From: km1h@juno.com
Received: (from km1h@juno.com)
by m3.boston.juno.com (queuemail) id D7SGVFES; Fri, 09 Apr 1999 13:45:18 EDT
To: towertalk@contesting.com
To: <towertalk@contesting.com>
Date: Fri, 9 Apr 1999 13:33:14 -0400
Subject: Re: [TowerTalk] Guy wire interaction - the devil is in the details
Message-ID: <19990409.134452.-5005.4.km1h@juno.com>
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>> >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.
Using a HP-4271B and 1/4" grips I measure 1.24pf @ 1MHz.
Using an antique Tektronix 130 LC Meter I measure the same within a
needle width on the 0-3pf scale.
The grips and 502 are supported and suspended on 2 very dry hunks of wood
12" off a wood table. The 502 is a new, out of the carton one, that has
not been exposed to moisture absorbtion.
>
>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.
21" of grip leads adds inductance.
>
>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?
Short of a NBS quality test fixture, no I cant. For this discussion I
believe that my own setup minimizes strays to a non-issue.
>
>> >
>> >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.
I always use rope at the ends of dipoles.
>
>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.
Sounds like something for Dean Straw, N6BV, to investigate.
>
>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.
I dont disagree with that premise Tom, just the C value. Based upon many
years of contesting with many side mounted antennas ( 7 10M yagis at one
point) I could not
notice any pattern degradation, SWR change, etc to speak of. Those
10/15/20M yagis that were on rotating sidearms showed only a very small
VSWR ( 1.2:1 up to 1.4:1 for instance) change during rotation. I
attribute that more to tower effect and 160/80M wires off the same tower
than to guy problems.
Another point that someone can model if they wish is that almost all side
mounted antennas, and all rotatable ones were mounted just 2-3' above the
guy stations.
10M towers 1990-98 were 70' and 100' of 25G, 15M was 100' of 25G and 20M
was 160' of 45G. All guys went out the full 75%. Sharp angles and
element tips very close to guys may give entirely different results.
BTW, the C drops down to .37pf with the grip replaced with an equal
length of #18.
73 Carl KM1H
>
>73, Tom W8JI
>w8ji@contesting.com
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