In a message dated 6/7/01 8:48:05 AM Pacific Daylight Time, n4kg@juno.com
writes:
<< 4KG comments inserted in text below
On Wed, 6 Jun 2001 K7GCO@aol.com writes:
> The posts below show the questions that still exist on the need of
> breaking up guy wires. A single insulator at the tower for all metal guys
is a must
and will eliminate RF Spill Over transfer to the guys from the tower and
coax shield. There can still be RF coupling from the beam to a guy
directly underneath even with the insulator at the tower and a resonant
length
of a guy like an inverted vee. I found minimum coupling with the
inverted vees
> 10' or more below the beam.
SNIP
>
> With a 1/2 wave of Phillistran from the tower down to a metal guy
> wire, it's totally out of any field that is of any concern. Scale
this in
> Eznec, rotate > the beam (that will take awhile). and see for your
self. Further more beams have a vertical directive pattern that points
> straight ahead. Therefore the pattern component pointing down at guy
wires is greatly attenuated and any reflected RF is attenuated again back
toward the beam.
> The higher the gain of the beam the more isolated the beam is from
> surrounding objects to the side/back and below. It just doesn't see
> them.
OK so far.
> The multiple-insulator installation all the way to the ground is
> mostly just another "TT Band Aid" for a problem not properly
> addressed like poor feed systems used in beams.
Here's where we disagree. What you say is true for Beams
AT THE TOP of the Tower ONLY. When you side mount other
antennas lower on the tower, then you need insulators on the
guys that pass close to them (within 1/2 WL) as well. For 20M,
this means +/- 35 ft of each side mounted antenna. N4KG
> Fix the source of the problem first. Unless Phillistran is used for
> a 1/2 > wave from the tower I'd suggest an insulator at the tower, one
at 5', one at > 15' and one at 30' in metal guys.
As stated before, my preference is to place the first insulator as close
to the tower as possible, typically 3 to 5 ft, depending if torque arms
are used plus the length of two preformed grips. This keeps the
resonance between insulators, through the tower, and directly under the beam
at the top of the tower well above 28 MHz.
Second and third insulators are then spaced 10 to 12 ft for isolation
on the HF bands. Your 'recommended' 15 ft is VERY close to a 10M
director length...too long for good isolation on 10M.
A real life example of improper top insulator placement was a friend
whose top guys were attached 10 ft below the top of this tower and
the first insulators were 10 ft out from the tower. His tribander worked
fine on 10 and 20 meters, but he felt "less competitive" on 15M where
the guy wires formed a nice 15M director (10 ft + 10 ft + 1 ft thru the
tower) 0.2 WL below the DE of this tribander. Keep that first insulator
CLOSE to the tower !
de Tom N4KG
>>
Tom: Your points are good. I didn't mention stacked beam as my 5 conditions
didn't include them and was a simplistic example. I assumed readers would
know that stacked beams need a clear shot also. If I had stacked beams I'd
have Phiilistran all the way to within 33' of the ground and make 4-wire 40M
verticals.
I suggested with a single beam on top and in absence of Phillistran for a 1/2
wave, an insulator at the tower as close as possible, at 5', at 15' and 30'.
That didn't mean spacing between the insulators --it meant "from the tower".
So no wire was over 10' long. No 15' wire was suggested which is close to a
10M director length. Even then it would be a ways down. Try 4 15' lengths
below a 10M beam in this location and see how much current is on it in the
screen with the current amplitude turned up max and in the current data.
Record the pattern and then remove the 4 wires and record the pattern again.
k7gco
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