[Elecraft] [K3] Noise from P3

[John Lemay]

I've spent a bit more time investigating this noise, following my initial
response to David.

The noise sounds rather like a buzz saw. It is quite audible on the K3
speaker, it is strong enough to move the S meter, and it's visible on the
P3. There is not just one frequency, but the noise is every 19kHz (approx)
across the band.

The noise goes away if:-

1. I turn the P3 off
2. I remove the 2m antenna

The noise reduces if:-

1. I disconnect the BNC at the K3 IF Out connector.

I already have chokes on the DC lead to the P3.

I think at this stage I should also make it clear that I have a highly
sensitive 2m system with masthead preamplifier, and four eight element yagis
which are at present quite close to ground level.

Regards

John G4ZTR



-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
Sent: 23 March 2011 19:20
Cc: elecraft at mailman.qth.net
Subject: Re: [Elecraft] [K3] Noise from P3

On 3/23/2011 11:56 AM, John Lemay wrote:
> It seems to me that the noise is being radiated by the screen itself, and
no
> amount of cabinet scraping will reduce this.

That's entirely possible -- but it's also possible that the cables are 
involved.  Many RFI problems have more than one cause, or more than one 
component, or a combination of causes, both of which must be present for 
the RFI to happen.  In this example, a hand or finger next to the 
display allows RF to capacity-couple to the body, and the resulting RF 
current radiates the noise. Take the fingers away, and if there's still 
noise, another coupling mechanism may be present. On the other hand, 
I've never felt the urge to have my fingers next to the P3 screen. :)  I 
don't operate 2M for weak signal work, because I'm blocked by a high 
ridge to all of North America, so I can't comment on noise from my own 
display. :)

I don't recall details of the P3 construction, nor the display, but the 
display is a potential break in shielding, and even a potential RFI 
source.  Ideally, the display should be bonded to the shielding 
enclosure and be constructed in a manner that there is no break in the 
shielding, and any current driving the display should flow in a loop 
with very small area to minimize any magnetic field.

73, Jim K9YC
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