[Elecraft] Elecraft: RFI Problems

[Stewart Baker]

On Fri, 28 Jan 2005 09:59:35 +0000, Martin.Evans at simoncarves.com wrote:
>
>
> Sorry this is "off topic" but there is so much experience here. I've been
> licensed for only 2 years, my only rig is K1 so I've been QRP CW only (and
> enjoyed it immensely). I've recently started using a PA to get up to 100W
> on 3.5 to 21MHz, but RFI problems have started to appear. Here are the
> symptoms:
>
> 50Hz hum on key down on hi fi
> Similar problem on mains MW radio.
> Freezing of PC modem on key down.
>
> These are all in different rooms of the house and all at least 15 feet away
> from the rig.
>
> The rig is on the ground floor right next to the outside wall where the
> coax feeder runs out at ground level. Feeder is about 70 feet long and all
> at ground level until it rises to a multi dipole at 30 feet up, cut for
> 80/40/20m. Feed point is at least 50 feet from house. The feeder shield is
> connected directly to the PA ground point and therefore to the mains ground
> wire. I have no Balun and no RF ground so I guess I've broken all the
> rules.
>
> I suspect RF is on the mains 240V house wiring and can see it on my scope
> (about 5mV P-P superimposed on the mains). I have a number of things to try
> but not sure in what order of priority, maybe someone could help, I want to
> tackle the source of the RFI first, rather than the affected domestic
> equipment.
>
> Here's the list:
> Make a coax choke balun in the feeder to reduce out of balance currents
> (quick and cheap)
> Fit a commercial balun (heavy and pricey)
> Construct an RF ground in the garden and connect to feeder sheath at entry
> into the house. (time consuming)
> Fit a ferrite ring choke to the PA mains lead
> Build an inductively coupled ATU to avoid connection of feeder sheath to
> the mains ground.
> Fit ferrite ring chokes to the affected equipment
> Fit a proprietary RFI filter to the telephone line (to cure the modem
> problem)
>
> Or maybe I should just go back to QRP hi.
>
> Any advice would be greatly appreciated as I don't want to run the risk of
> causing RFI to the neighbours.
>
>
> Why not take a look at our Web site?
> http://www.simoncarves.com
>
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Things that I would try first:-

a) Fit a balun (at the center of the dipole)
or
b) Make the antenna completely balanced by using open wire feeder, and a 
balanced ATU. You will notice a reduction in received noise as well as RFI 
reduction.

Good luck

73
Stewart G3RXQ