"OK1SIP" wrote
"Reg Edwards" wrote
Hi Reg,
unmistakably, the number of radials is not the only measure of the
goodness of RF grounding. Their length related to the working
wavelength is certainly significant as well - imagine 120 radials 1
meter long each at 160 m ... The soil structures underneath and beyond
the radials play an important role, too. An analysis of this problem
is given in the famous book by John ON4UN.

The following ranges are suggested:

Excellent : 2 ohms or less.
Good : 2 to 5 ohms.
Average : 5 to 15 ohms.
Poor : 15 to 50 ohms.
Awful : Greater than 50 ohms.

Are these in the right ball-parks? Depends on frequencies of
occurrence.

IMHO they are right at shortwaves. One problem: how to measure the
grounding resistance simply ? It should be measured at the working
band. Measurements with a ground resistance meter may apply to static
and lightning protection (which is VERY IMPORTANT, too), but not to
RF.

BR from Ivan OK1SIP

============================
99.9 percent of amateurs don't know what their ground connection resistance
is. But it's readily easy to determine.

Just erect a random height vertical antenna above the ground connection,
tune it, and measure the tuned RESISTANCE between the bottom end and the
ground connection using a simple, hand-held antenna analyser. In the tuned
condition the antenna input impedance ia always R + j*Zero.

Calculate the radiation resistance of the vertical wire. Deduct radiation
resistance plus loading coil loss resistance from the measured value and
there you have it.

Loading coil loss resistance is estimated from a crude guess of coil Q.
Crude is quite accurate enough if you havn't got a Q meter.

For heights less than 0.3 wavelengths, Radiation resistance can be
accurately enough estimated from -

Rrad = Square( 24 * Height / Wavelength ) ohms

Which is near enough exact when height = 1/4 wavelength.

Measurements need be made at no more than two frequencies, say 3.5 or 7 MHz,
and 14 or 21 MHz. This is because ground impedance changes fairly slowly
with frequency. And nobody is interested in ground connection resistance
above 30 MHz anyway.

What ground REACTANCE may be is neither here nor there because it is not a
loss component. In practice it is tuned out simultaneously with tuning up a
vertical antenna and merely affects the tuner settings. Ground reactance
may be either inductive or capacitative depending on the number and length
of radials and on soil resistivity and permittivity. At HF it can be of the
same magnitude as the resistive component. But what does a 10-ohm
capacitative reactance matter?

By the way, an example of an 'awful' ground connection is a single 4 to 8
feet rod - unless submerged in salt sea water. Depending on the antenna
they very often 'work' and, if it's your very first ground electrode,
inevitably you will place great faith in it.

Thanks Ivan for your comments. I would like a few more views from amateurs
with experience of known ground connection resistance values. Isolated
examples, especially with a description of the type of soil, would be
especially interesting and useful.

But everybody already knows that 120 radials, of any reasonable length, in
any sort of soil is "Excellent".
----
Yours, Reg, G4FGQ