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

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

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 -

Mark wrote:

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.. :/

I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over 'average
ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

On Sun, 03 Oct 2010 22:31:07 +0000, Owen Duffy wrote:

I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over
'average ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

Owen,

Based upon your findings above, have you thought of increasing the height
of your model to determine at what height would be necessary to equal the
same efficiency as your 120 radial reference?

Danny, K6MHE

danny wrote in :

Owen,

Based upon your findings above, have you thought of increasing the
height of your model to determine at what height would be necessary to
equal the same efficiency as your 120 radial reference?

I don't think it is a simple as that Danny. With sufficient height, the
pattern changes significantly and so you cannot compare the antennas on
efficiency alone.

It might seem intuitive that a ground plane a very long distance (say km)
above earth would approach 100% efficiency, and we tend to assume that for
VHF ground planes many wavelengths above ground, the model does not
indicate that. At sufficient horizontal distance above flat earth, some
rays must reflect off the ground and so warm to soil to some extent, no
matter how high the antenna.

The study was more to answer the question whether elevated radials were
effective, and how high they needed to achieve similar performance. The
model suggests that just three radials at 2m height is about 1dB down on
120 buried radials, or about 90% of the efficiency or EIRP (since patterns
are almost identical).

Owen

On 10/03/2010 07:31 PM, Owen Duffy wrote:

I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over 'average
ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png . The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

Thanks Owen for the Clip053a.png
The results are just amazing.

But I just cann't understand why the current in the radials, don't
induce a significant current in the earth, that ends as an important
lost power.
--
Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:
http://1fcr.com.ar

On Oct 3, 11:32*pm, Alejandro Lieber alejan...@Use-Author-Supplied-
Address.invalid wrote:
On 10/03/2010 07:31 PM, Owen Duffy wrote:





I have explored what you have said in an NEC4 model of a quarter wave
monopole with three quarter wave radials at varying heights over 'average
ground'. The results are summarised at
http://www.vk1od.net/lost/Clip053a.png. The reference for the graph is
the efficiency of the same antenna with 120 buried radials in the same
soil type.

If the models are correct, laying just a few radials on or very close to
the ground (eg the popular method of pinned into the turf) would appear
to be a very poor option. The model indicates efficiency improves with a
very small increase in height above the dirt, just 30mm is a 6dB
improvement of lying on the dirt, just half a metre achieves 90% of the
available efficiency.

Owen

Thanks Owen for the Clip053a.png
The results are just amazing.

But I just cann't understand why the current in the radials, don't
induce a significant current in the earth, that ends as an important
lost power.
--
Alejandro Lieber *LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map foF2:http://1fcr.com.ar

because the currents in the radials cancel out below them which
reduces the current in the ground. as long as there are enough of
them and relatively symmetric. look at a radial field from below, lay
on the ground and look up at the radials all branching out from the
center... now visualize the current in each one, note that they are
all in phase, the current flows out from the middle to the ends all at
the same time. now if you start adding up the vector representations
of the fields from each little piece you will see that ideally they
all cancel out. in the middle that is likely pretty good, but as you
move out from the middle you get farther away from the radials on one
side than the other so cancellation isn't as good, but the current is
also lower so not as much gets coupled to the ground.

Alejandro Lieber wrote
in :

....

Thanks Owen for the Clip053a.png
The results are just amazing.

But I just cann't understand why the current in the radials, don't
induce a significant current in the earth, that ends as an important
lost power.

The model results do not support the common Rules of Thumb (RoT) or
explanations that I have often seen on the subject of elevated radials.

The model suggests that as the 3 radial system is lowered, efficiency
(meaning total power radiated in the hemisphere divided by power input)
changes little until the radials are 100mm above ground, and the
efficiency drops quickly, more quickly as the height approaches zero.

I interpret that to mean that there is little elecric flux in the soil
due to antenna currents for this configuration until the radials are very
close to the soil, and then the loss effects grow very rapidly. I assume
that the losses are mainly due to dielectric effects in normal soils, but
that might be different if you had magnetic material in the soil.

Is this surprising? I think that if you wanted to make a device to warm
soil by dielectric heating using RF, you wouldn't expect it to be very
effective if the conductors weren't very close to the soil.

Using more radials reduces the losses.

If you can visualise the electric field flux in the case of the monopole
over three radials, and compare it to a horizontal dipole, you may get an
insight to why a low dipole is more effective at heating the soil.

Owen

The model suggests that as the 3 radial system is lowered, efficiency
(meaning total power radiated in the hemisphere divided by power input)
changes little until the radials are 100mm above ground, and the
efficiency drops quickly, more quickly as the height approaches zero.

All monopoles need an electrical reference point to be "driven
against."

Using a symmetrical arrangement of two or more 1/4-lambda-resonant,
horizontal wires elevated sufficiently above the earth provides that
by acting at their junction under the base of the monopole as a point
with constant electrical characteristics with respect to the current
flowing in the antenna system.

NEC shows the peak free-space gain of such a system using a 1/4-wave
monopole to be the same as that of a 1/2-wave dipole in free space,
e.g., 2.15 dBi. When that system is operating within a few electrical
degrees above a perfect ground plane then the peak gain rises to 5.15
dBi, because all of the radiation is re-directed/confined to one
hemisphere.

Horizontal wires lying on, or buried several inches below the surface
of the earth do not have the same electrical characteristics or
function as when they are elevated. Instead, they serve to collect
the r-f currents generated by the displacement field radiation of the
monopole -- which currents flow in the earth out to about 1/2
wavelength from the base of the monopole.

If the earth was a perfect conductor then those currents could travel
through the earth without loss, and a single, short ground rod would
serve as an electrical reference point for the r-f current flowing in
the antenna system. The sum of those r-f currents flowing in the
earth around the monopole, and collected by that ground rod will be
equal to the base current in the 1/4-wave, series-fed monopole. The
gain of this configuration is 5.15 dBi, the same as when using a few
elevated, resonant wires as a counterpoise.

But the earth is not a perfect conductor. For that reason it is
necessary, when using buried radials, to install enough of them in the
surface area out to about 1/2 wavelength to collect those r-f currents
before they have traveled through much of the lossy earth to reach
those wires.

The benchmark 1937 I.R.E. paper of RCA's Brown, Lewis and Epstein
showed that 113 x 0.412-lambda buried radial wires used with monopoles
of about 45 to 90 degrees in height produced a radiated groundwave
field when measured at 3/10 of a mile that was within several percent
of the theoretical maximum for a perfect monopole radiator with a zero-
ohm connection to a perfect ground plane -- and this despite the fact
that earth conductivity at/near their test site was not better than 4
mS/m.

As an aside: NEC analyses for far-field conditions show an elevation
gain of zero in the horizontal plane for a monopole over real earth,
and peak relative field gain at some elevation angle above the
horizontal plane. However the radiated, relative fields that exist
at, and relatively close to the edge of the near-field boundary of the
radiation hemisphere of all monopoles of 5/8 wavelength and less are
very nearly equal to the cosine of the elevation angle -- which value
is 1.0 at zero degrees (the horizon), and zero at the zenith. If they
were not, then the fields measured by BL&E would be much different
than they recorded in their 1937 paper.

It is only after those fields propagate a significant distance over a
real earth path that they depart significantly, and progressively more
significantly with distance, from the relative fields described by the
cosine value of the elevation angle.

RF