From: firstname.lastname@example.org (carltons)
Subject: Re: Should one worry about reflected current into untuned loads at low power?
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
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NNTP-Posting-Date: Sun, 08 Dec 2002 16:15:56 GMT
Date: Sun, 08 Dec 2002 16:15:56 GMT
In article <email@example.com>, firstname.lastname@example.org
(Tom Bruhns) wrote:
> Phil Seakins wrote in message
> > In your original post on this topic you referred to "reflected power
> > with RF signals" in a PA system. Further, the topic mentions "untuned
> > loads".
> > Clearly, "PA" stands for Public Address. PA systems are only used for
> > audio. I am puzzled why you and several of the respondents are
> > overlooking this fact. Their is no way you could get any "normal" PA
> > system to reproduce frequencies of 40MHz.
> > "Tuned loads" and "reflected power" have no relevance in audio
> > distribution. These are RF issues.
> 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.
> 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.
> 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.
My question is: "Why do you want to put up with a mismatch if you know you
have one?". Get a lower power pa and a decent load instead of more power
and a lousy load. Is there an application which would have a lousy return
loss and needs the pa to overcome the transmission loss? You can always
use a circulator to accomplish this also, providing the frequency band is