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Message-ID: <BFD4158E8A18420DBE973FDE9339735E@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <38A51B74B884D74083D7950AD0DD85E828AC77@File-Server-HST.hst.e-technik.tu-darmstadt.de>
In-Reply-To: <38A51B74B884D74083D7950AD0DD85E828AC77@File-Server-HST.hst.e-technik.tu-darmstadt.de>
Date: Tue, 15 Dec 2009 14:14:43 -0000
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Subject: LF: Re: "Gain" between qrss3 and qrss10 / PA design
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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" <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
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