Re: Q about balanced feed line

Big Endian wrote:
In article <1206oscg5noap5f@xxxxxxxxxxxxxxxxxx>,
Roy Lewallen <w7el@xxxxxxxxx> wrote:

Big Endian wrote:
>
> Here is something strange I discovered this weekend. While using a F.S.
> meter as a RF sniffer, I was probing along some lamp cords that are
> plugged into one of those extention outlet strips. I have a split block
> ferrite core which measures one inch square and a 1/2 inch ID hole. So
> while monitoring the FS meter I placed the core on the 115 to the house
> outlet thinking that I will see a decrease in FS reading, much to my
> surprise just the opposite happened, the FS meter pegged out. Why would
> adding the core cause the FS reading to increase?

There are at least three possible explanations.

The first is one I've come across many times in doing EMI work, and is
probably the most likely. What happens is that you've got two or more
radiating sources whose fields cancel or partially cancel at the field
strength meter. When you reduce the radiation from one of those sources,
the field at the meter increases.

The second can be a bit subtle. Suppose you have a wire near an antenna
and that wire is, say, 3/4 wavelength long. Very little current will be
induced in this wire because it's far from self-resonance. Now put a
choke in the wire 1/4 wavelength from one end. Presto, a lot of induced
current in the now-isolated 1/2 wavelength portion. This phenomenon can
cause common mode current to increase when you add a common mode choke
on the feedline, if the current is being induced in the feedline (as
opposed to conducted) and the wire length and choke position are
favorable for this to happen.

The third is that the core you're using is a high frequency ferrite. If
it is, it will act as a loading coil, which could make a previously
non-resonant system resonant. I don't think this is likely, though,
because most clamp-on cores are made from ferrites suitable for EMI
suppression. A common type of material for this purpose is type 43
ferrite, which has a Q of 1 at a few MHz. So this type of ferrite won't
cause resonant effects like a high frequency ferrite (e.g., type 61) would.

Roy Lewallen, W7EL

I suppose the right thing to do is to place ferrite cores on all the
conductors in the area. Could get expensive. I have so many wires all
over the place the job seems a bit hopeless.

Thanks for the information Roy.

Another option is to place a choke in the mains supply for the entire station. If the RF current path is down the feedline (in common mode) and then out into the mains, you can reduce it by inserting a choke at any point along its path, and the mains supply may be more convenient place.

The mains supply would need a different kind of RF choke, of course. One option is to wind the entire mains cable - one or both live conductors, neutral and safety ground - on a stack of large toroids (bearing in mind what Roy said above, about choosing the right material). Another option is to buy a ready-made mains filter that also has an RF choke in the ground lead.

The difficulty is to organize the mains wiring so that *everything* passes through the choke. If you leave even one sneak path to ground, the RF will happily use it! Also any other lines such as phone and network connections need RF chokes of their own. If this starts to read like the things you have to do for lightning protection... why, that's perfectly true because very similar considerations apply.

Groveling on your knees under the table is a chore, and of course it's totally beneath the dignity of a Licensed Radio Armature. Well, do it anyway. At my old QTH I found it well worth the effort. Re-grouping all the mains feeds through a single filter cleaned up my mains-borne TVI, and it also reduced the computer and other noise that was coming *up* the mains.

Another very useful tool is a clip-on RF current meter. This will tell you much more than a FS meter, because at last you can *see* where the RF current is. For details, see my "Best of 'In Practice'" pages or the MFJ catalog.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
.

Relevant Pages

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