[Elecraft] Computer noise problem

Mon Mar 24 14:33:01 2003

The K1 cannot directly hear what is travelling along the OUTSIDE of the coax shield where it attaches to the K1. It can only hear what is coming to it on the inside.

The PC is inducing noise voltage on the OUTSIDE of the coax INSIDE the shack. The noise travels away from the K1 and out to the end of the coax, where the outside of the coax is attached to the inside. Then the noise travels back to the K1 on the inside of the shield. 

The 10 to one transformer would have blocked the shield current from getting to the antenna wire. IF the coax shield was somewhat near a multiple of a half wave, then the coax turns out near the antenna would not have stopped the voltage. The noise voltage would have been impressed against the inside of the shield, with the center conductor at relative zero, hence a noise voltage difference. Hence you hear the PC.

If you add the counterpoise, the noise voltage going out has another 1/4 wave of wire to go to a hard end, the coax turns are at a CURRENT point, and become effective. The amount of voltage transferred from the outside of the shield to the inside of the shield is greatly reduced.

This kind of noise transmission issue is well known to those trying to get the last few DB of quiet from beverage listening antennas and the like. Common mode current on the outside of a shield out to the antenna, coupling in somehow, and coming back mixed in with the desired signal is a classic beverage killer.

73, Guy.

> 
> From: "Bozidar Benc" <[email protected]>
> Date: 2003/03/24 Mon PM 01:58:32 EST
> To: "Guy Olinger, K2AV" <[email protected]>, 
> 	<[email protected]>
> Subject: RE: [Elecraft] Computer noise problem
> 
> Thanks a lot for the reply.
> 
> I understand that a counterpoise changed things quite a bit, but what I
> still don't understand is how it can suppress noise created by the PC, which
> is 50 cm from K1 (and counterpoise is connected to the coax cable shield at
> the antenna side).
> 
> > Unless you have taken steps at the transformer to suppress common
> > mode current back onto the shield, the shield all the way back to
> 
> I forgot to mention that I have a choke (9 turns of RG58, some 30 cm
> diameter) one end connected to the transformer and the other to the cable.
> The choke was there all the time, before I added a counterpoise.
> 
> > the rig is part of the radiating antenna. Even grounding it at
> > the antenna end cannot prevent this in all cases.
> >
> > Adding the counterpoise has changed this dynamic.
> >
> > There is nothing magic about coax that makes it per se immune to
> > being part of the radiating system. Unless you have taken
> > specific steps to block it, it IS part of the radiating system.
> 
> Yes, I know that coax is a part of the system, but I thought that it has
> nothing with the noise created by the PC.
> 
> 73, Bole
> 
> 
> > 73, Guy.
> >
> > >
> > > From: "Bozidar Benc" <[email protected]>
> > > Date: 2003/03/24 Mon PM 12:43:53 EST
> > > To: <[email protected]>
> > > Subject: [Elecraft] Computer noise problem
> > >
> > > Hello,
> > >
> > > PC in my shack generated lot of noise on my K1, especially on 40 m band.
> > > When I replaced PC's power supply with an older one, the noise decreased
> > > almost by half.
> > >
> > > My antenna was 20m long wire with 10:1 transformer at the end, which is
> > > connected to my K1 with 20 m of RG58 cable.
> > >
> > > Now the interesting part.
> > > After I added a 10 m long counterpoise to the antenna, the
> > noise from my PC
> > > almost completely vanished!
> > >
> > > Does anyone have any explanation?
> > >
> > > Another question: Is it clever to ground my K1, and if it is,
> > where on K1 is
> > > the best place to connect ground?
> > >
> > > 73, Bole, 9A3RR
> > > k1 #1499
> > >
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> > _______________________________________________
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> >
> >
> 
> 

73, Guy
K2AV