A ground probe lets you measure your local ground permittivity and
conductivity for antenna analysis. Accurate models of verticals and low
horizontal wires need realistic values. The probe uses two metal rods
about a foot long spaced a few inches apart. You insert it in the ground
and connect it to a VNA. My Windows program converts measured S11 values
to ground permittivity and conductivity.
Ground probes are susceptible to common-mode current caused by any stray
RF path from the probe shield to ground. The path may include the cable
to the VNA, the VNA itself, or anything it connects to or couples to,
including your body. It is possible to suppress common-mode current at
the probe by adding a switch (requires two measurements) or a wideband
balun transformer (one measurement).
The latest program version provides a new method to suppress common-mode
current. You simply connect the rods between the VNA ports with two
cables. The program then uses S11 as well as S21 to extract the series
impedance when a shunt impedance is present at each port due to a stray
RF path to ground.
http://ham-radio.com/k6sti/gnd.htm
Brian
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