Message-ID: <000601c06915$2e8b88a0$8e2a883e@default>
From: "MAL HAMILTON" <MAL@g3kev.freeserve.co.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: RE: other reports
Date: Mon, 18 Dec 2000 17:06:00 -0000
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-----Original Message-----
From: Talbot Andrew <ACTALBOT@dera.gov.uk>
To: 'rsgb_lf_group@blacksheep.org' <rsgb_lf_group@blacksheep.org>
Date: 18 December 2000 09:16
Subject: LF: RE: other reports


>Still the same power, but wet surroundings may make the antenna less
>efficient.  The load R has risen so looks valid.  The BPSK message is
>stored in PIC so won't change in the immediate future, I was soak
>testing the PA and thought you'd prefer a message to a carrier.  Keeps
>within the spirit of the licence ID rules as well.
>
>May not persue the 2.5kW full bridge PA design.  I had a couple of
>rather frightening explosions yesterday that were a bit off-putting.
>NEVER, EVER EVER use a make before break switch for hot switching
>transformer taps

Hot switching RF under any circumstances is risking device failure. I
thought that was golden rule number one.!!!!!!!!
While make before break helps it is better to switch off, make the tap
adjustment and then turn it back on again. I use this procedure when
adjusting my PA power levels and so far have never blown any FETS !!!!!!!!!
It pays to be careful.

Regarding super ac/bridge amplifiers. I know a man using that procedure and
he can light up a 3kw electric fire as a dummy load. He is winding a new
loading coil for the antenna using copper microbore tubing.
Monitoring on a scope reveals a nice square wave into the electric fire but
when connected to a 50 ohm dummy load the wave goes triangular. Expect there
is an impedence mismatch/loading problem but the device is still in the
experimental stage and no doubt will be rectified. These amplifiers need a
man with a steady hand and a stout heart. It is important when testing to
get behind a screen to protect yourself from flying debris if there is a
bang. The electric fire DL versus the carbon resistor 50 ohm load is the
opposite to what one would expect. The electric fire wire would element
characteristics need investigating. The man says going beyond the 3kw o/p
level causes too much voltage sag and almost puts the house in darkness.
Expect he really needs a 3 phase supply.


, I hadn't spotted it was Mb4B.  That's the trouble
>with having a very extensive junk box - don't tend to look at components
>carefully enough.
>But the present half bridge design will upgrade to 800W reliably with
>IRF462 devices -cheaper than IRF450.   Still need a very fast acting OP
>short circuit protection circuit.
>
>Andy  G4JNT
>
>
>> Also Andy is on testing again at 13.00 but the same old
>> message - could send
>> out some sort of lowfer news for those without E-mail. You
>> are S8 with me
>> Andy, but not sure whether full power, sounded louder when I
>> first heard
>> you, but then went off before I could look at the S meter.
>> John, G4CNN
>> Reading, IO91ML24
>>
>>
>>
>>
>
>
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