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
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Tuesday, December 15, 2009 12:44 PM
Subject: AW: AW: LF: "Gain" between qrss3 and qrss10?
Yes, you are right. But i mean i USA, where the voltage is 110V, you get
about 155V and thus the 900V FET would have 6 times more. For a class e mode
PA, 4x the voltage can be reached when SWR is correct(!) so there would be a
goog savety factor.
73, Stefan / DK7FC
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