[Elecraft] Question for Wayne about your OCF Dipole

[Jim Brown]

On 9/19/2017 10:55 AM, Bob McGraw K4TAX wrote:
> Most HF antennas for 160M, 80M and 40M, based on average height above ground, have a feed Z less than 50 ohms at resonance.  Some as low as 20 ohms.

Yes, but that depends strongly on ground conductivity and electrical 
height. I'm part of a team that activates CA counties with few if any 
hams for the CA QSO Party, and do most of the antenna design work. The 
QTHs where we set up mostly have very poor ground, and I take that into 
account when modeling them in NEC. Our 40M dipole for the SSB station 
was rigged from trees at about 35 ft, and modeling predicted a feedpoint 
Z around 75 ohms, so we fed it with RG11 (it was a long run to minimize 
QRM to/from the CW station on 40M). When rigged, I checked SWR with an 
AEA SWR bridge calibrated to 75 ohms and the SWR was close to 1:1. That 
antenna at the same height over good ground modeled closer to 50 ohms.

Using a vector network analyzer and SimSmith software to transform 
measurements in the shack to the feedpoint, I've measured the feedpoint 
Z of my 120 - 140 ft high 80M dipoles over poor ground in the range of 
85-90 ohms.

Feedpoint Z of horizontal antennas includes the mutual impedance of the 
ground reflection, and that reflection varies in both magnitude and 
phase with electrical height and ground conductivity. This can be 
clearly seen in graphs of computed feedpoint Z vs height for horizontal 
antennas, which show low dipoles oscillating around 50 ohms and high 
dipoles oscillating around 75 ohms. One place I recall seeing such 
graphs is in ON4UN's Low Band DXing, much of which is about antennas and 
counterpoise/radial systems for 40-160M.

73, Jim K9YC