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 < [email protected]> 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 > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
