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Re: [TowerTalk] Antenna element diameter \ length calculation

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna element diameter \ length calculation
From: Dan Maguire via TowerTalk <towertalk@contesting.com>
Reply-to: Dan Maguire <djm2150@yahoo.com>
Date: Wed, 8 Oct 2014 01:09:30 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
If a "shortening factor" is defined as the ratio between the height of a 
resonant grounded vertical and a free space quarter wavelength, the following 
curves show the factors for 3.75 MHz and 28.5 MHz for element diameters ranging 
from 0.125" (~ #8 AWG) to 4".

http://ac6la.com/adhoc/VertFactor1.png

For 3.75 MHz the factors vary from 0.973 to 0.953; for 28.5 MHz, from 0.964 to 
0.917.  Note that the 28.5 MHz factors do not correspond to Table 1 in the 
Antenna Compendium Volume 2 article by Doty et al.  The Doty ACV2 measurements 
were done with the vertical element mounted over an elevated counterpoise, not 
grounded.

Calculations for the above chart are easy using the "Resonate" button of 
AutoEZ.  The procedure is shown here.

http://ac6la.com/adhoc/VertFactor2.png

If you'd like to duplicate these results, or run calculations for other 
frequencies, the following model file is suitable for use with the free demo 
version of AutoEZ (http://ac6la.com/autoez.html) in conjunction with either the 
free demo version of EZNEC (http://eznec.com/demoinfo.htm) or any other EZNEC 
v5 program type.  You'll also need Microsoft Excel.

http://ac6la.com/adhoc/VertFactor.weq

Download that file, use the AutoEZ "Open Model File" button, tab to the 
Calculate sheet, select all the "H" values (cells D11-D27), and click the 
"Resonate" button.  Or change all the frequencies to your new choice and set 
all the "H" values to approximately λ/4 at that frequency (initial value not 
critical).

To produce a plot of the results, tab to the Custom sheet and choose "Variable 
3" (Shortening Factor) for the Y axis and "Variable 1" (Diameter) for the X 
axis.

Dan, AC6LA
http://ac6la.com
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