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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