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[TowerTalk] Experimental Results: Element Shortening Due to Insulation

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Subject: [TowerTalk] Experimental Results: Element Shortening Due to Insulation
From: d@peak.org (5)
Date: Sat, 11 Aug 2001 19:59:31 -0700
Hello Antennaites:      August 11, 2001

A few months back I posted a question asking what might be the effective
shorting of shielded vs. bare element wire (such as in a dipole or
quad).
For those unfamiliar with this phenomenon, it is real and is the result
of capacitive coupling.  In planning the materials list for my quad I
wished to know how much the ideal element lengths (bare wire) would
deviate from the actual when I used insulated wire.  The responses
varied  from 1% to 6%.  The best suggestion was to go and do the
experiment.   So I did, and what follows are the experimental parameters
and results. 

Equipment:
· Wire type = 14 gauge stranded copper with THHN jacket.
· Measurement tool = MFJ 259B.
· Tested wire length = dipole with ~6.25 foot ¼ wave elements.  The plan
was for longer leads, but I tired of stripping wire.

Procedure:
· Four lengths of above wire was cut to specified lengths.
· Two of the four lengths were stripped of the THHN insulation.
· One at a time each of the similar wire pairs  were connected as a
horizontal dipole.  The dipole was suspended ~ four feet off the ground
and clear of obstacles.   Although this setup could lend itself to
possible coupling, I took care to have the comparative tests between
wire types performed under the same conditions.

Results:
· With THHN insulation - Resonance at 35.0 Mhz (60i+0j), SWR = 1.2:1
· WithOut THHN insulation - Resonance at 35.4 Mhz (60i+0j), SWR = 1.2:1
· 41 KHz / foot offset between wire types (this value also accounts for
the non element lead length used to feed the 259B)


AND the FINAL BIG ANSWER:  For this test setup there was found a delta
of 1.1% to 1.2% apparent length reduction (35.4/35.0) seen due to the
THHN insulation. 

If someone spots a flaw in my calculations or assumptions, please
respond.  This is not tricky science and results are approximations. I
just wanted to know how much wire to buy!  Thank you to those that
responded to my original posting.

Paul

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