Elevated vs buried radials
 

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

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.

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?

Also, what about the loss in any matching components needed (e.g. if you
had a real efficient narrow band antenna, then operating off nominal
means you'll need a different matching network, and the loss in it might
be worth considering)

Elevated vs buried radials
 

On Thu, 30 Sep 2010 03:46:40 -0700 (PDT), Richard Fry
wrote:

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

Hi Richard,

Solid piece of information - thanx.

Odd to notice none have acknowledged field data.

73's
Richard Clark, KB7QHC

Elevated vs buried radials
 

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.


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.


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.

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.



Also, what about the loss in any matching components needed (e.g. if you
had a real efficient narrow band antenna, then operating off nominal
means you'll need a different matching network, and the loss in it might
be worth considering)

Matching network loss was not included in my analysis because both
quarter wave options present fairly similar load impedances that need
transformation to 50 ohms. The shortened verticle is slightly lower R
(23 vs 38 IIRC), and slightly more loss can be expected, but it is
practical to match with a shunt coil of copper tube and matching loss
should be real low in the system context, and in comparison of elevated
vs buried radials.

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.

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?

Owen

Elevated vs buried radials
 

On Sep 30, 3:13*pm, Owen wrote:

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

The BL&E experiments were conducted using 3 MHz signals, so their
applicability to the 80m band is not a large uplift.

Elevated vs buried radials
 

On 01/10/10 06:39, Richard Fry wrote:
On Sep 30, 3:13 pm, wrote:

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

The BL&E experiments were conducted using 3 MHz signals, so their
applicability to the 80m band is not a large uplift.

I stand corrected. I have read the paper many times, and my recollection
was that it was below 2MHz. Must be time to read it again!

Owen

Elevated vs buried radials
 

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.




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.


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.





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.






Owen

Elevated vs buried radials
 

Referring to my earlier post in this thread with a link to the
measured field intensity data of a MW antenna system using elevated,
1/4-wave radials taken by a consulting engineering firm ...

On Sep 30, 1:53 pm, Richard Clark wrote:

Odd to notice none have acknowledged field data.

A non sequitur, possibly?

Elevated vs buried radials
 

On Fri, 01 Oct 2010 06:13:56 +1000, Owen 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

Elevated vs buried radials
 

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.

73's
Richard Clark, KB7QHC

Elevated vs buried radials
 

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.

73's
Richard Clark, KB7QHC