Topband: Measuring ground screen resistance

[Tom Rauch]

Hi Rick,

> There was some recent discussion about chicken wire ground screens
> vs conventional radials.  What I would like to know is:  how do
> you measure any proposed ground screen to determine its ground losses? 

The only accurate way to measure ground loss is through field 
strength measurements. With non-resonant (low Q) ground 
systems and reasonably tall antennas, base resistance does track 
loss resistance closely...but you have to be careful using feedpoint 
resistance as an indicator.  

As a general rule reasonably tall current fed verticals with fairly long 
radials that prevent high voltages (and high electric fields) from 
shunting the base terminals will provide good agreement with what 
really happens for terminal losses in the ground system. Of course 
this test does not tell you about the losses due to radiation into the 
lossy earth, which can amount to several dB.  

This test is mostly invalid for antennas with high voltage areas near 
the ground system, or ground systems with high voltage areas near 
the antenna.

(To see a good example of where ground loss was under estimated 
using the method you propose, look at the newer ARRL antenna 
books at mobile antennas under ground loss. A conclusion was 
reached a system mounted on a car on 80 meters had only two 
ohms loss resistance, and that loss resistance changed greatly 
with just a few feet movement of the antenna location on the car! 
That method does not work with any resonant system that has 
concentrated fields shunting current across the feedpoint.)  

The only accurate and foolproof way to measure "ground loss" is 
with field strength, and a comparison of FS change as only the 
ground system is changed. All you need to do is go out a few 
wavelengths and measure sample field levels in a few directions 
and watch for changes.


       
73, Tom W8JI
w8ji@contesting.com



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