Date: Wed, 16 Mar 2016 10:51:33 -0700
From: jimlux <jimlux@earthlink.net>
To: TowerTalk <towertalk@contesting.com>
Subject: Re: [TowerTalk] "Magic" length from tower to first insulator
On 3/16/16 9:47 AM, Richard (Rick) Karlquist wrote:
> On 3/16/2016 9:01 AM, Larry Loen wrote:
>
>> Comments? Dissents? What did I overlook?
>>
>
> What you overlooked was that if the wire is
> not insulated from the tower, the resonance
> is at 1/4 wavelength not 1/2 wavelength.
Seems reasonable (e.g. the tower is "big" compared to the wire, so it's
sort of like a 1/4 wavelength whip over a ground plane..)
I wonder if that's how it works in reality, though?
Maybe that top guy segment (connected to the tower) is really a very
long wire that happens to be bent at the end, and doesn't feature into
the circuit?
## Rick is correct. If the guy wire is bonded to the tower in the usual
manner,
like through pre-form dead ends.... then what the arrl books say to avoid is
either... Grounded 1/4 waves..... OR floating half waves.
## a 10 foot piece of guy wire to the 1st egg insulator constitutes a
grounded
10 ft piece of cable. 10 ft is resonant on 23.4 mhz. (234 / 10’ = 23.4
mhz)
## 23.4 mhz / 21.000 mhz = 1.11428 or 11.428% higher in freq
23.4 mhz / 21.450 = 1.0909 or 9.1% higher in freq.
## Dunno if 9-11% higher in freq is enough off freq, to not affect the
15M yagi or not ?
## The problem with guy wires, even with eggs, is IF yagis are mounted lower
on the
tower, you still have guy wires above the lower yagis...which could easily
create
a ‘shadowing’ effect in some cases.
## I know folks who have guyed a 100 ft tall tower with NO eggs at all, then
installed a single 2-el 80m yagi on top......with zero problems. In that
case, the
guy wire is bonded to the tower via pre-form dead ends.....and also terminated
at the bottom via pre-form dead ends + turn buckle + steel tube into
concrete.
## Come along and install a yagi part way up the same tower, then the lower
yagi is
shrouded by the upper real long guy wires. Those upper guy wire may well
not be resonant
on the lower yagis freq, but can still easily screw up the pattern.
## A combo of philly coming off the tower, then morphed into steel EHS
for the lower portion
of each guy line may well work.
## Long guy wires, even if they are not resonant on the lower yagis
freq....with these guy wires being
Phsically BELOW the lower yagi....can also be bad news. They look like a
giant groundplane to the
lower yagi..and can screw up the pattern. The lower yagi no longer looks like
its say 50 feet above the
ground, the lower yagi looks like its 1 ft above gound, if say the guy wires
are at the 50 ft level on the
100 ft tower. Plenty of ant and yagi installs have been botched up cuz of
guy wire lengths... and also
phy larger yagis placed above phy smaller yagis. It all has to be modeled
carefully, or you will end up
going 4 steps backwards, then u will have to model it anyway, then tear it
all apart and re-do the entire
mess.
Jim VE7RF
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