"Roy Lewallen" wrote in message
news:z8WdnThgM7y9_5fVnZ2dnUVZ_gWdnZ2d@easystreeton line...
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.

Here we are again forgetting that we are dealing with standing wave circuit
and cos/sin current distribution along the elements. Half wave vertical
might have low current at the base but quarter wave away it will be max
(assuming half wave elevated electrical radial). The radiation pattern is
formed between the radiator and radials (and how they are affected by ground
under).
Radials close to ground couple to it and depending on ground RF quality we
are dealing with decent reflecting mirror or "RF eating sponge".


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.


Dense radial field with electrical length of radials around wavelength has
shown remarkable imrpovement in low angle performance over "regular"ground.

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


Yuri, K3BU.us

Yuri Blanarovich wrote:
"Cecil Moore" wrote in message
...
Al Lorona wrote:
It's funny to think that really terrible ground can have an advantage
over pretty good ground.
Free space is just about the most terrible "ground"
that one can imagine. :-)
--
73, Cecil http://www.w5dxp.com

So much disinformation by W8JI School of DC circuitry :-)

Modeling various configurations shows benefits of good ground, especially
for taller than 1/4 wave radiators.
Myth that half wave radiators do not need ground is spreading like snake
oil wild fire. They need it but "looking" for it further out, not just at
the base.
I will anytime trade good ground (mirror) for lossy (RF sponge) ground.
Its just where the radiator is "looking" for the mirror, taller one -
further out, enhancing signals at lower angles.
3/8 vertical with some 3/8 physical length radials start morphing into
far field.

Yuri, K3BU.us

Previously, about BL&E's 1937 measurements:

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.

Anybody: Just wondering -- how does this conclusion flow from the findings
published in the 1937 I.R.E.paper of BL&E?

The theoretical (not measured) BL&E groundwave field at 1 mile for 1 kW
radiated from a perfect monopole over a perfect ground plane as shown in the
BL&E I.R.E. paper is not the equivalent/adjusted field they measured from
the monopole heights they tested. But, as BL&E published, the groundwave
fields they measured from these real monopoles over real earth was within
several percent of that theoretical maximum, when working against 113 buried
radials each of 0.41WL -- even for the poor conductivity at/near their
antenna site.

Also, I'm speaking of sky wave. Ground reflection isn't a factor in
determining surface wave, ...

But neither theory nor practice supports this, does it? If so, then the
groundwave fields that BL&E measured at 3/10 of a mile would have been at
least 29.3% less than that theoretical maximum field, which included a
perfect (3 dB) ground reflection -- not just the several percent they
measured. And this measured performance just beyond the near field radius
has been re-proven in thousands of groundwave r.m.s. field strength
measurements of AM broadcast stations over many decades since the BL&E work.

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.

Just to note that since the 1930s (at least), AM broadcasters have been
aware of the effects of the differing propagation characteristics of
groundwaves and skywaves. This is evident in the fact that most 50 kW,
fulltime, AM broadcast stations in the US use a radiator height that
minimizes the self-interference of their skywave with their groundwave, so
as to ~maximize their interference-free coverage areas when skywave
propagation occurs. The great majority of these stations use a monopole
radiator height of about 195 degrees.

RF

On Apr 20, 8:05 am, 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.)

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.

If you used 120 radials on the ground as optimum, even raising only
1/8 wave off the ground will enable one to reduce the number of
radials
to equal the same performance.
But... You will still probably need at least 60 radials at 1/8 wave up
to equal the 120 on the ground. So even at that low elevated height,
4 radials is better than 4 on the ground. But... Still not very
good.. :/
At 1/4 wave in height, you will need about 8-10 radials to equal the
120 on the ground. Only when you approach 1/2 wave in height can
you use 2-4 radials and have the same appx ground losses as the
120 on the ground.
You *must* think in terms of wavelength off the ground, not just feet
in general. A 160m vertical will need to be about 250 ft off the
ground
to be able to use 2-4 radials with optimum results.
A 10m vertical can be 16 ft off the ground with 2-4 radials and have
the same performance.
If you had the 20m vertical at 16 ft, "1/4 wave", and used 4 radials,
it would be equal to a ground mount using about 60 radials.
Pretty decent antenna.

In the case of your 20m vertical, it will need to be 32 ft high to
be able to use 2-4 radials and appx equal 120 on the ground.
But a 20m vertical at 8 ft off the ground with 4 radials will
be better than the same vertical on the ground with 4 radials.
But if you want that 8 ft high vertical to equal 120 on the ground,
you will need about 60 or so, being it is only 1/8 wave up at that
frequency. This would give pretty decent performance.
Much better than the 4 radials at 1/8 wave up where 4 radials
is equal to about 8-10 on the ground. Neither one of those
is going to set the woods on fire..

Obviously, an elevated multi band vertical with radials for
each band will have varying degrees of ground loss depending
on the band in use at the time.
If you had a multi band 1/4 wave vertical "GP" at 32 ft, and had 4
radials for each band, you would have much less ground loss on
10m, than on 80m.
On 10m, it's 1 wavelength, and just 1 radial will be enough to
make an efficient antenna, except you have a dipole.
If you use two radials 180 degrees apart, that should actually
be a tad lower ground loss than 120 radials on the ground
being it's at 2 wavelengths up.
On 20m, it's at 1/2 wave up, and still very low loss.
At this point the 4 radials should be very close to
the 120 on the ground mount.
On 40m, it's at 1/4 wave up, and the 4 radials will
be equal to about 50-60 on the ground.
The antenna will still work quite well.
On 80m, it's at 1/8 wave up, and the 4 radials
would be about equal to about 8-10 on the ground.
A good bit of loss on that band. You will be able to
operate, but not with the gusto of the higher bands.

Maybe this will give you an idea of the appx level of loss
for a given number of radials at certain heights, vs the 120
on the ground. The most important thing to remember is
to think in terms of wavelength off the ground for the band
to be used.


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.

Nope, I don't really agree. In fact, I think using the shorter
radiator makes the use of the lower ground loss radial set
even more important and worthwhile if you are trying to
approach full size performance.
You would see a difference I'm fairly sure.
BTW, a lot of the info I just wrote a novel about came
from the Bill Orr handbooks. He has sections on the subject,
and also graphs that match the levels of loss I mentioned
at the various heights.
In testing various verticals, including a full size 40m ground
plane which I could vary the height, I've never seen anything
to show his data is incorrect. It's from one of those blasted
books Art has problems with, but I happen to trust it as fairly
accurate.
All this pertains to the usual 1/4 wave elevated ground planes
vs a ground mount.

On Apr 20, 10:32 pm, wrote:

All this pertains to the usual 1/4 wave elevated ground planes
vs a ground mount.

BTW, using a larger number of radials will not
improve the ground conditions of the far field,
but being that improving the efficiency of a given
height vertical improves the gain equally in all directions,
you should see an improvement for all types of propagation.
The ground wave, space wave, and sky wave will all
improve by increasing antenna efficiency.

On Apr 20, 10:32 pm, wrote:

If you use two radials 180 degrees apart, that should actually
be a tad lower ground loss than 120 radials on the ground
being it's at 2 wavelengths up.

Make that one wave up for 10m.. :/ But the rest should still apply.
Two should be slightly lower loss than the 120 on the ground,
being 20m is the frequency where they should be about equal.
I was rereading all that to see if I molested any numbers..
I knew I'd find one..

Richard Fry wrote:
. . .

I've discussed the difference between sky and ground wave, and Brown,
Lewis, and Epstein's measurements a number of times on this newsgroup in
response to pretty much the same questions by Richard, so there's no
need to do it again. Anyone interested in my comments can do a search of
my postings which include "ground wave" or "surface wave".

Roy Lewallen, W7EL

On Sun, 20 Apr 2008 14:07:59 -0700, Roy Lewallen
wrote:

Buck wrote:

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

Long ago, I discovered that anyone can become an "expert" on the
internet by making a statement and having someone else agree with them
or back them up. This is, of course, why I included the statement
"but not confirmed."

Very recently I heard a conversation on the air between a ham who was
"elmering" a new ham and another ham. The "elmer" was trying to help
the new ham convert a "vertical dipole" so it would operate on HF. It
was rather confusing for a bit until he better described it. It
turned out to be some kind of VHF ground plane antenna.

I understand ignorance.... but this worries me.


--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

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