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[K6STI]

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

[N0GV]

Useful tool for site selection!

Grover

[K6STI]

Useful tool for site selection!

The main drawback is that a ground probe samples only the surface soil. If the ground characteristics change with depth, you won't know about it. But your antenna will. It's surprising how far the fields from an antenna penetrate ground in some cases. It varies with frequency and ground characteristics, but it can be tens of feet. Not all ground is inhomogeneous, and even when it is, the characteristics of the soil can be expected to bear some relation to what's below. Most antenna models are not critically sensitive to ground quality. It affects them much less than even small changes in antenna dimensions. Getting permittivity and conductivity values within the ball park is often good enough.

To give you some idea of where you stand, the program reports and plots skin depth, the distance where antenna current drops to 37% of its surface value. Most of the effect of ground occurs above this point. Some people use two skin depths (13.5%) to be conservative. So if the skin depth is 3 feet, you can feel more confidence in the numbers a foot-long probe provides than if it is 60 feet.

There are ways to probe the ground at depth, such as measuring the impedance of a carefully constructed test dipole at a well-defined height, but they are narrowband, not portable, and tedious to build. See the Low Dipole section here:

http://ham-radio.com/k6sti/hifer.htm

Brian

[AC6LA]

It's surprising how far the fields from an antenna penetrate ground in some cases. It varies with frequency and ground characteristics, but it can be tens of feet.

Here's something I don't think I've ever looked at before.  For a horizontal 3.5 MHz dipole at a height of 20 ft fed with 100 watts, this chart shows the above and below ground field strengths for three different ground characteristics.  The sampling point is offset by 1" from the center of the dipole wire.



Notice that a) the "below ground" field strengths fall off much more rapidly for good ground and b) the "above ground" field strengths also vary by ground quality up to about twice the dipole height.

Dan, AC6LA