[TowerTalk] New N6LF Ground Probe Designs

[Jim Brown]

On 3/2/2024 12:10 PM, Brian Beezley wrote:
> I should point out the the FCC ground conductivity map is generally not 
> useful for hams. It applies to the AM broadcast band only, not to HF. It 
> does not show ground permittivity. Both conductivity and permittivity 
> influence antenna performance and both can vary greatly greatly with 
> frequency. In addition, the resolution of the map is way too coarse to 
> be useful. For example, on the BC band where the map is supposed to be 
> valid, Rudy measured ground conductivity 4-5 times higher at his QTH 
> than the map indicates. In this case the map is highly misleading.

The FCC map is ancient -- I'm guessing late '40s-'50s. It's also rather 
coarse-grained. Development is the enemy of good soil conductivity.

To the extent that it's accurate or not out of date, I would expect the 
map to be a decent first approximation on 160M for assessing a vertical 
antenna's possible radiation efficiency in areas that are reasonably 
homogeneous, but close to meaningless in areas that are well-developed. 
By this I mean the extent to which soil in the far field does or does 
not attenuate or reinforce the direct signal to result in strong 
low-angle field strength. Simply put -- if the map shows high 
conductivity, verticals have a good shot at being effective, if low 
conductivity, try to rig horizontal antennas as high as practical.

A serious radial system can minimize loss for a vertical antenna's 
return current.

Extensive modeling studies have shown that while horizontal antenna 
impedance characteristics are dependent on the soil underneath them, 
their radiation efficiency and vertical pattern are almost entirely 
dependent on their height and on ground slope. In the last decade, N6BT 
has done work to show that the field strength and vertical pattern of 
vertical antennas is strongly dependent on ground slope.

73, Jim K9YC