We are accustomed to using the *number* of radials as a measure of the
goodness of a system of shallow-buried or surface-laid radials. But what
really matters is the resistance to Mother Earth as is effective at the
system center or off-center as the case may be.

We are familiar with associating soil resistivity data with 'goodness' of
the soil or the terrain.

Forgetting all about what the regulations may specify about commercial
transmitting stations, what are the ranges of ground electrode resistance
which amateurs consider to be excellent, good, average, poor, awful. Or
perhaps some other descriptions.

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.
What do you think?

Anyway, it makes a change from circulators and S-parameters at SHF.
----
Reg, G4FGQ

"Reg Edwards" wrote in message ...

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

"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

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.

Hi Reg, I just went outside and measured the impedance of my 8' mobile
antenna. It's center loaded with a high Q loading coil and small capacity hat.
It is mounted on a full size pick-up truck. At 7.2 MHZ it reads 37+j0 ohms.
The antenna analyzer has always been accurate within a few percent. Does this
mean that my truck provides an "excellent ground" at 7.2MHZ? If it does, I
need to use my truck for an rf ground for the rig inside my house. It may be
that the truck does indeed provide a good ground, I get good results on 40M
mobile.

73 Gary N4AST

I expect that the radiation resistance of your antenna is closer to 10
to 15 ohms. That implies that your truck has a 20 +/- ohm ground system
and your antenna has several ohms of resistance loss.

BTW, that's a typical value of Rr. I measure about the same for my
Ranger on 40 meters.

DD, W1MCE

JGBOYLES wrote:

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.

Hi Reg, I just went outside and measured the impedance of my 8' mobile
antenna. It's center loaded with a high Q loading coil and small capacity hat.
It is mounted on a full size pick-up truck. At 7.2 MHZ it reads 37+j0 ohms.
The antenna analyzer has always been accurate within a few percent. Does this
mean that my truck provides an "excellent ground" at 7.2MHZ? If it does, I
need to use my truck for an rf ground for the rig inside my house. It may be
that the truck does indeed provide a good ground, I get good results on 40M
mobile.

73 Gary N4AST

Hi Reg, I just went outside and measured the impedance of my 8' mobile
antenna. It's center loaded with a high Q loading coil and small
capacity hat.
It is mounted on a full size pick-up truck. At 7.2 MHZ it reads 37+j0
ohms.
The antenna analyzer has always been accurate within a few percent. Does
this
mean that my truck provides an "excellent ground" at 7.2MHZ? If it does,
I
need to use my truck for an rf ground for the rig inside my house. It may
be
that the truck does indeed provide a good ground, I get good results on
40M
mobile.

73 Gary N4AST

====================================

Gary, if your coil-loaded 8-feet vertical had been mounted above a ground
radial system then the radial system would have to be classified as "poor".

Radiation resistance is about 4 ohms for a height of 8 feet at 7.2 MHz.
Coil loss = 3 ohms. Subtract 4+3 from your measured 36 ohms which leaves 29
ohms for the ground loss resistance. Which is a poor value for a radial
system. But it may not be poor for your particular vehicle-mounted system
because the vehicle itself forms a large physical part of the antenna and so
increases radiation resistance.

( The vehicle plus whip behaves as a short, off-centre fed, half-wave
resonant vertical, there being fewer ground losses to worry about.)

But the calculating procedure is the same for all vertical ground-mounted
antennas. Subtract calculated radiation resistance from measured input
resistance to obtain ground loss resistance. If there's a loading coil,
subtract its loss resistance too.
Also subtract something for wire-loss of a long wire.
---
Reg, G4FGQ

Hi Reg, thanks for your valuable advice.

73 Ivan OK1SIP

"Reg Edwards" wrote in message ...
============================
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

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

Careful! When you get your tongue that far into your cheek,
it's
sometimes hard to get out. Tongue 'cramps' are terrible...
'Doc