The link below leads to a NEC-2 study of a short vertical monopole
for two mounting/operating conditions stated in the study.

It shows about a 13 dB advantage in the surface wave launched by
the roof-mounted configuration in this comparison.

Similar affects should apply to verticals used in the ham bands,
so this result might be useful to some readers here.

http://www.keepmyfile.com/download/3ef1691784487

RF

Richard Fry wrote:
The link below leads to a NEC-2 study of a short vertical monopole
for two mounting/operating conditions stated in the study.

It shows about a 13 dB advantage in the surface wave launched by
the roof-mounted configuration in this comparison.

Similar affects should apply to verticals used in the ham bands,
so this result might be useful to some readers here.

http://www.keepmyfile.com/download/3ef1691784487

RF


It would be more accurate to bill this
as "Mediocre radial system vs 20 m x 30
m solid aluminum ground screen ".

I suspect much the same results would
obtain for the roof mount, even if the
roof were at ground level.

73,

Chuck





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"Chuck"
It would be more accurate to bill this as "Mediocre radial system vs 20 m
x 30 m solid aluminum ground screen ".

I suspect much the same results would obtain for the roof mount, even if
the roof were at ground level.
_____________

When the comparison is made between the roof-mounted version as shown in my
PDF and an earth-mounted vertical using a 2-ohm "broadcast quality" r-f
ground system of 120 buried radials each about 1/4-wave long, then the
advantage of the roof-mount version drops to about 5 dB.

N.B. these radiators are electrically very short. Using taller radiators
would reduce the performance difference.

RF

Richard Fry wrote:
"Chuck"
It would be more accurate to bill this as "Mediocre radial system vs
20 m x 30 m solid aluminum ground screen ".

I suspect much the same results would obtain for the roof mount, even
if the roof were at ground level.
_____________

When the comparison is made between the roof-mounted version as shown in
my PDF and an earth-mounted vertical using a 2-ohm "broadcast quality"
r-f ground system of 120 buried radials each about 1/4-wave long, then
the advantage of the roof-mount version drops to about 5 dB.

N.B. these radiators are electrically very short. Using taller
radiators would reduce the performance difference.

RF

Richard Fry wrote:
"Chuck"
It would be more accurate to bill this as "Mediocre radial system vs
20 m x 30 m solid aluminum ground screen ".

I suspect much the same results would obtain for the roof mount, even
if the roof were at ground level.
_____________

When the comparison is made between the roof-mounted version as shown in
my PDF and an earth-mounted vertical using a 2-ohm "broadcast quality"
r-f ground system of 120 buried radials each about 1/4-wave long, then
the advantage of the roof-mount version drops to about 5 dB.


Sounds reasonable. My point was that the
entire 13 dB difference could probably
be explained in terms of the difference
in ground resistance ( 15 ohms vs 2 ohms
vs 1 ohm? for the aluminum roof) and
the extremely low radiation resistance.

N.B. these radiators are electrically very short. Using taller
radiators would reduce the performance difference.

RF

Chuck

Richard Fry wrote:
The link below leads to a NEC-2 study of a short vertical monopole
for two mounting/operating conditions stated in the study.

It shows about a 13 dB advantage in the surface wave launched by
the roof-mounted configuration in this comparison.

Similar affects should apply to verticals used in the ham bands,
so this result might be useful to some readers here.

http://www.keepmyfile.com/download/3ef1691784487

I don't think too many hams are using 10 foot high verticals on 160
meters -- not for transmitting, anyway.

Roy Lewallen, W7EL

On Aug 5, 12:06 pm, "Richard Fry" wrote:
"Chuck" It would be more accurate to bill this as "Mediocre radial system vs 20 m
x 30 m solid aluminum ground screen ".

I suspect much the same results would obtain for the roof mount, even if
the roof were at ground level.

_____________

When the comparison is made between the roof-mounted version as shown in my
PDF and an earth-mounted vertical using a 2-ohm "broadcast quality" r-f
ground system of 120 buried radials each about 1/4-wave long, then the
advantage of the roof-mount version drops to about 5 dB.

N.B. these radiators are electrically very short. Using taller radiators
would reduce the performance difference.

RF

I think that comparison would be flawed though. That is not an optimum
"elevated" radial system, and I can see it losing to a 120 radial
ground system. For one thing, the radial system is not resonant.
This is quite important for an elevated system. Fairly critical
really.
Also, the manner of the wires being laid out like chicken wire mesh
is not an optimum use of wire. You would have better results using
the usual "spoke radial" system, due to more wire being concentrated
at the feedpoint, vs farther out.
If you compared a ground mount with 120 radials vs a roof mount
with 120 radials, the roof mount would smoke the ground mount
handily. For sure, as far as the local "ground/space" wave.
Also, if this antenna was for MW, and roof mounted, which is
very low height in wavelength, you will need a lot of radials to
equal the 120 on the ground. But if you had the antenna at 1/4
wave up, about 10 radials would equal the 120 on the ground.
I often see and hear about MW comparisons using ground mount
vs elevated, and almost to a tee, most ruin the comparison by
applying substandard radial systems on the elevated antenna.
Either that, or they ignore the number of elevated radials needed
to equal the 120 on the ground. That of course varies with height
in wavelength.
At 1/2 wave, 3-4 radials will equal 120 on the ground.
At 1/4 wave, about 8-12 will be needed.
At 1/8 wave, about 60 or so at least..
And it just gets worse from there as you get lower and
lower. Less than 1/8 wave, and you will need a load of radials
to equal the 120 on the ground. Probably 80-100... ??
Anyway, I've done quite a bit of comparing elevated vs ground
mount on 40m, and there is absolutely no doubt in my mind
that elevated is the best way to go. Totally smokes a ground
mount, as long as the number of needed radials per wavelength
is not ignored.
Even if you equal the ground losses between the two, the
elevated still wins due to more clearance of ground clutter,
and a much better local ground/space wave signal, which
I assume is due to the radiator being high and in the clear.
More line of sight so to speak.. This also greatly improves
the lower angle DX coverage, being both use the same fairly
low angles of radiation.
MK

wrote
I think that comparison would be flawed though. That is not an optimum
"elevated" radial system, and I can see it losing to a 120 radial
ground system. For one thing, the radial system is not resonant.
This is quite important for an elevated system. Fairly critical
really.
Also, the manner of the wires being laid out like chicken wire mesh
is not an optimum use of wire. You would have better results using
the usual "spoke radial" system, due to more wire being concentrated
at the feedpoint, vs farther out.

However the study was not intended to model an ideal elevated radial system,
but one using an existing metal roof to serve as the "other conductor" of an
electically short, vertical dipole.

The mesh wires used in the study to simulate the metal roof have a density
of well below 1/10 wavelength, and for these conditions the roof would
appear essentially solid to r-f.

I often see and hear about MW comparisons using ground mount
vs elevated, and almost to a tee, most ruin the comparison by
applying substandard radial systems on the elevated antenna.
Either that, or they ignore the number of elevated radials needed
to equal the 120 on the ground. That of course varies with height
in wavelength.
At 1/2 wave, 3-4 radials will equal 120 on the ground.
At 1/4 wave, about 8-12 will be needed.
At 1/8 wave, about 60 or so at least..

And it just gets worse from there as you get lower and
lower. Less than 1/8 wave, and you will need a load of radials
to equal the 120 on the ground. Probably 80-100... ??

A NEC-2 model of a 1/4-wave vertical monopole (base at earth level) in the
AM broadcast band using just four, 1/4-wave radials elevated 12-15 feet
above a perfect earth shows a peak h-plane gain within tenths of a decibel
of the theoretical peak value for a 1/4-wave monopole over a perfect ground
plane, ie, at least 5 dBi.

Such systems have been installed by commercial AM broadcast stations where
the earth at the antenna site makes it impractical to use the standard 120
buried radials. These elevated radial systems permit those stations to
produce at least the minimum groundwave field at 1 km for 1 kW of applied
power that is required by the FCC for AM broadcast stations -- and with far
fewer than "80-100...??" radials elevated less than 1/8-wave, fortunately.

Here is a link to a paper about this
http://www.nottltd.com/ElevatedRadialSystem.pdf

Even if you equal the ground losses between the two, the
elevated still wins due to more clearance of ground clutter,
and a much better local ground/space wave signal, which
I assume is due to the radiator being high and in the clear.
More line of sight so to speak.. This also greatly improves
the lower angle DX coverage, being both use the same fairly
low angles of radiation.

My models were done over a perfect ground plane in order to show the field
of the monopole radiator as it is launched. This is the method used by the
FCC in AM broadcast practice.

Terrain, obstructions, r-f ground loss, nearby parasitic radiators etc --
and in the case of ham antennas, their height above the earth -- will have
an affect on system performance. But all of that needs to be evaluated
separately for the installation conditions, based first on a knowledge of
the field pattern launched by the radiator itself.

RF http://rfry.org

On Aug 6, 8:06 am, "Richard Fry" wrote:


A NEC-2 model of a 1/4-wave vertical monopole (base at earth level) in the
AM broadcast band using just four, 1/4-wave radials elevated 12-15 feet
above a perfect earth shows a peak h-plane gain within tenths of a decibel
of the theoretical peak value for a 1/4-wave monopole over a perfect ground
plane, ie, at least 5 dBi.


Dunno. I'll have to ponder that a while.. But something doesn't seem
right to
me.. My red flag is going off.. By "perfect ground plane", I assume
you mean
120 radials? I have a hard time seeing 4 low elevated radials within 1
db
of 120 buried radials over real ground. I realize the ratio should be
equal
if converted to "perfect ground" but still, this just doesn't seem
right to me.
I know I've never seen results like that here on real earth. I've
tried four
slightly elevated radials quite a few times on 160m, and I've never
had the
illusion of performance nearly equaling 120 on the ground.
In fact, I've heard of a few others that complained how lousy that
type
of system worked overall, and went to many more buried radials with
better results.
Lets say the two were over poor earth. I would think the 120 radial
system would still be pretty low loss, but the 4 radial system quite
stunted
in comparison. I have a hard time seeing them within a few tenths of a
db
of each other. I dunno if I trust that particular modeling... :/
MK

wrote
Dunno. I'll have to ponder that a while.. But something doesn't seem
right to me.. My red flag is going off.. By "perfect ground plane",
I assume you mean 120 radials?

No, this assumes that the earth is an infinite, flat, perfect conductor --
in which case no radials are needed for a monopole radiator to reach its
theoretical performance. But careful measurements done by Brown, Lewis and
Epstein in 1937 in New Jersy (conductivity 4 mS/m or less) show that a
1/4-wave monopole using113 buried radials about 0.41 wavelengths each will
produce a groundwave field that is within a few percent of that when using a
perfect ground.

A 1/4-wave monopole using such a perfect ground has a peak h-plane gain of
5.15 dBi, and will generate a groundwave field of about 314 mV/m at 1 km
with 1 kW of applied power.

As a point of calibration, the FCC requires Class B regional AM stations to
generate a groundwave r.m.s. field of at least 282 mV/m for 1 kW at 1 km.
Using 120 each 1/4-wave buried radials with a 1/4-wave (or somewhat shorter)
monopole can do that for almost all real ground conditions.

Class A (50 kW full time) AM stations need to generate at least 362 mV/m
r.m.s. for 1 kW at 1 km -- which means they must use a radiator longer than
1/4-wave. Most of them use 195-degree radiators.

I have a hard time seeing 4 low elevated radials within 1 db
of 120 buried radials over real ground.

Understandable, but when properly done this is the case, as described
in the paper I linked to earlier. Buried radials behave much differently
than elevated ones. NEC models also show this.

RF