On 01/10/10 07:44, Jim Lux wrote:
Owen wrote:
On 01/10/10 03:51, Jim Lux wrote:

Thanks Jim.


I would think that the buried radials are more convenient (broad band,
etc.)

Yes, I understand that there are advantages to buried radials, but I
don't understand the preponderance of cases where I see 120 radials
pinned on the top of infertile dirt. They still present a trip hazard,
and less money spent on just a few elevated radials may perform just
as well.

never underestimate the power of tradition. It was written by BL&E that
120 radials work, and the FCC accepts that for broadcast, so by golly,
that's what we do. Why 120? it was at the point of diminishing returns
or practicality back when the study was done (e.g. there was no
detectable change from going to more)

As for laying on ground.. I think that's more the laying on grass, and
eventually, the wire sinks into the grass/turf.

There's also the whole "the radials must be resonant" misconception..



Look at the performance of your ankle biting radials when the dimensions
are changed slightly.. For instance, if you shorten them by 5%, does it
make a big difference? For the buried radials, the length is very, very
non critical.

Yes, of course the feedpoint impedance is more sensitive to change in
length or conversely change in frequency.

While for a buried radial system (probably because of the losses) it's
going to be less frequency sensitive.

I expect so.




Something else to look at is the sensitivity of "efficiency" (and your
definition of radiated power in the hemisphere/power into antenna is
fine) to soil properties.. if the soil conductivity or epsilon changes
(as it will with changing water content) does the efficiency change
rapidly?

Yes, efficiency is sensitive to soil parameters... for both types, but
not very sensitive.

Maybe less sensitive for the buried radials? Or, it was "good enough"
for BL&E, so being so written, so shall it be done.

BL&E were measuring ground wave, I think solely. My efficiency measure
is the hemisphere, so ground losses play a different role.



Because of the impedance change mentioned above, the impedance
transformation needs adjustment for wide range frequency operation.
Not such an issue in the intended application, the DX window on 80m
here is just 50kHz.




If I haven't got something quite wrong in the modelling, it would seem
worthwhile to prototype the shortened version with a view to extending
the system to a four-square if suitable.

The shortened version will, of course, aggravate the tuning sensitivity.

Yes, but the model suggests that the variation in R is very small, and
variation in VSWR (with shunt coil match) is small... in that band segment.





I have still to read Rudy's papers... I am away from home (less
bandwidth) and I will download them later today when I get home. I
suppose that the proposed design challenges the norm of a very large
number of buried radials. In our case, part of the property is quite
rocky, and a configuration with just a few elevated radials offers
deployment opportunities that aren't suited to buried radials.

So, my original question is no so much suggesting everyone else got it
wrong, but why don't I seem more people doing it this way. Could I be
forgive in thinking that the popular, nearly universal, way is to
uplift the BL&E research at MF and apply it to 80m?

Tradition is a powerful force. Look how many years it took for someone
(e.g. Rudy) to put the substantial work into doing a real quantitative
experiment. For most hams, they're only going to do something once, and
if works ok, that's how it stays. Almost none are going to do a well
controlled A/B study, especially if there's a (not necessarily valid)
tradition that says A works better (where better is ill defined and
probably a combination of radiation efficiency and installation
convenience)

Until recently, modeling tools available to most amateurs were not
suitable for making the call, although there have been some people who
did models and published it, but, in the face of decades of "lay down
120 radials" it was a tough sell.

The other thing is whether the difference is big enough to "make a
difference" in observed system performance. For a lot of operators, a 1
dB change in performance might not be noticeable. If you're in a "either
propagation is there, or it isn't" situation the difference between good
and bad is 10s of dB. There are relatively few people who work at 0dB
SNR (where tenths count) on a regular and continuing basis, and they're
not necessarily the ones who are interested in doing experiments on
antennas on the scale needed.

Yes, there will be differing view on what is significant difference. I
am not in the school of declaring less than one or two S points is
insignificant in general.

In the case of a four square in the DX segment, users are looking for
performance... and it seems to me that the elevated three radials, eight
wave vertical with capacity had is very close to quarter wave over
buried radials... depending of course on the soil type.

You mention the modelling tools, I am not so much concerned as to
whether the elevated radials model is good, but whether the NEC4 buried
radials model is good, and likewise for radials on and just above the
ground because those models are setting the benchmark for the
performance of the alternative.

Owen






Owen

On 01/10/10 08:17, Richard Clark wrote:
On Fri, 01 Oct 2010 06:13:56 +1000, wrote:

Could I be
forgive in thinking that the popular, nearly universal, way is to uplift
the BL&E research at MF and apply it to 80m?

The original field research was done at 3 MHz - very much closer to
80M than to 300M.

73's
Richard Clark, KB7QHC

Yes, I accepted that advice from Richard Fry an hour or so earlier... Owen

On Sep 30, 4:44*pm, Jim Lux wrote:

never underestimate the power of tradition. *It was written by BL&E that
120 radials work, and the FCC accepts that for broadcast, so by golly,
that's what we do.

A minor point, but in the interest of accuracy - the greatest number
of buried radials used in the BL&E experimental work was 113.

There's also the whole "the radials must be resonant" misconception..

While the physical lengths of the buried radials in the BL&E
experiments were stated in free space wavelengths, that does not mean
that those physical lengths will behave the same when buried as they
will when not buried.

As shown in the link I posted earlier in this thread (and by NEC), a
few elevated wires used as a counterpoise in place of the BL&E buried
wires need to have an electrical wavelength of 1/4-lambda for best
antenna system radiation efficiency, even with "short" vertical
monopoles. And even when those elevated counterpoise wires are close
to the earth in terms of a free space wavelength, their electrical
length is not much different than their physical dimension in terms of
a free space wavelength -- as is the case when they are buried.

Theory and practice both show that such wires perform differently when
they are buried than when they are elevated above the surface of the
earth.

RF

LA4RT Jon wrote in :

....
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...les-on-ground-
system-experiments.html

Hi Jon,

All of the articles are interesting. I have previously read the last, but
wasn't aware it was only one of a series.

Looking at Article 3, Fig 1 suggests that efficiency improves very slowly
beyond about 32 radials lying on the surface, and 4 such radials are
about 5.4dB below 32 radials.

In Fig 2, he shows 4 radials just 6" (150mm) above ground as about 5dB
better than 4 radials on the ground.

My interpretation of Fig 1 and Fig 2 then is that 4 radials at 6" are
about 0.5dB behind 32 radials lying on the ground. That is the type of
effect I was referring to when I said "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" in my first post.

Rudy reports some further small improvement (1dB) in raising the radials
to 4'(1.2m).

These are very similar effect to those predicted by my NEC4 model. In the
case of my model of radials from 100mm depth to some distance above
ground, the improvement was mostly in the range of heights from 0mm to
about 20mm. Obviously, the model is sensitive to soil type, and different
soil types can be expected to yield different response... but it would
seem that just 3 radials at 1 to 3m height give similar system efficiency
to 16+ radials shallow buried for a range of common soil types.

I know my interpretation of Rudy's measurements and my NEC4 model don't
fall in line with some traditional thinking, and will not appeal to many.

I think it is time to build a prototype.

Thanks for the links.

Owen

Owen Duffy wrote in
:

....

I should have noted that the efficiency figures I spoke of from NEC models
are technically not directly comparable with Rudy's |S21|, the error in
interpeting |S21| as system gain is likely to be small for the kind of load
impedances encountered.

Owen

On Oct 1, 12:45 am, Owen Duffy wrote:
Obviously, the model is sensitive to soil type, and different
soil types can be expected to yield different response... but it would
seem that just 3 radials at 1 to 3m height give similar system efficiency
to 16+ radials shallow buried for a range of common soil types.

The two links below may be of interest in comparing NEC modeling with
empirical results.

The first link is a clip from the "benchmark" 1937 I.R.E paper of
BL&E, showing that the radiated fields measured 3/10 of a mile from
monopoles ranging from about 45 to past 90 degrees in height, and
using 113 each 0.412-lambda buried radials is within several percent
of the theoretical maximum for a perfect monopole of those heights
when driven against a zero-ohm connection to a perfect ground plane.
The BL&E tests were conducted in the sandy soil of New Jersey, where
earth conductivity was/is 4 mS/m or less. Those measured results
indicate those systems were radiating 90% or more of the applied
power, and that the conductivity of the earth in which those radials
were buried is relatively unimportant.

The second link is a NEC model of a 1/4-lambda monopole driven against
four, elevated counterpoise wires with no antenna system connection to
a perfect ground plane, showing that its peak gain is 5.15 dBi --
which is the theoretical maximum for a perfect 1/4-lambda monopole
driven against a zero-ohm connection to a perfect ground plane.

Adding an ohm or two in the connection from the source to the four
elevated radials reduces the gain/field of the NEC model such that it
is approximately what was shown in the BL&E study, indicating that a
similar value must have been present in their buried radial ground
system consisting of 113 each 0.412-lambda wires.

Using NEC-4 to incorporate buried (or elevated) radials into the model
should show groundwave fields within 1 km of the monopole that are
very close to the theoretical maximum for the applied power when
radiated along a perfect ground plane, if the model is optimal, and
accurate.

The theoretical maximum inverse distance voltage field intensity at 1
km for 1 kW of radiated power from a perfect 1/4-lambda monopole
system is about 313 mV/m.

http://i62.photobucket.com/albums/h85/rfry-100/G.gif

http://i62.photobucket.com/albums/h8...tedRadials.jpg

RF

Owen wrote:

So, my original question is no so much suggesting everyone else got it
wrong, but why don't I seem more people doing it this way. Could I be
forgive in thinking that the popular, nearly universal, way is to uplift
the BL&E research at MF and apply it to 80m?

There are lots of reasons that people don't use elevated radials versus
buried ones. But before that, keep in mind that comparing elevated to
buried is not really comparing 4 elevated to 120 buried. 120 is
overkill, and not many of us who have verticals have that many. I found
that 32 was getting into diminishing returns for me, so stopped.

Also buried radials are more forgiving of length variations. My case was
that the antenna had to be located a good bit closer to one end of the
yard than the other. So the radials on one side were from 10 to 25 feet
shorter.

Also, many of us are married, and the spouse doesn't like all that many
wires running around. My wife also mows the yard, something that would
be relegated to me if I had a lot of wires running around the yard.

And I've used and been around an elevated radial system. It was a royal
pain. You have to declare a rather large area off limits, we added
little ties to it to warn people, and it didn't really help at night.
That experience told me that elevated HF radials was not the way I
wanted to go - ever.

Even if you are way out in the middle of nowhere, an elevated radial
vertical is a liability unless you put a wire link fence around it -
check with your insurance company.

Just my opinion of course, but it seems to be shared by many.

So with the buried radials not being all that much more work,(unless you
insist on 120 of them) the greater flexibility of buried radials when
dealing with real estate limitations, the appearance and liability
issues, just makes a buried radial system a more attractive and
practical option to many of us.

- 73 de Mike N3LI -

Richard Fry wrote in
:

On Oct 1, 12:45 am, Owen Duffy wrote:
....
Using NEC-4 to incorporate buried (or elevated) radials into the model
should show groundwave fields within 1 km of the monopole that are
very close to the theoretical maximum for the applied power when
radiated along a perfect ground plane, if the model is optimal, and
accurate.

It may do, I can not comment. My interest is for an antenna for sky wave
path, and I have not explored ground wave performance.

In the cases of 32 buried radials and three elevated radials, the
patterns are similar, efficiencies are similar, and maximum gain is
similar. Reducing the number of buried radials degrades its performance
significantly.

The elevated radials configuration allows a shortened radiator with
capacity hat with negligible degradation in performance. I haven't
modelled the same thing over buried radials, but I expect performance
degradation would be significantly worse.

Owen

Richard Clark wrote:
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux
wrote:

It was written by BL&E that
120 radials work,

2, 15, 30, 60, and 113.

No discussion whatever of 120.


I stand corrected.. thanks..
So they extrapolated to 120 as a "nice round number" for the future
purposes of the FCC.

Richard Clark wrote:
On Thu, 30 Sep 2010 14:44:53 -0700, Jim Lux
wrote:

For a lot of operators, a 1
dB change in performance might not be noticeable.

That 1dB is at the periphery of a radius where surface area
(customers) mounts up by the square.


For broadcasters, sure. But the discussion is in reference to the
potential performance difference for ham use, and I would think that
there is more than 1 dB variation in the "other end" of the link. The
hard core DXer or QRPer digging the signal out of the noise will care,
but that's a small fraction of the overall ham population.