Ground conductivity's effect on vertical
 

On 19 Apr 2008 21:35:28 GMT, "Ed_G"
wrote:

In my case, I am considering the use of a vertical at a new residence
built on sand. Since I am not concerned about low angle radiation
characteristics, the Half Wave may be something to consider..... giving me
a fairly efficient vertical operation with some NVIS characteristics.

Hi Ed,

Efficient? A vertical has almost no Near Vertical radiation for Near
Vertical Incidence Skywave. You can get along with "almost no," or
you can simply use a low horizontal which would exhibit "a lot of"
Near Vertical Incidence Skywave.

Good ground, bad ground, radials, no radials won't change efficiency
much for the vertical's incidence overhead (there's a hole in that
pattern).

73's
Richard Clark, KB7QHC

Ground conductivity's effect on vertical
 

Richard Clark wrote in
:

On 19 Apr 2008 21:35:28 GMT, "Ed_G"
wrote:

In my case, I am considering the use of a vertical at a new
residence
built on sand. Since I am not concerned about low angle radiation
characteristics, the Half Wave may be something to consider.....
giving me a fairly efficient vertical operation with some NVIS
characteristics.

Hi Ed,

Efficient? A vertical has almost no Near Vertical radiation for Near
Vertical Incidence Skywave. You can get along with "almost no," or
you can simply use a low horizontal which would exhibit "a lot of"
Near Vertical Incidence Skywave.

Good ground, bad ground, radials, no radials won't change efficiency
much for the vertical's incidence overhead (there's a hole in that
pattern).

73's
Richard Clark, KB7QHC


Richard,

By "efficient" I was referring to the transfer of power.... to a
presumed 50 ohm antenna input, not to any radiation characteristics !
As I understood it, a half wave vertical can give me this, with a
little effort. I also understood it to have a fairly high take off
angle.... which will certainly give me better in-state coverage than a
good low angle takeoff would..... wouldn't it?

Yes, I know a proper NVIS antenna would be far better than this....
that is why I used the term "some NVIS" characteristics.

TNX


Ed

Ground conductivity's effect on vertical
 

Ed,

It won't be suitable for NVIS, as you can see from a model.

Roy Lewallen, W7EL

Ed_G wrote:
Very nicely put, Roy. Although I "knew" this in the recesses of
memory, the refresher will stick with my memory more, now. Thanks.

In my case, I am considering the use of a vertical at a new residence
built on sand. Since I am not concerned about low angle radiation
characteristics, the Half Wave may be something to consider..... giving me
a fairly efficient vertical operation with some NVIS characteristics.



Ed K7AAT

Ground conductivity's effect on vertical
 

Ed_G wrote:

Richard,

By "efficient" I was referring to the transfer of power.... to a
presumed 50 ohm antenna input, not to any radiation characteristics !
As I understood it, a half wave vertical can give me this, with a
little effort. I also understood it to have a fairly high take off
angle.... which will certainly give me better in-state coverage than a
good low angle takeoff would..... wouldn't it?

Yes, I know a proper NVIS antenna would be far better than this....
that is why I used the term "some NVIS" characteristics.

TNX


Ed

All the radiation from an antenna isn't concentrated at some "takeoff
angle", but radiates at all angles at various amounts. That distribution
is known as the "elevation pattern" and trying to replace it with a
single "takeoff angle" value loses nearly all the information about how
and where the antenna radiates. The half wavelength vertical radiates
very little above about 60 degrees elevation angle regardless of the
ground characteristics.

Roy Lewallen, W7EL

Ground conductivity's effect on vertical
 

"Roy Lewallen"
However, you can't compensate for this factor when the ground is poor by
improving the ground system. The reason is that the reflection takes place
much farther from the antenna than nearly any ground system extends. And
low angle radiation, where the improvement is most needed, reflects the
greatest distance away.
___________

Roy, didn't the experiments of Brown, Lewis & Epstein of RCA in ~1937 show
that the h-plane field measured 3/10 mile from a vertical monopole of about
60 to 88 degrees in height, over a set of 113 buried radials each 0.41 WL,
was within several percent of the theoretical maximum for the applied power
as radiated by a perfect monopole over a perfect ground plane? And
conductivity at the NJ test site was poor -- 4 mS/m or less.

That tends to show that the fields radiated at very low elevation angles
also will be close to their theoretical values when measured at this radial
distance, even though ground conductivity at the antenna site is poor. The
relative field (E/Emax) for radiators of these heights and propagation paths
approximately equals the cosine of the elevation angle.

The greatest radiated fields always will be directed in or near the
horizontal plane when measured/calculated for such conditions. This also
will be true for any monopole from infinitesimal to 5/8 wavelength in
height, although the elevation pattern of monopoles from /4- to 5/8-WL no
longer are described by the cosine function (see
http://i62.photobucket.com/albums/h8...omparison.jpg).

Elevation patterns show maximum relative field centered at various elevation
angles above the horizon, when those fields are measured at progressively
longer radial distances from the monopole, due to the propagation loss for
the surface wave over other than a perfect, flat, infinite ground for those
ranges. Earth curvature and terrain diffraction add to those losses for
longer surface wave paths over real earth, and for very great distances the
h-plane relative fields falls to ~zero.

But that pattern shape is not the pattern shape originally radiated by the
monopole, it also includes the effects of the propagation environment at the
range where it was measured (or calculated).

If this were not true then MW broadcast stations would have essentially zero
coverage area for their groundwave signals.

RF

Ground conductivity's effect on vertical
 

On Sat, 19 Apr 2008 14:20:33 -0700, Roy Lewallen
wrote:

There are two quite separate ways which ground affects a vertical
antenna's performance.

The first is loss due to current returning to the antenna base when the
antenna is grounded, or induced in the ground under an elevated radial
system. To minimize loss, you want as much of the current to flow
through radial wires as you can. The power loss is I^2 * R. For a given
power input, I is much lower for a half wave bottom fed vertical than a
quarter wave bottom fed vertical. So the loss due to the conducted or
induced current is much less, and you can get by with a much simpler
ground system with the half wave vertical and still have low loss.

This ground loss is usually the chief determining factor of a vertical's
efficiency.

The other effect of ground is that the field from the antenna reflects
from it some distance from the antenna. The reflected field adds to the
directly radiated field to form a net field which is different at each
elevation angle. This is a major factor in determining the antenna's
elevation pattern. The conductivity and permittivity (dielectric
constant) of the ground affect the magnitude and phase of the the
reflected field, so the pattern changes with ground quality. In general,
the more conductive the ground the better the low angle radiation.
However, you can't compensate for this factor when the ground is poor by
improving the ground system. The reason is that the reflection takes
place much farther from the antenna than nearly any ground system
extends. And low angle radiation, where the improvement is most needed,
reflects the greatest distance away. The only way to improve the
situation is to move the antenna to a location where the ground is
better, which usually isn't possible or practical.

Because of the two separate effects, the overall field strength might be
better or worse as the ground conductivity improves, and it might even
be better at some elevation angles and worse at others.

Roy Lewallen, W7EL


Maybe I should change the subject line, but here goes.
First of all, i am fishing for information, not challenging anyone's
intelligence.

I understand from books I have read, that a ground mounted vertical
antenna needs many radials. IIRC, the point of diminishing returns on
adding radials falls somewhere between 64-128 radials. I imagine the
best radial-based ground I could have for 20 meters would be a solid
copper disk with about 16 feet radius, give or take. However, I
recall in the ARRL Antenna handbook, not the latest version, but one
prior to this one, there is no noticeable difference between a raised
ground plane antenna with 4 elements as opposed to 128. (From here,
or another antenna forum, I heard for the first time that it holds
true for two radials.)

I am still trying to figure out why so many radials are needed on the
ground and a few feet higher so few are needed.

Actually, more important than the why, is how high is high enough to
reduce the optimum number of radials? For example, i want to build a
20 meter vertical. I understand the best place for it is on top of a
100 foot+ tower, but somewhere in between, there has to be a place
where 4 radials above ground is noticeably better than the same 4
radials on the ground.

Another point I have heard in the forums, but not confirmed, is that a
reduced size vertical element doesn't gain much by adding radials
longer than the antenna is high.

73 for now,
N4PGW
Buck




--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

Ground conductivity's effect on vertical
 

I understand from books I have read, that a ground mounted vertical
antenna needs many radials. IIRC, the point of diminishing returns on
adding radials falls somewhere between 64-128 radials. I imagine the
best radial-based ground I could have for 20 meters would be a solid
copper disk with about 16 feet radius, give or take. However, I
recall in the ARRL Antenna handbook, not the latest version, but one
prior to this one, there is no noticeable difference between a raised
ground plane antenna with 4 elements as opposed to 128. (From here,
or another antenna forum, I heard for the first time that it holds
true for two radials.)
================================================== =
Instead of an approx 16 ft radius disc ,you might consider an area
covered with chicken wire mesh with the antenna in its centre.If that's
laid on a lawn the grass will quickly grow over it . In a few weeks time
the wire mesh will no longer be visible and the lawnmower can just run
over .
A Butternut (vertical)antenna with a mesh ground plane compared with
none gave an improvement of 1 to 2 S points on 20 ,15 and 10 m.


Frank GM0CSZ / KN6WH

Ground conductivity's effect on vertical
 

Richard Fry wrote:
"Roy Lewallen"
However, you can't compensate for this factor when the ground is poor
by improving the ground system. The reason is that the reflection
takes place much farther from the antenna than nearly any ground
system extends. And low angle radiation, where the improvement is most
needed, reflects the greatest distance away.
___________

Roy, didn't the experiments of Brown, Lewis & Epstein of RCA in ~1937
show that the h-plane field measured 3/10 mile from a vertical monopole
of about 60 to 88 degrees in height, over a set of 113 buried radials
each 0.41 WL, was within several percent of the theoretical maximum for
the applied power as radiated by a perfect monopole over a perfect
ground plane? And conductivity at the NJ test site was poor -- 4 mS/m
or less.

That tends to show that the fields radiated at very low elevation angles
also will be close to their theoretical values when measured at this
radial distance, even though ground conductivity at the antenna site is
poor. The relative field (E/Emax) for radiators of these heights and
propagation paths approximately equals the cosine of the elevation angle.

I believe we've discussed this before, so I'll be brief.

Their calculation of the field at the receiving site when the radial
system is perfect was adjusted for the effect of ground wave attenuation
caused by the imperfect ground conductivity. If the ground between the
antenna and receiving site were perfect, the field strength would have
been greater.

Also, I'm speaking of sky wave. Ground reflection isn't a factor in
determining surface wave, which is what they measured and which isn't of
interest to most amateurs.

The greatest radiated fields always will be directed in or near the
horizontal plane when measured/calculated for such conditions. This
also will be true for any monopole from infinitesimal to 5/8 wavelength
in height, although the elevation pattern of monopoles from /4- to
5/8-WL no longer are described by the cosine function (see
http://i62.photobucket.com/albums/h8...omparison.jpg).

Elevation patterns show maximum relative field centered at various
elevation angles above the horizon, when those fields are measured at
progressively longer radial distances from the monopole, due to the
propagation loss for the surface wave over other than a perfect, flat,
infinite ground for those ranges. Earth curvature and terrain
diffraction add to those losses for longer surface wave paths over real
earth, and for very great distances the h-plane relative fields falls to
~zero.

As I thought you were aware, the surface wave propagates considerably
differently than the sky wave.

But that pattern shape is not the pattern shape originally radiated by
the monopole, it also includes the effects of the propagation
environment at the range where it was measured (or calculated).

If this were not true then MW broadcast stations would have essentially
zero coverage area for their groundwave signals.

It would be a mistake to design HF antenna systems based on optimizing
surface wave propagation as AM broadcasters do, unless you desire
communication for distances not exceeding a few miles.

Roy Lewallen, W7EL

Ground conductivity's effect on vertical
 

Buck wrote:

Maybe I should change the subject line, but here goes.
First of all, i am fishing for information, not challenging anyone's
intelligence.

I understand from books I have read, that a ground mounted vertical
antenna needs many radials. IIRC, the point of diminishing returns on
adding radials falls somewhere between 64-128 radials. I imagine the
best radial-based ground I could have for 20 meters would be a solid
copper disk with about 16 feet radius, give or take. However, I
recall in the ARRL Antenna handbook, not the latest version, but one
prior to this one, there is no noticeable difference between a raised
ground plane antenna with 4 elements as opposed to 128. (From here,
or another antenna forum, I heard for the first time that it holds
true for two radials.)

When a ground system is a long distance above ground, only two radials
are needed for high efficiency and a circular pattern at zero elevation
angle. The pattern does become non-circular at higher angles, which can
be prevented by adding two more radials. A ground system which is above
but close to the ground requires fewer radials for good efficiency than
one with radials which are buried or much closer to the ground.

I am still trying to figure out why so many radials are needed on the
ground and a few feet higher so few are needed.

The ground is very lossy. When the radial system is on or in the ground,
current flowing to the radials is forced to flow through the lossy ground.

Actually, more important than the why, is how high is high enough to
reduce the optimum number of radials? For example, i want to build a
20 meter vertical. I understand the best place for it is on top of a
100 foot+ tower, but somewhere in between, there has to be a place
where 4 radials above ground is noticeably better than the same 4
radials on the ground.

That's a good question without a single good answer. It depends at least
on the ground conductivity and permittivity (down to a considerable
depth), frequency, and radial length. Modeling can give you a good idea
of the tradeoffs, although the very simple minded ground model might not
be adequate to make a very accurate comparison.

Another point I have heard in the forums, but not confirmed, is that a
reduced size vertical element doesn't gain much by adding radials
longer than the antenna is high.

I don't believe that's true. I'll gladly consider any supporting
evidence. Hearing something on forums is among the worst justification
for believing it, in my opinion.

Roy Lewallen, W7EL

Ground conductivity's effect on vertical
 

On Sun, 20 Apr 2008 14:26:26 +0100, Highland Ham
wrote:

================================================= ==
Instead of an approx 16 ft radius disc ,you might consider an area
covered with chicken wire mesh with the antenna in its centre.If that's
laid on a lawn the grass will quickly grow over it . In a few weeks time
the wire mesh will no longer be visible and the lawnmower can just run
over .

Thanks, Frank, but the example was for illustrative purposes only.
I'd hate to think of the cost of that solid sheet of copper hihi.


--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."