[RFI] Computer RFI problem

[Ian White, G3SEK]

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