From: Paul Burridge
Subject: Re: Should one worry about reflected current into untuned loads at low power?
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com>
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Date: Mon, 09 Dec 2002 11:43:00 +0000
NNTP-Posting-Date: Mon, 09 Dec 2002 13:11:22 GMT
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On 8 Dec 2002 02:28:22 -0800, firstname.lastname@example.org (Tom Bruhns) opined
>Clearly from the context, PA means "Power Amplifier" in this case.
>I'm familiar with the term dating back to the 30's or perhaps before,
>when many transmitters were of the "MO-PA" type: two stages, master
>oscillator and power amplifier.
Indeed. This is nothing to do with audio frequency.
>As for Paul's question about a recommended transistor: you didn't say
>as I recall whether it's to be a linear amplifier or a class-C stage,
>and if it's class-C whether it's to be amplitude modulated or not.
I thought class C would be the best way to go, really. The input
signal is already FM so an additional power stage shouldn't affect the
signal information adversely as it would had it been AM or SSB. We're
just simply seeking to up the power of a carrier wave, effectively.
>Those are important considerations. Some RF transistors are
>specifically rated with respect to the output mismatch they can safely
>handle, but if you arrange things properly, you'll be able to tune the
>antenna to provide the appropriate load to the amplifier stage, and
>not have to worry overly much about mismatch, unless you disconnect
>the antenna. It's common in RF power amplifiers, especially solid
>state ones, to sense whether the load is correct or not, and power
>things down if it's not. In the case of a fixed-tuned system designed
>to operate into a 50-ohm load, the protection might well be a built-in
>50-ohm reflected power meter feeding an "AGC" or a shutdown signal
>back to the amp.
Well, maybe there's a simple network one can build to match even a
random length of wire?
"I was going to help him up, but then I thumped him."
Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975