Hi Dave,
There is a very accurate formula to calculate radiation resistance of
vertical radiators that are 1/8 wavelength or shorter in physical length
that's in the ARRL Antenna Book, and this formula provides values very
close to those you would obtain using 4nec2 (you could use this formula to
compare the radiation resistance of a short vertical to your 36 ohm
radiation resistance value for a 1/4 wavelength over perfect ground to
determine the approximate difference in feedpoint impedance (assuming
similar ground losses in both cases).
Unfortunately the use of top loading wires complicates the issue as the
radiation resistance will also be dependent on the length of the top
loading wires, the number of top loading wires, and the angle of the top
loading wires. I suspect there is already published data regarding the
radiation resistance for various top loading cases. As Clive mentioned,
EZNEC or 4nec2 is the best way to estimate the radiation resistance of the
shorter top loaded vertical.
73,
Don (wd8dsb)
On Tue, Jun 13, 2017 at 5:08 PM, David Cole via Topband <
topband@contesting.com> wrote:
> Hi all -
> Assuming a standard 1/4 wave vertical is 36 ohms - is there any formula
> that will calculate the Z if the vertical is short and brought to resonance
> by top loading wires - I understand the Z will be less - but how much by ?
> I am trying to use the formula on ON4UN disc to calculate the approx LC
> ratios for a Lahlum-Lewallen phasing system.
> Thanks Dave g3rcq
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