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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