[Amps] 10 kw CCS on 6M - High Power Amplifier Installations

[John Lyles]

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 at sbcglobal.net>
> To: amps at 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
>