At 10:49 AM 10/16/2005, Robert Chudek wrote:
>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?
Hi, Bob--
Height may be an issue to some extent but I can confirm that plastic jacket
house wiring will lower the frequency as well due to its effect on the velocity
factor on the wire -- just as coax cable dielectric defines the
VF. In my college
days I had a good friend who built a 4L quad for 20M and used house wiring.
It had a very bad pattern and VSWR. I told him to strip off the
plastic jacket
and he did, and all was well from that time on.
The other issue with house wiring is that it is soft-drawn copper wire and will
stretch some under tension.
So you have three things that want to lower the resonant frequency. The low
height, the jacket, and wire stretch. I would be surprised if it DID match the
SIMPLISTIC theory but it seems to be doing just what it should under the
circumstances of a more complete theory.
One way to make the soft copper not stretch any more is to pre-stretch it
to make it hard drawn. This used to be down (in MY good old days) by tieing
it to the bumper of a car and and a tree pulling on it. With plastic bumpers
this is probably not a good idea! ;-)
73--John W0UN
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