Re: Output vs. frequency behavior QCX 30m
|
If you are trying to operate on both 30/40, the answer is likely that the
output stage, which is a class E amplifier, needs to operate reasonably close to
its optimum resonant frequency.? The series tuned part of the circuit (C30
and L4) can’t be tuned to a “compromise” frequency without degrading one or the
other or both frequencies.? Although broad enough for a single band, it
just won’t work that well if you are away from the resonant frequency because
its efficiency depends on its switching point of the output fets occurring at
near zero current.? As soon as its resonant frequency is moves
significantly away from it’s optimum single band frequency, the three output
fets will experience much higher current at the switching point and end up
dissipating a lot of power, reducing the output power.?
-Steve K1RF
toggle quoted message
Show quoted text
Sent: Wednesday, December 13, 2017 9:03 AM
Subject: Re: [QRPLabs] Output vs. frequency behavior QCX
30m
?
73
Enno,
I too find the same as you. I don't know the answer. It sure works
on both 30/40, which also is unexpected, after all I was trying to make mine
work on 20/30 !
I would like to have higher power output but have not
found the way to do it but I've not been trying very hard. I console myself that
the difference is only maybe 1dB. A doubling of the power would only be 3dB. I'm
pleased though that it works even better on 40m, which was
surprising.
However the little extra power on 40m, in practice, remember
of course that on 30m any antenna is going to perform better generally for any
given height, than on 40m, so in reality of on air practice, I think no
disadvantage in the slightly less power on 30m, aside from the fact it is only
around 1dB in my case, e.g. getting 2W instead of 3W, or 3W instead of 4W,
depending on the voltage. So I'm happy with it. I'd like to find some cheap PA
that would be a separate kit that would take power up to say 10W or so but could
not find any such from searching eBay.
|
