Topband: Measuring ground screen resistance
Mon, 13 Sep 1999 19:47:28 -0400
> 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
(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
FAQ on WWW: http://www.contesting.com/topband.html
Administrative requests: topband-REQUEST@contesting.com