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Re: [TowerTalk] ARR Preamp Failures

To: <>, "VR2BrettGraham" <>
Subject: Re: [TowerTalk] ARR Preamp Failures
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 6 Oct 2004 08:58:26 -0400
List-post: <>
> That we do not hear from a number of people about
RF-switched ARRs
> failing a lot does tend to suggest there may be something
about the
> specific installation in Jim's case.

The problem is not only preamp design critical, it is also
influenced by the particular radio and even the length of
feedline between the radio and amplifier.

Using RF switching is *always* a bad system, because by
definition the RF must always hit the preamplifier for some
length of time before the preamp bypasses. The clamp system
in the preamp as well as currents and  voltages in sensitive
components like the FET are subject to wide variations
depending on how long the feeder is (what fractional
multiple of a wavelength), the radio transmitter source
impedance, the radio envelope rise time and slope, and so

It isn't surprising at all only a small number of people
complain, because there are:

Not many masthead preamps used
Fewer still using the same amplifier with RF keying
Fewer still with the same transmitting equipment and cable
lengths using RF keying

RF keying ANY receive amp is a huge problem. Especially with
the poor ALC systems and large leading edge power overshoot
of many radios.

73 Tom


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