HF-Ground
 

Owen Duffy wrote:
Whilst there are articles around about the performance of shallow
buried radials, I have not seen any that deal quantitatively with
radials laid on the ground, or pinned to the ground as you describe,
and the effects of those different installations on antenna
efficiency.

Will NEC-4 accurately model radials at different depths?
--
73, Cecil http://www.qsl.net/w5dxp

HF-Ground
 

Owen Duffy wrote:
One of the things that intrigues me is the common "expert" advice to
cut radials for 7MHz to 33' long and bury them. It seems to me that
when buried and considering the wire as a transmission line, the
velocity factor will be somewhere between 0.3 and 0.8 depending on the
soil type, so that 33' is likely to be closer to a half wave
electrically, and present a relatively high and reactive impedance at
the antenna base if it were not for the attenuation of the wave on the
radial.

You seem to be referring to the feedpoint impedance of a
single radial the virtual impedance of which would depend
upon the magnitude and phase of the forward and reflected
wave on the radial wire. The single-wire transmission line
formula gives a Z0 for each radial as less than 100 ohms.
Given the probability of a high degree of attenuation and
the number of radials in parallel, the impedance presented
at the base is likely to be relatively low no matter what
the length of the radials assuming an electrical length
of longer than 1/4WL.
--
73, Cecil http://www.qsl.net/w5dxp

HF-Ground
 

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

HF-Ground
 

"Roy Lewallen" wrote
2. Making elevated radials too long, even if the elevation consists of
being just above the ground, can seriously reduce the antenna efficiency.
Buried radials, on the other hand, are insensitive to length provided
they're sufficiently long. This latter fact is well known. I've found in
other modeling I've done that making elevated radials shorter than a
resonant quarter wavelength doesn't negatively impact the efficiency. So
if you have to guess, guess on the short side.
_____________

NEC studies of a 1/4-wave vertical radiator working against three 1/4-wave
horizontal radials at 120 degrees, when all elements are elevated 12 feet
above a perfect ground plane show virtually identical peak gain as when the
same radiator minus the radials is mounted with its base at the perfect
ground plane, and connects to it though two ohms (about the same ground loss
as produced by 120 buried radials, each 1/4-wave long).

As few as four elevated radials have been used at AM broadcast sites where a
typical system of 120 buried radials was impossible due to rocky terrain.
The FCC "efficiency" of these radiator systems meets/exceed FCC
requirements for radiation at 1 km.

Conclusion: a few elevated radials can be the electrical equivalent of a
classic "Brown, Lewis & Epstein" system of 113 (or 120)buried radials.

RF

HF-Ground
 

"Owen Duffy" wrote
Whilst there are articles around about the performance of shallow
buried radials, I have not seen any that deal quantitatively with
radials laid on the ground, or pinned to the ground as you describe,
and the effects of those different installations on antenna
efficiency.
========================================

Owen, you may be interested in Program RADIALS3.

Once radials are buried in the ground, just below soil surface, I
don't think depth of burial matters very much. But things happen very
fast when resting on the soil surface and conductor height is raised
only slightly.

Consider a single counterpoise wire. When resting on the soil surface
propagation velocity along the wire is about half of the free space
velocity. Propagation velocity increases fast as height increases.
When height is greater than length the VF is very nearly 1.

When resting on the ground there is an equivalent loss resistance
distributed along the wire due to the mutual impedance with the
ground. The input resistance at resonance is high. Q is very small.
But a lot depends on soil resistivity.

With increasing height the coupling with ground decreases and so does
the input resistance. When height is greater than length the input
resistance is due only to conductor loss resistance and resonant Q is
high.

Resonant frequency increases with height due to the increase in VF.
The increase in VF is due to decrease in capacitance to ground when
considered as a transmission line.

Changes in capacitance, VF, resonant frequency, and induced loss in
the ground, mostly occur in first few inches of height above ground
but cannot be neglected until height is roughly greater than length.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

HF-Ground
 

"Richard Fry"
NEC studies of a 1/4-wave vertical radiator working against three 1/4-wave
horizontal radials at 120 degrees, when all elements are elevated 12 feet
above a perfect ground plane... etc
____________

I neglected to include that the frequency in these studies was 1 MHz, so
elevation of the system was about 0.012 wavelengths (Reg).

RF

HF-Ground
 

"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

HF-Ground
 

On Fri, 20 Jan 2006 13:56:28 +0000 (UTC), "Reg Edwards"
wrote:


Owen, you may be interested in Program RADIALS3.

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
--

HF-Ground
 

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

HF-Ground
 

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