Rob Atkinson, K5UJ wrote:
>I was plagued by rf coming back on my coax shield on transmit last year
>when I fired up a 1.2 kw amp for the first time (I am on a small lot so
>my vertical antenna could not be sited far enough away from the radio
>shack to eliminate the problem).
Me too, so I've been forced to learn more about this subject than I'd
ever wish to!
>Since then I've begun to discover that this is a fairly common problem.
>Some hams try running amps, experience distortion on transmit, give up
>and get rid of their amp and simply conclude they can't run high power.
- or else they blame the amp, and it shuttles back and forth uselessly
to the manufacturer.
Part of the difficulty is that RF feedback can be a threshold-type
effect, and if the stray currents are below a certain level they may not
be noticeable. This makes it very easy to conclude that you don't "need"
any precautions. But in some situations it only needs a few dB increase
in the power level, and suddenly RF feedback is running riot.
> Others operate with fuzzy audio not knowing they have a problem.
Bob Heil certainly finds that's so. In his business a lot of people
bring him their TX audio problems, and he finds that most of them are
due to un-diagnosed RF feedback.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
|