is similar except that the case of the FET
becomes very warm after a transmitting for some tens of seconds. It
settles down to the heat sink temperature (slightly above ambient in
the room) within 10 seconds or so after I stop transmitting.
On MF it will make about twice as much power into the antenna before
it has problems.
Usually LF should be easier than MF. If MF is easier, than it could be
saturation of cores (at LF) or too low time constants of RC components.
But if the cores stays cold and the voltages are constant, this should
not happen.
Voltage is very constant.
On MF it is much more efficient with the FET running almost cold
compared to very warm at half the power level on LF.
Then you should use more turns on your 1:9 transformer!! E.g. 3x10 turns
instead of 3x5 turns.
That is the one thing I have not tried yet! Today I will rewind that
transformer with 3x10 turns. I will try to use slightly thicker wire
if I can find something suitable around here. Let us see what happens...
It is still a mystery why these problems do not occur when running
into a dummy load. Could not having enough turns on the transformer
cause the PA to be more sensitive to slightly reactive loads or
loads that are very reactive out of band?
Paul
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