[TowerTalk] Determining coax length?
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
> 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)
> 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.
> 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.
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)
More information about the TowerTalk