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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