Many probably don't bother or notice, if the SWR is not killed, many
would not care as long as a yagi seemed to have a pattern and took
power. And someone can make the case that for much amateur activity,
it just doesn't matter. No argument from me.
What does happen rather easily, monkeying around with models of
miscellaneous wires around yagis, is that nulls in transmission are
moved or reduced. Nulls are the result of fairly precise cancellations
of radiation from the antenna elements, and include important uses
like front-to-back, and are neighbors to sensitive angle ranges like
extremely low incoming signals.
Much dx would seem to be worked in a yagi's low pattern which is
already well down from the maximum due to the unavoidable ground
reflection cancellation. It is possible to make the drop worse, by
adding unwanted null loss from a miscellaneous conductor in the near
field. Hence stories of DX popping up when metal guys were replaced.
The maximum forward gain is hardly changed at all, but an already
handicapped low arrival angle can be made worse by adding an
additional unwanted null to the existing cancellation.
Other wire-yagi combinations can be shown to introduce unwanted
vertically polarized reception which may raise noise levels on
reception.
Loss of front-to-back or front-to-side can be a real handicap in
contest operations.
But then again, I'd guess the placement and depth of nulls is probably
not noticed much, if the SWR looks OK. Mapping nulls in all directions
is quite a bit more work than reading a meter.
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Guy Olinger, K2AV" <k2av@contesting.com>; "Bill Hider NN3RR""
<n3rr@erols.com>
Cc: "tongaloa" <tongaloa@alltel.net>; <towertalk@contesting.com>
Sent: Tuesday, June 25, 2002 6:51 AM
Subject: Re: [Towertalk] Guy wire resonance. Break it up or use stubs?
> > Not that my not hearing about it means it's wrong. No, I'm not
saying
> > that. What I am saying is that, since this is the first time I've
> > heard anyone say what you've said, and I've been a ham for over 40
> > years and plugged into publications, listserves and clubs where
these
> > types of things might be brought up, I'm wondering if it's a
> > wide-spread problem or not. I suspect not, since I've not heard
of
> > it. And if it's not widespread, what then?
>
> I just modelled a two element phased array, and adding a "non-
> resonant" wire right in the middle of a recommended "safe length"
1/8
> wl below the phased array center decreased the null from almost
> infinite to less than 20dB. It made a very profound pattern change
in
> null directions.
>
> Obviously it is also important guy wires be short compared to 1/2 wl
> at the operating frequency, at least near a directional antenna.
>
> I agree with the insulator capacitance problem. The grips and
> insulators can also change things, I'd bet in some cases a large
> amount.73, Tom W8JI
> W8JI@contesting.com
>
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