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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