Hello
Jim!
First
i want to say that i am glad to meet you here. Do you remember our QSO in 2004?
And mni tnx for your article to the tuning meter (and others)! I have built it
2003 and it is for me the most useful measuring device on LF at all!
Yes,
on 137kHz a half bridge is also a good idea since the losses are quite small. But,
for a half or full bridge like in these decca transmitters, one needs an output
ferrite core that can handle the whole output power. I think these are
expensive (?), not easy to get and could be driven into saturation when dimensioned
well. In my class e PA I use a single (!) IRFP360 that costs 1,9 Euro in the
moment. This PA can easily handle 1kW with an efficiency of more than 96%. In
the whole PA circuit, no ferrite cores are used, just two iron powder cores for
the output filter. The driver is one cheap ICL7667, available for 1,1 Euro. All
Capacitors are of the type wima FKP1
(2kV version), costs are below 1 Euro. So, I think a half bridge is more
expensive?
Best
73 and hope for another QSO!!!
Stefan
/ DK7FC
-----Ursprüngliche
Nachricht-----
Von: [email protected] [mailto:[email protected]]
Im Auftrag von James Moritz
Gesendet: Dienstag, 15. Dezember 2009 15:15
An: [email protected]
Betreff: LF: Re: "Gain" between qrss3 and qrss10 / PA design
Dear Stefan, LF Group,
A much better choice of circuit topology for QRO
transmitters using an
off-line rectified mains PSU of about 330V is the
half- or full bridge,
voltage-mode class D, as used in the "Decca"
PAs, and G4JNT's off-line
design. Here the peak voltage across each MOSFET is
only equal to the DC
supply voltage, so cheap 500V Mosfets designed for
off-line SMPSUs are
ideal. The class E and push-pull current-mode class D
designs are better for
relatively low voltage supplies, say < 40V.
The limitation on TX power that can be used at LF with
a vertical antenna is
the antenna voltage, roughly Iant * XCant. In my
experience, corona
breakdown becomes a problem in the 10 - 20kV range
with a wire antenna. So
to use high power, and so achieve high antenna current
and ERP, you need
high antenna capacitance. Also, the voltage will be
increased if the antenna
loss resistance is low. I found that, with an 10m high
inverted L antenna of
about 50m of wire in total, I could run 1.2kW
reliably, provided care was
taken to eliminate sharp ends, narrow spacings, etc.
in the antenna and
tuner. This resulted in up to 5A antenna current, and
about 17kV on the
antenna wire. Even so, I have to increase the antenna
height in order to
approach 1W ERP. At 500k, the antenna impedance is
much lower, and
efficiency also usually higher, so TX power is less of
a problem.
In my experience, DX contacts using QRSS30 are quite
practical, given a
large supply of strong coffee! But with longer dot
periods, good propagation
does not last long enough to complete even a minimal
QSO.
Cheers, Jim Moritz
73 de M0BMU
