[TowerTalk] Guy wire interaction - the devil is in the details

[Tom Rauch]

Hi Carl,

This reflector has really been pretty pleasant to be on. I hope it 
doesn't get like the amps reflector.

> >The measured capacitance of a 502 insulator with the strand-loops 
> >only (grip cut off at the point where it starts to lock) is 12 pF. 
> 
> 
> My measurements today say you are off by a factor of 10 at least. Are you
> using a Wavetek, MFJ or a real instrument? So I can duplicate please
> detail what you are doing.

I used a HP-4291B. There are always problems measuring low 
capacitance loads made from "big things", because stray C and 
series L tends to increase C readings.

I doubt the readings are off by a factor of more than ten, as you 
measured. I would expect an error of no more than 50% given the 
size of the loops and their self-capacitance.

I measured on a HP-4291B.

I just double checked by grid-dipping a small 1 uH inductor placed 
across the insulator (that way I don't need to get near the insulator 
with test leads or a test fixture). The inductor dipped at 52.4 MHz 
with the insulator and grip. Substituting a 9 pF capacitor caused a 
dip at 54 MHz.

That seems to be close to what the HP indicated.

Of course I could still be mistaken. Can you (or anyone else) think 
of a better way to measure the capacitance? 
 
> >
> >Why do you think AM BC stations break up lines every 50 feet or 
> >so on the AM band? Because of insulator capacitance, something 
> >totally overlooked in the "non-resonant length" tables we vainly 
> >follow.
> 
> Gee Whiz.  After all these years Johnny Ball capacitance is important? Why
> didnt the "experts" mention this years ago?

Don't know. We don't use strain-type insulators on the ends of 
dipoles. We make efforts to keep the ends away from other 
metallic objects. As a matter of fact, we often use rope to insure 
low coupling across the insulator.

Let's assume capacitance is 5 pF, not 12. That's still 1.13 K ohms 
Xc at 28 MHz.

We can be positive 1.13 k ohms Xc tied across the end of a long 
hunk of wire will shift the resonant frequency down quite a bit.

Unless they included the effects of the insulator and wire loops in 
tables, we can be sure longer lengths are in appreciable error at 
the higher end of HF. 

I sure don't want to fight, I'm just suggesting this needs to be 
considered.
 
> IMO...HOG WASH.

You are welcome to your opinion.

My opinion is, I'd never use longer guy lines near higher frequency 
antennas. Banking on a table being correct that doesn't account for 
end-capacitance, no matter how many years its been out, is not a 
good choice.

It seems reasonable to me the model should allow for end 
capacitance. Food for thought anyway.

73, Tom W8JI
w8ji@contesting.com

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