HF-Ground
 

"Owen Duffy" wrote

For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp

Reg, did I miss something. RADIALS3 doesn't seem to be in the index
at
http://www.btinternet.com/~g4fgq.reg...3.html#S301%22

Owen
----------------------------------------------------------------------
----

Very sorry Owen. My mistake.

The name of the program is RADIALS2.

It deals only with buried radials.

I am at present trying to write a program about coil-loaded
counterpoises and artificial grounds at low heights. But it's a beast
to model mathematically.
----
Reg.

HF-Ground
 

"Cecil Moore" wrote
It would appear that 1/2 wavelength
buried radials do NOT present a high impedance.

===============================
Cec,

.. . . . as demonstrated by program RADIALS2

which treats radials as transmission lines. As they truly are.

The permittivity of soil surrounding buried radials is high. It is due
to the moisture content of the soil. Water has a high permittivity K
= about 80. If the moisture content is 20 percent then the
permittivity is roughly K = 16 plus a little bit for the dry content.

The poor, low conductance of the dielectric material, in conjunction
with wire inductance, also has an effect.

The soil is mainly, minute rock particles and a little air. Rock has
K = 4 or 5. Air = 1.

Velocity factor of any transmission line = 1 / Sqrt( Permittivity ).

In some circumstances, there may be no point in having radial lengths
longer than 1/10th or 1/12th of the free-space wavelength.

If the soil has any magnetic material in it then the velocity is even
lower. But it's a waste of time trying to tune buried radials by
sprinkling iron filings around your garden.

The attenuation along buried radials is usually so high that even
1/4-wave resonance doesn't show up. Impedance versus length at low HF
is a smooth curve approximately equal to Zo. But input impedance of a
set of radials is NOT equal to the impedance of the individual wires
all in parallel. They interact with each other. The Law of Diminishing
Returns applies.

In perfectly dry desert sand with a resistivity of 5,000 or 10,000
ohm-metres and K = 3, the 1/2-wave resonance may appear on an
impedance vs frequency curve. Program RADIALS2 shows this effect as
evidence of reasonable modelling accuracy.

This is a case of ground loss decreasing as soil resistivity increases
further. It appears attenuation is a maximum when soil resistivity
is around several hundred ohm-metres (377?). Which is quite a poor
soil.

(I once had a garden of sandy soil. Resistivity was 400 ohm-metres
even in wet weather. Eventually I moved house. SR fell to 70. On the
160m band 7 radials, each about 10 feet long, plus the cold water
pipe, were good enough with a 3/8 wave inverted -L. I never tried
B,L,E's 118 radials, 1 wavelength long.)

But in bone dry sand-desert soil, just rock mixed with air, at low HF
one would not use a system of radials under a 1/4-wave vertical. The
antenna could be a horizontal dipole lying on the ground. ;o)
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

HF-Ground
 

Reg Edwards wrote:

. . . . as demonstrated by program RADIALS2

which treats radials as transmission lines. As they truly are.
. . .

I disagree. Transmission lines have two conductors. Radials don't. The
fields from transmission lines are confined to the region between the
conductors. The fields from radials can couple quite strongly, altering
their performance a great deal. It might be possible to model a radial
system as a system of coupled transmission lines. Is that what your
program does?

Roy Lewallen, W7EL

HF-Ground
 

On Fri, 20 Jan 2006 01:14:18 -0800, Richard Clark
wrote:


such illusions. The association with the necessity of being a quarter
wave long comes by the field data obtained by Brown, Lewis, and
Epstein. This was simply an arbitrary selection born more of the
available wire being portioned out in binary increasing counts
(2,4,8,16....) such that 119 radials depleted their stock (short of
that magic 128). Their work has been offered on the web through the
interests of our discussions here, and by one or several
correspondent's scanning and posting their report. Google this
newsgroup for that link using the authors as a keyword search. This
was offered last summer.

Thanks, yes I have read the BLE paper.

Owen
--

HF-Ground
 

On Sat, 21 Jan 2006 00:55:13 +0000 (UTC), "Reg Edwards"
wrote:


Very sorry Owen. My mistake.

The name of the program is RADIALS2.

It deals only with buried radials.

Ok, I have had a play with it (again).


I am at present trying to write a program about coil-loaded
counterpoises and artificial grounds at low heights. But it's a beast
to model mathematically.

I wish you would explain the models a bit more in these tools, so that
the user can have a better understanding of the approach and its
applicability to the problem.

Owen
--

HF-Ground
 

On Thu, 19 Jan 2006 22:19:04 -0800, Richard Clark
wrote:


I found a much more compelling report in:
UNITED STATES DEPARTMENT OF AGRICULTURE
Rural Electrification Administration
REA BULLETIN 1751F-802
SUBJECT: Electrical Protection Grounding Fundamentals
Which is vastly more comprehensive and directly answers these
questions when viewed in the terms of the resistivity of the earth
connection.

Thanks Richard, I have read the document quickly overall, and a bit
more detailed in some key areas.

It covers similar material to documents I collected when studying
power earthing and lighting protections in years past, but it is a bit
more comprehensive... so a good read and a good reference document.

One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning protection.
I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m, the graph is at
http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt, this
graph does not predicting the RF characteristics of the radials.)

Tks...

Owen

PS apologies for the delay in responding, I have been up to the big
smoke (the city... Sydney) over the weekend... reminds me of why I
left there thirty something years ago.
--

HF-Ground
 

Your mind is already made up. Do as you like.

I note that you decline to substantiate the reasons underlying your advice.
Hello sir, maybe YOU should nip in the yard with a shovel and some wire
(don't forget your strippers) and do some tests yourself! old man

HF-Ground
 

"Owen Duffy" wrote
One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning protection.
__________________

For those using buried radials as one terminal of a vertical monopole...

The only path consisting of physical conductors that can exist between a
series-fed vertical monopole and buried, uninsulated radials is through the
PA output, and the antenna matching network at the tower base. This is not
adequate to control/prevent system damage from lightning.

Three added techniques are used in most MW broadcast applications:

1. A "static drain choke" is installed between the tower base and the
junction of the radials.

2. An arc gap is installed across the base insulator and set to flash over
at some margin above normal peak voltage

3. The tx contains r-f phase sensors that kill tx output for a few
milliseconds after an arc is sensed, so as not to sustain it.

RF

HF-Ground
 

"Owen Duffy" wrote
One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning
protection.
I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m, the graph is at
http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt,
this
graph does not predict the RF characteristics of the radials.)
=========================================

Owen, I assume the curves in your graphs have been obtained by
treating the conductors as transmission lines. As far as I am aware
there's no other way of doing it. Except perhaps EZNEC
number-crunching mathematical modelling methods.

At VLF the inductance of the conductors and the capacitance due to
relatively high permittivity of the dielectic material (soil) can be
neglected.

This leaves only conductor resistance and conductance (or resistivity)
of the soil. It is then quite simple for single wires.

To predict performance at RF it is necessary to take inductance and
capacitance into account. What is unknown is the way in which soil
permittivity and resistivity change with frequency. But this hardly
matters as the uncertainty at 60 Hz is sufficient to swamp it.

I won't ask you what you did about calculating the effects of multiple
radial wires, and the interaction between individual wires, which
causes "The Law of Diminishing Returns" to be followed.

There is sufficient information in your graph to demonstrate that
Magician Marzipan's magic high number of 120 is never necessary for
amateur purposes.
----
Reg.

HF-Ground
 

"Roy Lewallen" wrote -
I disagree. Transmission lines have two conductors. Radials don't.
=======================================

Roy, try using your imagination!
----
Reg.