Date sent: Fri, 9 Apr 1999 14:11:04 -0400 (EDT)
From: Bill Fisher - W4AN <w4an@contesting.com>
To: towertalk@contesting.com
Subject: [TowerTalk] BOUNCE towertalk@contesting.com: taboo
header: /^from:\s.*\bkm1h\b/i
(fwd)
>
> From: km1h@juno.com
> Received: (from km1h@juno.com)
> 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.
I don't know what frequency the 130 runs on, but it is best to make
the measurement as close to the operating frequency as possible.
Stray inductance, in series with a capacitor, decreases the amount
of capacitive reactance and effectively increases the value of
capacitance.
That's a well established effect.
> 21" of grip leads adds inductance.
That is true, but as I said earlier I cut the grips off just after the
started to wrap in the normal spot by the second wrap marking.
> >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.
Then we have to agree to disagree.
Calculations, based on standard formulas and ignoring strays,
indicate more capacitance than you measured.
> I always use rope at the ends of dipoles.
Me too, because I worry about coupling across the insulator
affecting the antenna.
> >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.
It's tough to measure big things that have low values. That's why
things are checked and cross checked, and measured at or near
the important frequency.
It is more time-efficient for me to simply not take chances. I've
decided to break up all lines in front of or right below HF antennas
with links or short guy sections (less than 1/2 wl at the highest
frequency). What others do is up to them.
Paying attention to details helps on 160, I can't imagine it means
less on ten meters.
73, Tom W8JI
w8ji@contesting.com
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