----- Original Message -----
From: "Nat Heatwole" <nat@ajheatwole.com>
To: <towertalk@contesting.com>
Sent: Tuesday, December 16, 2003 11:16 PM
Subject: [TowerTalk] Antennas "Looking Through" Other Antennas
> I've been wondering, what is the effect, if any, on the pattern of an
> antenna if it is radiating in the direction of another antenna?
In the classic phrase: it depends...
For
> example, suppose that a station consists of 2 towers spaced 100ft apart,
> the first with stacked tribanders and the second with a pair of 40m
> yagis. Now suppose that when beaming Europe, the tribanders are pointed
> directly at the 40m tower and subsequently, directly at the 40m yagis.
> Will this effect the performance of either the 40m yagis or the
> tribanders? Does the interaction depend on frequency or any other
> factors?
yes... it does depend on the physical dimensions of the objects as well as
the frequency. Throw in traps and it gets even more exciting..
This is why antenna modeling programs exist.. to examine just such problems.
>
> Also, is there any effect on the pattern of an antenna if it looks "down
> on" or "up to" another antenna that is for the same band. For example,
> suppose that one has a tower with stacked 40m beams at, say, 75 and
> 150ft. Now suppose that there is a wire 40m yagi running between the
> tower and a nearby tree at 100ft high (between the two yagis). Will this
> arrangement effect the performance of any of the yagis? Does it matter
> where the rotatable yagis are pointed or how far the wire yagi is out
> from the base of the tower? Any help would be greatly appreciated.
A wire yagi or just a dipole? In any case, since these things are all tuned
to the same frequency, and they are within 1/2 wavelength (20meters = about
66 ft) the potential for interaction is definitely there. It's almost a
worst case scenario... if you had asked about the impact of a TV broadcast
antenna or a DBS dish, you could almost answer off-the-cuff and say,
negligble impact, because it's so much smaller than a wavelength.
Yes, it will matter which direction the stacked yagis are pointing. When
the elements are parallel to the wire, the coupling is highest. Whether it
makes any significant difference is another story (it might actually help).
One way to check would be to hook a power detector on the feedline from the
wire yagi and put power into the other antenna. If there's mutual coupling,
then some of the power will show up at the detector. (Don't use an
expensive sensitive receiver, unless you only run real low power to the
stack.. the coupling might be more than you think...)
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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