[TowerTalk] Foam cables and velocity factor
Robert Chudek
k0rc at pclink.com
Fri Nov 11 21:27:53 EST 2005
Tom,
Once you explained how to calculate the Velocity Factor, I went "duh, I
should have know that". Thanks for the refresher.
This evening I tested the inverted vee with the new RG-8x 1/2 wavelength
feedline attached. Boy, the numbers really changed! They are so different I
can hardly believe the only thing I changed was the coax!
I uploaded the new PDF charts to the http://chudek.aberon.net website and
labeled them "160m-2". All 7 charts are available in the 160m folder, for
anyone interested.
I am a true believer now! These new numbers paint quite a different picture
of this antenna. In brief, instead of a minimum vswr at 1830 KHz, the
analyzer is showing a vswr maximum. The other measurements have drastically
changed too. For example, the impedance and resistance dropped from 43 to
about 2.3 ohms. Thinking about this for a moment, these numbers make sense.
If I am feeding a 2.3 ohm antenna with 50 ohm coax, I should have a pretty
high vswr.
It appears my original random chunk of Belden 8214 was a pretty good
impedance transformer after all! I know a 160 meter dipole or inverted vee
at 70 feet is a significant compromise. Next summer I will look into shunt
feeding the tower. Right now I have several other antennas that need my
attention.
Everyone, thanks for the help and advice. Additional comments are always
welcome!
73 de Bob - K0RC
----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>
To: "Robert Chudek" <k0rc at pclink.com>; <towertalk at contesting.com>
Sent: Friday, November 11, 2005 6:29 PM
Subject: Re: [TowerTalk] Foam cables and velocity factor
>> The cable wasn't rated per se. It is a foam dielectric so I guestimated
>> .8
>> as a starting point. Can the true VF be determined from the data I
>> collected?
>
> Sure. You can find the electrical length in feet, fractions
> of a wavelength, or degrees with an analyzer and the
> physical length in the same basic unit with a physical
> length measurement. The measured physical length (say 100
> feet or .20 wavelength) over the measured electrical length
> (say something like 150 feet or .30 wavelength) would be the
> actual velocity factor.
>
> You'll find foam dielectric cables are all over the place.
> Some around .90 down to near .66.
>
> 73 Tom
> _____________________________________________________
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