Towertalk
[Top] [All Lists]

Re: [TowerTalk] Determining coax length?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Determining coax length?
From: Dan Maguire <djm2150@yahoo.com>
Date: Wed, 13 Nov 2013 00:30:39 -0800 (PST)
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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/tldplot2.png

Also, the OP wanted to know how critical it is to have equal coax lengths.  
Using AutoEZ I put together a model with two generic 10m 4-element Yagis 
stacked at 35 and 70 feet (1 WL and 2 WL on 10m).  Each is initially fed with 
100 ft of Bury-Flex from the respective feedpoints back to a simulated 
StackMaster II, modeled as an ideal transformer.  From there an additional run 
of 50 ft of Bury-Flex goes back to the rig.

All of those dimensions are adjustable.  Here's a screen grab of the AutoEZ 
"Variables" sheet.

http://ac6la.com/adhoc/10mStack1.png

And here are all the "non-antenna" parts of the model.

http://ac6la.com/adhoc/10mStack2.png

The Zo/VF/Loss values for Bury-Flex were automatically entered using this 
dialog window.  All three characteristics will change if the frequency is 
changed.

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.

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

To demonstrate use of the spin button, here's an animated gif.  The changing 
value of variable "H" can be seen in the lower-right corner.  The outer ring 
has been frozen so that the pattern magnitudes as well as shapes may be 
compared.

http://ac6la.com/adhoc/10mStack7.gif

Documentation and download links for both TLDetails and AutoEZ may be found 
here.

http://ac6la.com

If you'd like to experiment with the free demo version of AutoEZ here's the 
model file used in the above study.  (Note that you can't just click on the 
link to open the model.  Save it to your computer then open it from within 
AutoEZ, as explained in Step 3 of the Quick Start guide.)

http://ac6la.com/adhoc/10mStack.weq

Because of the large number of segments you won't be able to use the demo 
version to calculate this model.  However, you _can_ modify the model geometry 
by manually changing one or more variable values, perhaps changing the antenna 
heights and/or the coax lengths to match your situation.  You can then use the 
"View Ant" button to view the modified model, which means that AutoEZ will 
create a temporary .ez format file, pass that file to EZNEC, and invoke the 
EZNEC viewer.  You can then use the EZNEC "FF Plot" button to show the pattern 
for that particular set of variable values.

As mentioned, the antennas in this model are just generic 10m Yagis that I 
happened to have on hand.  If you have an EZNEC file for your particular 
antenna please contact me off-list and I'll be happy to help you create a 
"stacked, fed, and variable-ized" model for use with AutoEZ.  And that applies 
even if you are using the demo version.

Dan, AC6LA
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>