[TowerTalk] Antenna element diameter \ length calculation

[Dan Maguire via TowerTalk]

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