# [TowerTalk] Determining coax length?

Jim Lux jimlux at earthlink.net
Wed Nov 13 08:59:31 EST 2013

On 11/13/13 12:30 AM, Dan Maguire wrote:
> As K9YC said, coax VF changes as the frequency changes.  The
> TLDetails program allows you to plot VF vs Frequency for any of about
> a hundred different types of line, including Bury-Flex.  Here's a
> sample plot for two different types of RG-58, one more lossy than the
> other.  The higher the loss, the more the VF drops off at lower
> frequencies.
>
>
>
> http://ac6la.com/adhoc/10mStack3.png
>
> Then I used this dialog to calculate what 30 electrical degrees would
> be in feet, using the VF of Bury-Flex at 28.1 MHz.

about 2 feet or thereabouts? (10 meters = 360 degrees -> about 1 foot =
10 degrees in free space)
>
> http://ac6la.com/adhoc/10mStack4.png
>
> Finally, I set up a "variable sweep" to change the length of line to
> the upper antenna by steps equivalent to 30 degrees.  That means I'm
> simulating a mismatch in the line lengths, starting with no mismatch
> and ending at 180 degrees of mismatch.
>
> http://ac6la.com/adhoc/10mStack5.png
>
> After calculating all the rows this is what the "Patterns" sheet
> shows.  You can use the spin button to "watch the movie" of the
> changing patterns as the mismatch in line lengths grows.
>
> http://ac6la.com/adhoc/10mStack6.png
>

The big null at 30 degrees is your "height above ground" effect
What's interesting is to watch the other nulls move around.  And how
that low lobe turns into a null with not very much change in length
(>10dB change with a 30 degree step in length)

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