Owen Duffy wrote:

Bill, I often see the assertion that it is better to not bury radials.

Can you point me to any reputable texts or experimental evidence that
shows the difference between shallow buried radials and radials lying
"on top of the ground"?

Corrosion is often cited as a reason to use insulated buried radials,
but is corrosion a significant risk in most locations. We widely use
buried copper water pipes here, and copper clad earthing electrodes
for the MEN power supply earthing, yet they don't seem to suffer
significant corrosion in most places.

It seems to me that insulated buried radials are likely to be less
effective in a lightning protection role.

Owen

You're asking some good questions.

Many years ago, I was doing some ground system experiments with a
vertical antenna. I had strung a bunch of radials, consisting of small
insulated hookup wire, on the surface of the ground and I was measuring
antenna input impedance. I was surprised to discover distinct resonance
effects as the radial lengths were changed, something which I hadn't
expected -- they were acting more like elevated than buried radials. It
was summertime and the clay ground was pretty dry.

Then I took some pieces of mild steel wire and "stapled" the wires down
to conform with the ground and bring the wires into close contact with
it. The resonant effects disappeared and the radials acted more like
buried ones. I concluded that even a very small air gap between the
radials and the ground provided some independence from the ground.

More recently I've done some modeling to try and understand the
phenomenon a bit better. I'll give those results in my response to
another of your recent postings.

Oh, as for corrosion -- I'm sure it depends on the soil. But there's no
harm in using insulated wire, as far as RF ground effectiveness goes. I
suppose it would limit the lighning protection voltage to the insulation
voltage, however.

Roy Lewallen, W7EL

Owen Duffy wrote:
I often see the assertion that it is better to not bury radials.

Here's a data point for all. I'm taking down my 102 ft dipole
and putting back up my 130 ft dipole. I measured the resonant
frequency and feedpoint impedance of the 102 ft dipole both
while hanging in the air as a V and laying on the ground.

Hanging in the air: Resonant at 4.52 MHz with a feedpoint
impedance of 22 ohms.

Laying on the ground: Resonant at 2.17 MHz with a feedpoint
impedance of 108 ohms.

Laying the insulated wires on the ground resulted in a
reduction of VF of about 50%. The marked increase in
feedpoint impedance was due to the attenuation of the
reflected waves arriving back at the feedpoint and
agrees closely with my calculations of such.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Laying the insulated wires on the ground resulted in a
reduction of VF of about 50%. The marked increase in
feedpoint impedance was due to the attenuation of the
reflected waves arriving back at the feedpoint and
agrees closely with my calculations of such.

I forgot to say: Note that the dipole laying on the ground
was close to one wavelength, yet the feedpoint impedance
is not all that high. It would appear that 1/2 wavelength
buried radials do NOT present a high impedance.
--
73, Cecil http://www.qsl.net/w5dxp

"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
.................................................. ..........

"Bill Turner" wrote in news:xn0ehew7u3qjhj000
@cnews.newsguy.com:

wrote:

Question:

Is you are making a HF-ground (radials just below the surface) Should
these radials be insulated or not?

73 John
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Insulated will reduce corrosion. Don't bury them any deeper than
necessary. Lying on top of the ground is better. Dirt is not a good
antenna element.

Electric fence wire is adequate and cheap. I use it right on the surface,
as you say. It's a bit springy so you may have to work it to straighten
it, though.

--
Dave Oldridge+
ICQ 1800667

Ok, but do you use steel or aluminum fence wire? Galvanized steel fence wire
will rust in a few years.

"Dave Oldridge" wrote in message
9...

Electric fence wire is adequate and cheap. I use it right on the surface,
as you say. It's a bit springy so you may have to work it to straighten
it, though.

--
Dave Oldridge+
ICQ 1800667

"John, N9JG" wrote in
news:OVTzf.501279$084.368852@attbi_s22:

Ok, but do you use steel or aluminum fence wire? Galvanized steel
fence wire will rust in a few years.

Copperclad steel, usually.


--
Dave Oldridge+
ICQ 1800667

If you are also going to use the radials for a lightning protection
ground then bare wire is much better.

73
Gary K4FMX


On 19 Jan 2006 06:04:29 -0800, wrote:

Question:

Is you are making a HF-ground (radials just below the surface) Should
these radials be insulated or not?

73 John

I used about 1300 ft of insulated copper wire for my 72 radials. I chose to
pin mine to the ground witrh fenc e staples. I did this work in the dead of
winter. I then also covered them with a thin layer of soil. Come spring the
grass grows and the radials are hidden and beneath the applied soil layer so
as to not interfere with the lawn mower.

Pictures of my installation for my Hustler 5BTV at this web page

1. http://www.ad5th.com/5-BTV.html

--

Charlie-AD5TH
www.deepsouthnet.net


"Gary Schafer" wrote in message
...
If you are also going to use the radials for a lightning protection
ground then bare wire is much better.

73
Gary K4FMX


On 19 Jan 2006 06:04:29 -0800, wrote:

Question:

Is you are making a HF-ground (radials just below the surface) Should
these radials be insulated or not?

73 John

On Thu, 19 Jan 2006 23:31:32 -0600, "Charlie" wrote:

I used about 1300 ft of insulated copper wire for my 72 radials.

So they're about 18 feet long, giving you a short, dense pattern?

bob
k5qwg

I chose to
pin mine to the ground witrh fenc e staples. I did this work in the dead of
winter. I then also covered them with a thin layer of soil. Come spring the
grass grows and the radials are hidden and beneath the applied soil layer so
as to not interfere with the lawn mower.

Pictures of my installation for my Hustler 5BTV at this web page

1. http://www.ad5th.com/5-BTV.html