Topband: Velocity factor RG6U

[Raoul Coetzee]

Thanks to all who replied directly off the reflector.
Some say believe the quoted figure, I think rather not as I want to build a
small spacing 2 element receiving array.
Harry, N4HT, suggested using a proffesional Time Domain Reflector in
reverse.That will work very well, but, Alas, I do not have one now.


Tom, as usual, gave a very good explanation and mentioned that true RG6U
will have a solid dielectric, not a foam core like mine.I will quote him
here, I am sure he will not mind:

quote:

G indicates it is a Radio Guide number. Real RG-6/U is a
solid dielectric cable. It is NOT made in foam. It has a
.66vF

Many people now call all RG6 size  75 ohm cables RG-6, but
they are actually not RG-6.

If it is a foam cable the vF can be anything from just over
.7 up to the .9 range. The exact value depends on the
mixture of air to solid material in the foam.
(It is also really just  RG-6 "size", not really true RG-6
cable.)

If you know the length is 330 feet you can put a small
coupling link across  the end and grid dip it with the other
end open. I "think" you will get a dip every odd 1/4 wave.
I'm not exactly sure how a GDO would behave since I haven't
used one for 30 years. If I am wrong you will be off by a
factor of two, so it will be obvious. Let me know.

If you have a dip at 2 MHz and another at 2.5 MHz and 3 MHz
and 3.5 MHz, the dips are .5 MHz apart. A .5 MHz  quarter
wave in feet is 245.75 /  F = 491.5 feet long.  That is the
electrical length of the cable.    330 / 491.5 is .67 vF

You can work in meters. I only work in feet. :-)

The accuracy limit will be in how close you can read the
frequency of dips.

73 Tom.
Unquote
Thanks to everybody and hope to hear you better soon, not easy from a small
lot with VERY limited space for rx antennas.
73 ,Raoul, ZS1REC

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