[TowerTalk] Foam cables and velocity factor

[Robert Chudek]

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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