I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication is
that while it may require a large number of buried radials (16) before
efficiency levels off a bit, similar efficiency can be obtained with
just three radials elevated more than 100mm above the soil.

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Comments?

Owen

On Sep 29, 10:44*pm, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication is
that while it may require a large number of buried radials (16) before
efficiency levels off a bit, similar efficiency can be obtained with
just three radials elevated more than 100mm above the soil.

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Comments?

Owen

because they are easier to install in most cases than raised radials,
unless you get them far enough up to walk under them like i do.

On Sep 29, 3:44*pm, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication is
that while it may require a large number of buried radials (16) before
efficiency levels off a bit, similar efficiency can be obtained with
just three radials elevated more than 100mm above the soil.

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Comments?

Owen

Because my vertical is located out in a pasture. Deer, horses, and the
tractor will trip over the wires. Even buried, the tractor tires
brought some up when I mowed the pasture late this summer, especially
when I turned over the radial field.

Paul

On Sep 29, 5:44*pm, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication is
that while it may require a large number of buried radials (16) before
efficiency levels off a bit, similar efficiency can be obtained with
just three radials elevated more than 100mm above the soil.

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Comments?

Owen

I don't think the model is totally valid.. Partially though..
I agree that for a given number of radials used, slightly
elevating off the ground is better than buried.
But I don't agree that a small number of slightly elevated
radials is equal to a large number of buried radials, and
most certainly not equal to the same number of radials
highly elevated. IE: vs 1/4 or 1/2 wave or more up..
Three slightly elevated radials are not sufficient to lower
ground losses down to a low level over mediocre soil.

I always think in terms of wavelength when calculating
the approximate efficiency of an elevated radial set.
For instance, three radials at 1/2 wave up will be pretty
much equal to about 120 on the ground.
Three at 1/4 wave will be equal to about 50-60 on the ground.
Three at 1/8 wave might be equal to 15-20 on the ground.
Three at cigarette pack height will be equal to about twice
as many as actually used at best. "slightly guessing
on that one, but my real world tests seem to pretty much
agree".
So being as the increase is fairly small at such low heights
in wavelength, it is probably practical to just bury them so
people won't trip over them.. :/
If tripping is no issue, then it might be worthwhile to get
the slight edge in performance. But the increase over buried
will be fairly small with them only 100mm up.

On 30/09/10 12:05, wrote:
....
So being as the increase is fairly small at such low heights
in wavelength, it is probably practical to just bury them so
people won't trip over them.. :/
If tripping is no issue, then it might be worthwhile to get
the slight edge in performance. But the increase over buried
will be fairly small with them only 100mm up.

I am interested in the electrical performance rather than trip hazard.
Once electrical performance is known, issues like trip hazard, mowing,
rock etc can be dealt with for each application scenario.

So, back to the electrical performance, do you have measurement data, or
can you refer me to articles that contain sound objective measurement
data that would suggest that my NEC4 model is not valid.

If I have modelled the situation correctly, the models suggest that the
portion of energy lost in heating the soil drops very rapidly with
increase in height from zero to just tens of mm, and with very little
change to the pattern distribution.

Owen

PS: I perhaps need to clarify the meaning of 'efficiency' in my post, I
mean the total power radiated in the far field in the hemisphere,
divided by the power input to the antenna.

Owen writes:

I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication
is that while it may require a large number of buried radials (16)
before efficiency levels off a bit, similar efficiency can be obtained
with just three radials elevated more than 100mm above the soil.

N6LF made extensive measurements and essenstially confirmed this. He
wrote a 7 part series of articles for QEX. You can download them at
his site:
http://www.antennasbyn6lf.com/2009/1...periments.html

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Practicality. In most cases, you either want to be able to walk above
the radials, (i.e. bury them or leave them on the ground) or below
them. This means at least 2 - 2.5 m up, and there will be some
sagging. Essentially, your vertical just got that much shorter. But if
what remains is tall enough, it's a great choice. If a friendly farmer
lets you borrow a field in wintertime, stringing four elevated radials
is a lot less work than rolling out 32 on the ground.

Jon LA4RT, Trondheim, Norway

On 30/09/10 17:49, LA4RT Jon wrote:
writes:

I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication
is that while it may require a large number of buried radials (16)
before efficiency levels off a bit, similar efficiency can be obtained
with just three radials elevated more than 100mm above the soil.

N6LF made extensive measurements and essenstially confirmed this. He
wrote a 7 part series of articles for QEX. You can download them at
his site:
http://www.antennasbyn6lf.com/2009/1...periments.html

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Practicality. In most cases, you either want to be able to walk above
the radials, (i.e. bury them or leave them on the ground) or below
them. This means at least 2 - 2.5 m up, and there will be some
sagging. Essentially, your vertical just got that much shorter. But if
what remains is tall enough, it's a great choice. If a friendly farmer
lets you borrow a field in wintertime, stringing four elevated radials
is a lot less work than rolling out 32 on the ground.

Hi Jon,

Noted.

One of the designs I am exploring is an eighth wave vertical over
elevated quarter wave radials, three top guy ropes the the radial
straining posts, and the top section of the guys are top hat wires. The
whole thing is tuned low so that it is matched to 50 ohm line with a
shunt inductor. Modelled performance is within 0.5dB of a ground mounted
quarter wave with 32 buried radials. Elevating the radials with the
shortened vertical is not a big structural challenge, even to 2.5+m to
allow a bit of sag in the radial catenary. (I never said I wanted to rig
the radials at 100mm, just that above that, the efficiency was
relatively high and didn't vary much between 100mm and a few metres.)

But, such a design does not conform to the Rules of Thumb commonly
trotted out for low HF Marconis.

I will look at Rudy's articles, always an interesting read.

Thanks
Owen

On 29/09/2010 23:44, Owen wrote:
I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication is
that while it may require a large number of buried radials (16) before
efficiency levels off a bit, similar efficiency can be obtained with
just three radials elevated more than 100mm above the soil.

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Comments?


People generally don't have the room to have several elevated radials
1/4 wave long for 20/30/40/80m whereas the buried ones can be laid in
any shape required to fit.

--
Conor

I'm not prejudiced. I hate everyone equally.

On Sep 30, 1:24 am, Owen wrote:
On 30/09/10 12:05, wrote:
...

So being as the increase is fairly small at such low heights
in wavelength, it is probably practical to just bury them so
people won't trip over them.. :/
If tripping is no issue, then it might be worthwhile to get
the slight edge in performance. But the increase over buried
will be fairly small with them only 100mm up.

I am interested in the electrical performance rather than trip hazard.
Once electrical performance is known, issues like trip hazard, mowing,
rock etc can be dealt with for each application scenario.

So, back to the electrical performance, do you have measurement data, or
can you refer me to articles that contain sound objective measurement
data that would suggest that my NEC4 model is not valid.

I haven't done any careful measurements, but one of the books
I have has an article and graph that pretty much matches the
figures I gave as far as amount of elevated radials needed to
match a certain amount on the ground.
I'm not sure which book it is..
Maybe one of the Bill Orr radio or antenna handbooks..
I don't think it was the ARRL handbooks.
But I have tried exactly what you are proposing. And it didn't
really pan out too well. I could hardly tell the difference between
having them on the ground, and slightly elevated.
Sure, it worked OK, but it didn't mimic a large number of radials
by any means. I also tried the elevated ground plane at various
heights using the same four radials. You could tell an obvious
difference between 1/4 wave up, and 1/8 wave up using the same
four radials. The real world results I've seen seemed to fairly
closely match the graph I have in that book.

BTW, I know it was one of the Bill Orr handbooks which
suggesting trying the elevated low radials.. I think the
same one that had the graph. He seemed to suggest it
was better than having them in the ground. But on the
other hand his graph dealing with elevated radials
suggested the results would be fairly lackluster.
Which they were at this QTH.
Now that I think about it, I think it was the Bill Orr
antenna handbook that had that article and graph
pertaining to the radials.

Being as you are talking about radials which are very low,
I assume the base of the radiator will be low also.
Myself, I think a large part of the lack of success I saw
was due to not enough radial density at the base of the low
vertical. I think one would probably be better off taking
the three or four 1/4 wave radials and chopping them into
many short radials and laying them on the ground.
And this is indeed a fairly common practice when laying
them on the ground.

I think having the higher density of wire at the base works
better than have just a few radials elevated. Most of the
ground loss seems to be in the area of the base.
I know metal density under the radiator is quite critical as
far as short mobile whips.
I saw that when I tried mounting my mobile whip on a
piece of angle iron that was running across the bed of
my truck. It didn't work too well, and being I was used
to using that antenna on various mounts, it was obvious
something was wrong. And the angle iron was very
well grounded as far as bonding to the truck.
That wasn't the problem.
I then moved it over to the top of the side mounted
utility bed box, which is a part of the truck body and
a good bit wider, and my usual performance was back.
Just by adding more metal under the base of the whip
did the trick.

I think this is one reason why I always had so much
better results with elevated vs ground mount verticals.
At 1/4 wave up, the base of the antenna was much
farther from ground, and the antenna started to
perform more like a vertical dipole, than a grounded
monopole.
With the ground mount, I was up to 32 radials at
one point. And the performance was still mediocre,
and barely better than my dipole at 36 ft on longer
paths.
Only when I got it up at 1/4 wave did it really come
alive. After that, it ate the dipole for lunch on longer
paths, and with only four radials.
All of my comparisons were done using a full 1/4 wave
radiator.

Anyway, maybe your model is more accurate than I
think. But I've already tried doing that, and wasn't too
impressed with the results.
I've heard others that tried it also, with the same lackluster
results. I think elevated or not, it's just too few a number
of radials to really be effective at that low height in WL.
Too much lossy dirt between the few radials is my
theory.

I just don't believe in too much of a free lunch when it
comes to just a few radials at low heights in wavelength.
My stance comes from actually trying it, vs modeling
it. I don't totally trust the modeling programs in
this area. I've seen too many differences comparing
the models vs the actual vertical antennas.
For instance, using "average" ground, most of the
programs underestimated the performance of my 1/4
wave high ground plane. To get a model that more closely
matched real life vs my dipole, I had to set the ground at
a much higher conductivity.
And in the case you are modeling, it seems to be
over estimating the performance vs real life.
Or at least what I have seen after trying it.

I wished it would have panned out better. Would save
a lot of wire. But for me, the improvement was minimal.
But.. You are welcome to try it. Maybe you will have
better luck than I did.

On Sep 30, 1:24*am, Owen wrote:
So, back to the electrical performance, do you have measurement data
or can you refer me to articles that contain sound objective measurement
data that would suggest that my NEC4 model is not valid.

This topic was investigated experimentally quite some time ago by a
broadcast consulting firm in the US, which generated measured data.
Here is a clip from their paper describing the system tested, and the
results (note that the convention used for "efficiency" here is that
of the FCC practice based on the groundwave field intensity at 1 km
with respect to the power applied to the antenna system):

\\ In November of 1988, our firm supervised the construction of a
temporary antenna system in
Newburgh, New York under FCC Special Field Test Authority using call
sign KPI-204. The antenna
system consisted of a lightweight, 15 inch face tower, 120 feet in
height, with a base insulator at the 15
foot elevation and six elevated radials, a quarter wave in length,
spaced evenly around the tower and
elevated 15 feet above the ground. The radials were fully insulated
from ground and supported at the
ends by wooden tripods. Approximately ten feet above ground, a T
network for matching the antenna
was mounted on a piece of marine plywood to isolate the components
from contact with the lower
section of the tower which was grounded. Power was fed to the system
through a 200 foot length of
coaxial cable with the cable shield connected to the shunt element of
the T network and to the elevated
radials. A balun or RF choke on the feedline was not employed and the
feedline was isolated from the
lower section of the tower. The system operated on 1580 kHz at a power
of 750 watts.

The efficiency of the antenna was determined by radial field intensity
measurements along 12 radials
extending out to a distance of up to 85 kilometers. The measured RMS
efficiency was 287 mV/m for 1
kW, at one kilometer, which is the same measured value as would be
expected for a 0.17 wave tower
above 120 buried radials.

The Newburgh tests gave empirical proof that the elevated system
worked although, in an abundance
of caution, we used six radials instead of four. For the limited time
that the system was operational, the
system was stable as determined by monitoring the field intensity at
selected locations each day. The
measured base impedance was in general agreement with a tower of this
height above a standard,
buried, ground system. Results of the KPI-204 tests were submitted to
the FCC in January of 1989.//

The complete paper is available at this URL:

http://www.commtechrf.com/documents/nab1995.pdf

RF