[Elecraft] P3 RFI

[Ian White GM3SEK]

John Lemay wrote:
>Good morning all
>
>I raised the issue of noise coming from my P3 several months ago.
>
>It is noticeable on 2m, using an EME array that is about 50ft from the 
>rig. There is a combination of azimuth and elevation in which the array 
>"looks" directly at my shack and under those circumstances the noise 
>level on 2m rises several dB.
>
>I was advised to put chokes on the cables at the P3 end, and to revisit 
>the cleaning of metal panels on the P3 where they meet. I've done this 
>till I'm blue in the face, but it doesn't help because the P3 radiates 
>a small amount of hash from the display screen. It's not practical to 
>screen this !
>
>Regards
>
>John G4ZTR

It seems rather unlikely to be a radiating EM field launched directly 
out of the panel cut-out of the P3. The aperture is too small to act as 
an efficient radiator at a wavelength of 2 metres, and the internal 
noise source is unlikely to be driving it efficiently either.

It seems more likely to be starting out as conducted interference - 
internal RF currents escaping perhaps through that hole or through other 
defects in the shielding (which unfortunately are endemic in this 'flat 
pack' style of construction). Those RF currents will then flow over the 
outer surfaces of the metalwork and the connecting cables, and that 
could be where the radiation is actually taking place.

Have you checked which of the cables are actually carrying the noise 
currents? A clamp-on ferrite bead with a 10-turn secondary winding is a 
very good diagnostic tool, coupled into a spectrum analyser or a totally 
independent VHF receiver with an S-meter. Another option to use with the 
clamp-on transformer is W7ZOI's AD8307 microwattmeter, which is so 
sensitive it's scary.

An interesting experiment would be to blank off the entire screen 
aperture with a metal plate, solidly grounded to the front panel, just 
to see what difference it really makes. It obviously isn't a solution, 
but it could be a useful as a diagnostic.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek