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

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

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

On Oct 1, 3:02*pm, Owen Duffy wrote:
Richard Fry 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.

Just to point out that for vertical monopole heights of 5/8-lambda and
less, the peak elevation plane relative field (E / E max) _always_
occurs in the horizontal plane, regardless of the r-f losses in the
buried radial system or counterpoise wires they are driven against,
and the conductivity of the earth in which those radial wires are
buried, or above which they are elevated.

IOW, the relative field actually "launched" at all angles above the
horizontal plane from such antenna systems _always_ is LESS than that
in the horizontal plane. The reason for this is related to the r-f
current distribution, and its relative phase along the lengths of
those monopoles.

NEC analyses showing low to zero relative field in the horizontal
plane being launched by a monopole of 5/8-lambda height and less and
regardless of the r-f ground they are driven against need to be
understood in due context. The link next below leads to further
development of this ...

http://i62.photobucket.com/albums/h8...at_Compare.gif

The longest, great-circle, single-hop, skywave paths are related to
the relative fields launched by a monopole system at elevation angles
of less than ten degrees (see Figure 55 in the link below) -- where a
NEC analysis may show very low relative field.

But if such low relative fields really were true for the fields
actually launched by such monopoles, then the nighttime skywave
coverage of MW AM broadcast stations would be much different than is
shown by real-world experience (and applicable theory).

http://i62.photobucket.com/albums/h8...Comparison.gif

RF

Owen Duffy wrote:
LA4RT Jon wrote in :

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 you can take the fact that Rudy's measurements match the model
pretty well as experimental validation of the model. indeed, NEC4 works.

Traditional thinking (or more accurately, mindless repetition of
tradition) could well be wrong, eh?


I think it is time to build a prototype.

Thanks for the links.

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.

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)

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