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 10/02/2010 07:12 PM, Jim Lux wrote:
On Oct 2, 4:28 am, Alejandro Lieberalejan...@Use-Author-Supplied-
Address.invalid wrote:
On 10/01/2010 06:13 PM, Jim Lux wrote:

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

A bigger effect on a phased array is the relative phasing. For a 4
element array, you can have pretty big errors in phase on transmit
without changing the forward gain much (30 degree phase error on one
element might give you a 1dB change). But a 30 degree phase error on
receive could turn a -30dB null into a -7dB one..

How come ?
Can you elaborate how can these differences happen ?


it's the difference between the effect on a peak vs effect on a null.

consider a simple 2 element array.. for sake of argument, say it's 1/4
wavelength apart and phased 90 degrees, so it has a cardioid
pattern.... a gain of 2 in one direction (where the signals from the
two antennas align), and a gain of zero in the opposite direction.
The gain is 1+cos(phi - spacing*cos(theta)) where phi is the feed
phasing, and theta is the direction.. in the preferred direction
1+cos(90 - 90*cos(0)) = 1+cos(0) = 2
in the 45 degree direction: 1+cos(90-90*cos(45)) = 1+cos(90-90*.707) =
1.895
in the 90 degree direction: 1+cos(90-90*cos(90)) = 1+cos(90) = 1
in the 180 degree direction: 1+cos(90-90*cos(180)) = 1+cos(90-90*-1) =
1+cos(180) = 0

Now spoil the feed phase (phi) by 10 degrees... (80
on boresight: 1+cos(80-90*cos(0)) = 1+cos(-10) = 1.984
on 45: 1+cos(80-90*cos(45)) = 1.959
on 90: 1+cos(80-90*cos(90)) = 1.174
at 180: 1+cos(80-90*cos(180)) = 1+cos(80+90) = 1.52E-2

The gain on boresight didn't change much... from 2 to 1.984 (0.03dB)
But the null in the back came up from zero to 1.5E-2.. (instead of -
infinity, it's now -18dB)

Change the phase error to 45 degrees...)
@theta=0: 1+cos(45-90*cos(0)) = 1.707
@theta=180: 1+cos(45-90*cos(180)) = .292

So, from the 10 degree error case, the forward gain went from 1.984 to
1.707, about 0.6dB...
but the null went from 1.52E-2 to .292 (from -17dB to -5 dB)..


The thing to remember on any gain antenna is that it takes very little
power to disrupt a null (after all, a -30dB null means that if you're
radiating 1kW in the forward direction, you're radiating 1 W in the
null.. so just another watt will double the energy in the null,
turning it from -30dB to -27dB...)

(And, you can see why making antennas with sidelobes-60dB is VERY
challenging... )


Now, change the phasing to, say, 80 degrees.. in the preferred
direction, the gain is now 1+cos(10degrees)

Thank you Jim for the explanation. Sorry I wasn't more specific.
I was refering to the difference between receiving and transmiting gain.
--
Alejandro Lieber LU1FCR
Rosario Argentina

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

Alejandro Lieber wrote:

Thank you Jim for the explanation. Sorry I wasn't more specific.
I was refering to the difference between receiving and transmiting gain.

The gain effect is the same, but for a lot of radio applications, gain
is important on transmit, but less so on receive, where good back/side
performance (e.g. low gain in undesired directions) is important.

That is, my transmitter doesn't care about a strong interfering signal
from a different direction, but my receiver sure does.

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

On Oct 6, 6:29*am, Richard Fry wrote:
*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 etc

CORRECTION: change "5/8 wavelength" to 1/4 wavelength.