[Elecraft] Elecraft: RFI Problems

[W3FPR - Don Wilhelm]

The short answer to your problem is - keep the RF inside the coax until it 
gets to the antenna.
With an unbalanced feedline and no balun, you likely have considerable RF on 
the outside of the coax.
Your list of possible cures is a good one, but add one more at the top - run 
the feedline away from the antenna at right angles for as far as possible - 
that places the feedline in a position where the fields should cancel and 
prevents the feedline from picking up radiation from one side of the 
antenna.
Then start at the top of your list and try each item one at a time until you 
have found what is effective.

If your antenna is located close to the equipment you are having difficulty 
with, as might be the case with a low antenna or an attic antenna, then QRP 
may be you only solution.  Those devices are supposed to be unaffected by an 
RF field, but we all know what is supposed to be and what really is are two 
different things.

Your neighbors should be further away from the antenna than your own 
equipment is located, so your local problem should be greater than theirs.

73,
Don W3FPR

----- Original Message -----
>
>
> 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.
>