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
