[TowerTalk] Dipole resonates lower than expected.

[Rex Lint]

The 2005 handbook says,
"If insulated wire is used, a 3 to 5%
shortening beyond the standard 468/f
length will be required to obtain resonance
at the desired frequency. This is caused by
the increased distributed capacitance resulting
from the dielectric constant of the
plastic insulating material. The actual
length for resonance must be determined
experimentally by pruning and measuring
because the dielectric constant of the insulating
material varies from wire to wire."

Rex Lint, K1HI
26 Brek Dr.
Merrimack, NH 03054
PH:   603-860-7651    FAX: 603-882-6612


-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Robert Chudek
Sent: Sunday, October 16, 2005 11:50 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Dipole resonates lower than expected.

I slapped together an 80m dipole using the L=468/F formula and find that it
resonates about 200 KHz lower than expected.

My target frequency was 3.775 MHz. That's 124 feet on my calculator.

I built it using some left-over solid copper 12 gauge plastic jacketed house
wire. I center feed it with 100' of RG-59/a coax. One end is up 60 feet, the
other end about 40 feet. Using my AEA HF Analyst, it dips at 3.585 MHz with
a VSWR of 1.6:1. This is a 5.3% difference in length from the formula.

I can easily trim the length to meet my design goal, but I am curious why
theory doesn't match reality a little closer. Does the plastic jacket affect
the length? I would expect the resonant point to drop somewhat if the
antenna was "dragging on the ground". Or is 50' of height not adequate?

Thanks for any insight!

73 de Bob - K0RC

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