[TowerTalk] "Magic" length from tower to first insulator

[Jim Thomson]

Date: Wed, 16 Mar 2016 10:51:33 -0700
From: jimlux <jimlux at earthlink.net>
To: TowerTalk <towertalk at 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