[Antennas] Question on open wire feeders and RFI/TVI

Tue, 22 Apr 2003 14:46:37 -0500

Harvey, there is one notable exception to your second sentence that should
be pointed out.

While I agree that proximity of the antenna to the offended equipment is the
most critical consideration, it is still possible to cause serious RFI even
with the antenna hundreds of feet away. The source of signal is the feedline
itself if it is routed near the offended device or any of its wiring, etc.

The mechanism, of course, is Common Mode current on the line with the
attendant radiation it provides. This produces the same effect as if the
antenna itself were located in a similar position.

Since feedlines, especially coax, tend to be routed for convenience to get
from the antenna to the shack equipment, it is not unusual for them to pass
quite close to TV antennas and feedlines, telephone lines, etc. Any
radiation from such a line find a direct path into the associated devices to
produce RFI.

The usual correction for this problem is proper transition between balanced
and unbalanced loads and lines to maintain balanced currents, both in
magnitude and phase.

Open wire lines can be particularly challenging because of the relative ease
with which they can be unbalanced by their immediate environment, as
compared to coax where the principle consideration is prevention of CM
current on the outer braid - a task usually handled well by an appropriate
balun.

So, I can agree with your statement provided that adequate steps are taken
to ensure that CM currents are nil on the line, be it open-wire or coax.

Seldom do we differ on anything, Harvey - this one came as a surprise!   <:}

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:[email protected]>

----- Original Message -----
From: "Harvey&Bessie" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Tuesday, April 22, 2003 2:12 PM
Subject: Re: [Antennas] Question on open wire feeders and RFI/TVI

> I Vote with the other comments (Peter-N9QZD and Robert-W9DMK). Whether
> fed with co-ax or open-wire line has no effect on RFI/TVI. Its a matter
> of antenna location relative to the equipment or antenna that could be
> interfered with (how much RF field it is being subject to).
> It might be interesting to note, however, that antennas erected close to
> large metal objects that may have poor electrical contact between their
> parts (such as sheet-metal roofs, fire escapes, etc.) can cause harmonic
> generation and therefore terrible RFI/TVI problems.
> Harvey/W4TG
> - - -
>
> Your moderator for this list is:
> Larry Wilson KE1HZ [email protected]
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