[TowerTalk] (no subject)

[K7GCO@aol.com]

In a message dated 01.05.00 04:51:57 Pacific Daylight Time, 
k8bk@speedconnect.com writes:
<< 
     I thought I had sent this before but I will try again.  I have a quick 
question.
I am putting up a new C4SXL antenna on my 75' Rohn tower.  I am wondering if 
it is worth it to run about 50' of Phillystran off the top of the tower then 
finish with regular guy wire.  Is 50' worth the expense?  I will be running 
my 80 and 160 antennas out of trees so I have no plans of running other 
antennas off the tower at this time.  Thanks de Barry/K8BK
  >>

Barry: That is a great idea.  A couple months ago there was a rash of posts 
all concerned about the affect of guy wires and insistence of insulators 
every 13' or whatever.  It sounds logical.  Howsomever.

I modeled a tower, guy wires and beam and found exactly what I had expected.  
My recommendations from actual observations for years has been to use 
Phillystand originating from the tower for a 1/2 wave of the beam frequency, 
Then you can use even resonant wires from there to the ground like a 80 or 
40M ground plane originating at the ground guy point if the tower is high 
enough.  

With continuous guy wires connected to the tower I found very little RF on 
them.  One can see relative amplitude of the current loop in Eznec.  I can 
amplify it many times so as to see very weak levels.  I did a full bore 
amplify to where the current loops of the beam elements went out the top of 
the screen all the way to the ceiling before any RF current could been seen 
on the guys connecting about 5' below a 20M beam.  So much for insulators in 
guys.  In the Eznec test there is no RF Spill over on to the coax shield as 
there isn't any.

If you have no change is SWR as the beam is rotated one could basically 
ignore the guys although one at the tower can't hurt anything.  If there is 
some change you have to decide if some surrounding object is the cause.  One 
main cause of SWR change with rotation is when a beam like the TA-33 is fed 
without a balun.  The coax shield is hot with the "Dreaded RF Spill Over" and 
that really excites the tower and guy wires to where you can see a SWR change 
with rotation.  That problem has never been recognized.  Adding inuslators to 
the guys can eliminate or reduce the affect but NOT THE CAUSE!

I then added guys to the tower in Eznec that extended to within 1/2 wave of 
the tower.  I increased the amplitude full bore of the current loop and the 
shortened guy wires were literally stone cold.

Consider this.  Lets take a 3 element yagi.  It has a directive vertical 
pattern in the field of the guys which limits the amount of RF they see.  If 
RF is reflected back at the beam, the directive pattern limits the amount it 
sees in return to upset the beams pattern and Z.  If you add another director 
it sharpens the vertical pattern even more which limits the guys or other 
objects affect even more on and on.

Now consider this.  In order for the additional director to squeeze almost 1 
dB more gain, here is what has to happen.  This director has to be in the 
same plane on the boom, of the same exact polarization, of a specific and 
optimum spacing and an exact length to an inch.  If so it will also change 
the Z but progressively less as the number of directors is added.  As the 
number of elements increases the beam gets progressively pattern and Z 
independent of surrounding objects even in the front.  In some multi-element 
yagi's I have it actually goes up.  The critical nature of these 4 
requirements I was fully aware of from tuning yagi's manually on a test range 
and in Eznec.  If anyone of the 4 concepts is not optimum the gain cannot be 
maximized.  Even if maximized you will barely get 1 dB gain and less as each 
director is added.

Now this being the case all those who lost all that sleep over breaking up 
guys have never tuned a beam manually with no RF Spill Over on to the coax 
shield, tower of guys or in Eznec.  The guys are not in the same plain, the 
right polarization, the right spacing or the right length.  Not any of the 4 
factors that must be fully maximized is anywhere near.  Tell me how they are 
going to affect the beams pattern to get all concerned about.  If the guys or 
horizontal wires are no closer than 1/2 wave from the beam and below it, it 
for all practical purposes it's pattern interference free of these objects as 
I see it in Eznec.  The tower does have a little RF on it but this is 
unavoidable.  This RF can be removed with some 1/4 wave stubs connected to 
the tower at the right spot--1/4 wave below the beam pointing down.  I call 
them "RF Trick Sticks."  
 
I had a TH4 on my tower with a balanced feedpoint fed directly with coax.  
The SWR changed as it was rotated.  I blamed it on the power lines 50' from 
it at the same height.  I added 2 RF ammeters in the feedpoint and recorded 
their values every 30 degrees.  They read like one at 3 amps and the other 4 
amps due to RF Spill Over and varied 20% as I rotated the beam.  The coax ran 
20' down and then parallel to the beam for 15' and then down again.  RF was 
induced on the shield more or less as the beam was rotated and it affect the 
RF currents and SWR as the beam was rotated.  

I added the Telrex balun and the currents balanced within 5% or right on and 
the SWR hardly changed at all with rotation.  RF currents induced on the 
shield were isolated from the feedpoint by the balun depending how good it 
was.  I think the power lines in front may have had some small affect along 
with the power and telephone lines that came from the poll to the house.and 
the slopping ground high and low around the house.  I didn't worry about it 
from then on because I couldn't change it.  I had rope guys on this 20' 
wooden tower on the house. I had a great signal with the balun through the 
power lines in front of the house and the ones in back about 100' away.  When 
I got the TH4 new and installed the balun 20 M was open to Europe that night. 
 I filled 2 log sheets with consecutive DX contacts--right through the 
horizontal guys the same height and polarization 100' away.  HyGain printed 
the first log sheet with my testimonial as an Advertisement.  So much for 
power lines.  Barry do you see how good your idea is now?  Case closed.  If 
Attachments could be sent I could show the relative values of RF current.  
The actual values of RF current at anyone point on any conductor are given in 
the Eznec.

I have all these patterns from Eznec and am writing this all up for one for 
the mags. 
 K7GCO

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