To compliment what Karl-Arne said about MW and HF broadcasting
transmitters, VHF high power transmitters have cavity resonators
(sometimes called line circuits) that are in the range of 1/4 to a full
wavelength long. Standing waves from a poorly matched loads can create
dangerous RF voltages and currents inside the amplifier itself, and even
into the tube due to the size. Transmitters are less tolerant of this
kind of abuse. It can be an antenna with ice or off channel, for FM or
television, or particle accelerator cavity that is out of tune, etc. The
Q of the cavities in our particle accelerator at >50,000, so the
bandwidth is a few KHz for a 201 MHz resonant cavity. We have automatic
tuners that use stepper motor driven slugs that engage and move to keep
the line VSWR low. One of them runs about 100 kW reflected out of 2.6 MW
forward power. Still considered a reasonable match... We have slow ramp
up of power on turn-on, to allow the tuners to do their thing. Also have
reflected trip settings.
When i worked in FM BC transmitter design, we would find that icing
would cause the screen current on tetrode amplifiers to rise to
dangerous levels and trip off. Many stations in the northern states use
de-icers in their antenna, or have radomes that protect the elements
themselves from getting thick with ice formation. But some stations down
south didn't expect icing, but got it and were knocked off the air. I
suggested that we have a screen voltage rollback circuit, that would be
servo'd to work with reflected power. We tested this in transmitters up
to 30 kW and it became a standard feature, that eventually appeared in
our competitors transmitters as well. Hams already had it in solid state
finals in Japanese rigs. Its known as VSWR foldback, of course.
73
John K5PRO
Date: Fri, 16 Sep 2022 18:54:55 -0500
From: Kim Elmore <cw_de_n5op@sbcglobal.net>
To: amps@contesting.com
Subject: Re: [Amps] 10 kw CCS on 6M - High Power Amplifier
Installations
I want to thank everyone for sharing their wonderful experiences with
very high power transmitters.
My curiosity was piqued especially when I read about the 600 kW British
Aspidistra MW transmitter used for "black propaganda" in Geoffrey
Pidgeon's "The Secret Wireless War." What struck me about it was that it
had frequency agility and could be tuned to different MW frequencies to
avoid jamming, etc. I have no idea if it was channelized but that seems
likely. Still, the antenna matching network had to also be adjusted when
the frequency was changed and I have to wonder how the match was kept
acceptable as the wind blew or when the antenna accumulated ice.
I can only imagine that AM BC stations must have to deal with that. How
much of a mismatch can big transmitters tolerate? And I've always
wondered how the huge shortwave BC stations were switched back on based
on their schedules. With this much power, it's a delicate business!
73,
Kim N5OP
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