At 11:26 PM 9/6/96 -0400, you wrote:
>John:
>
>What material would u use to sideguy the booms on your antennas? I am
>considering doing this on a KLM 40M4 (42' boom) and am afraid to use a
>metal cross strut for supporting the philystran, due to probable
>interaction.
>
>Thanks very much.
>
>73 de Jeff WA2SYN
>wa2syn@li.net
Of course the ideal material to use is a dielectric material. I
recently picked up at a swapfest about 30 sections of military
surplus fiberglass mast for a $1 each (probably 42 inches long)
and will probably use these for some of my side guying.
But conductive material may be used as long as the two
modes of interaction are understood.
1) Any conductor that is on the order of a 1/2 wavelength long
any where near the antenna will have a large interaction with the
antenna and should be avoided.
2) Conductors that are very short compared to a 1/2 wavelength,
which applies to boom side guy supports, affect the antenna
by capacitive loading (or read detuning) of the nearby elements
if placed very close to the elements. Computer model a typical
Yagi and add a very short director where the boom side guy mount
should be and see the effect as it is placed close to an element.
Not much effect until it comes within a foot or so. My modeling
indicates that a 4 ft long sectionoin each side of the boom when
placed a few feet away from the dual driven elements on the KLM
will have virtually no performance effect on the pattern. (BTW I have
a stack of the 4L40 KLMs and both booms have a permanent
bends, which I plan to correct the next time I take down the two
antennas.
I recommend that you do the modeling for each individual antenna
to satisfy your own curiosity, but as long as the separation is
more than about 1/2 of the length of each side guy support the
interation will be minimal.
GL John
John Brosnahan W0UN
24115 WCR 40
La Salle, CO 80645
"Radio Contesting IS a Contact Sport"
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