I rebuilt the PA with bifilar wound choke feeding DC to the
push-pull drains and the primary winding on the toroid has
no center tap. With no load and with 50 ohms directly on
the secondary the drain waveforms are not bad at all - pretty
square with some ringing which is not a problem, the FETs are
600V so I won't bother to snub the spikes.
Output filter is a T circuit to present a hi-Z load to the PA
at the harmonics. Drain waveforms remain OK with the dummy
load attached via the T filter.
MOSFETs, output transformer and DC supply choke don't get even
slightly warm with 120W input, so that's a success thanks to
everyone's advice.
The coils of the T filter wound on ferrite toroids get very
hot very quickly. It's the ferrite producing the heat not
the wire. I must have the wrong flavour of ferrite so I
ordered some different ones (nice blue EPCOS ones) but they
wont arrive till Monday. Meanwhile I'll wind some air cored
coils for the filter.
A bit of work to do tomorrow on the antenna match and I need
to bury some metal for a better RF ground. Then will try some
carrier on air.
Gary wrote:
> Do you realise that [137.5 kHz +- 50 Hz] is already the
> established center frequency for Opera & WSPR ??
Convenient for EbNaut also, because the files recorded for use
by OPDS can also be used for EbNaut via a conversion program
written by Markus, hence Stefan's suggestion.
To start with though, I'll probably put some carrier in the QRSS
band because that's where the narrow band grabbers seem to be.
--
Paul Nicholson
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