John Popelish wrote:

Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. . .

It appears that what I've been writing the past few days either isn't
being read or isn't being believed. Among it is an explanation of why a
"ground plane" doesn't produce an "image" of the vertical.

Since you appear to continue to believe this, please explain the
mechanism by which you think a half wave disk produces an "image" of the
vertical.

Roy Lewallen, W7EL

John - KD5YI wrote:

Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from
4 or more radials. EZNEC shows very little change in terminal impedance
and pattern by removing two radials from a 4 radial ground plane.

I once used copper tape on a window to make a ground plane vertical like
that for 70cm. It worked very well.

George Brown, the inventor of the ground plane antenna, found that only
two radials were necessary. But when his company went to sell it, the
marketing department decided that no one would buy a two-radial ground
plane antenna in the belief that it would be omnidirectional. So they
added two more to make it "look" more omnidirectional. The four-radial
ground plane persists to this day.

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper tape
on a Duroid dielectric material, a lot like the window antenna John
described. An omnidirectional pattern was a requirement, and I was
concerned that either the flatness of the tape or the presence of the
dielectric might have some impact on the circularity of the pattern. So
I had it tested at a local lab. It was the most circular pattern they'd
ever seen, having about 1 dB maximum difference between any two directions.

Roy Lewallen, W7EL

Roy Lewallen wrote:
John Popelish wrote:


Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. . .


It appears that what I've been writing the past few days either isn't
being read or isn't being believed. Among it is an explanation of why a
"ground plane" doesn't produce an "image" of the vertical.

Since you appear to continue to believe this, please explain the
mechanism by which you think a half wave disk produces an "image" of the
vertical.

The disk forms an image by allowing the electric field lines to
terminate perpendicular to the "mirror" surface on exactly the same
lines as if they were heading toward a lower half of a dipole, while
the radial currents in the "mirror" allow the magnetic field lines to
encircle the monopole in the same pattern they would form if the
missing half of the dipole were in position.

This same pattern of electric and magnetic fields above the "mirror"
produces (half of the) photons that the full dipole would have
produced. A half wave diameter disk is about the minimum size
"mirror" that will keep the field patterns close enough to those of
the dipole to launch those photons. A larger disk would do better,
but not a lot better.

Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper tape
on a Duroid dielectric material, a lot like the window antenna John
described. An omnidirectional pattern was a requirement, and I was
concerned that either the flatness of the tape or the presence of the
dielectric might have some impact on the circularity of the pattern. So
I had it tested at a local lab. It was the most circular pattern they'd
ever seen, having about 1 dB maximum difference between any two directions.

Did this antenna include any provision to prevent current on the
outside of the feed line?

Which direction did the feed line exit the antenna?

On Mon, 10 Jul 2006 14:27:05 -0400, John Popelish
wrote:

A larger disk would do better, but not a lot better.

Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.

the "mirror" produces (half of the) photons that the
full dipole would have produced.

Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

73's
Richard Clark, KB7QHC

John Popelish wrote:
Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper
tape on a Duroid dielectric material, a lot like the window antenna
John described. An omnidirectional pattern was a requirement, and I
was concerned that either the flatness of the tape or the presence of
the dielectric might have some impact on the circularity of the
pattern. So I had it tested at a local lab. It was the most circular
pattern they'd ever seen, having about 1 dB maximum difference between
any two directions.


Did this antenna include any provision to prevent current on the outside
of the feed line?

Which direction did the feed line exit the antenna?


I don't know about Roy's antenna, but this subject has come up before,
and at the time I made a two meter vertical ground plane with only two
radials. No matter how I oriented the antenna, radially, I got the same
signal strength on my field-strength meter. And yes, I took precautions
to make sure the feedline wasn't radiating. (Many ferrite beads at
strategic places on the feedline to the point that feedline radiation
was undetectable.) If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.

It's pretty easy to understand. Any two radials,
180 degrees apart and high enough, should theoretically
cancel each other's radiation in the far field.
--
73, Cecil http://www.qsl.net/w5dxp

Richard Clark wrote:
On Mon, 10 Jul 2006 14:27:05 -0400, John Popelish
wrote:


A larger disk would do better, but not a lot better.


Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.


the "mirror" produces (half of the) photons that the
full dipole would have produced.


Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

Do you deny the photonic nature of radio waves?

I just realized that the sentence you quoted s easily misinterpreted.
When I said "the "mirror" produces (half of the) photons that the
full dipole would have produced." I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line. I didn't mean that the mirror
produces half of the total photons that are radiated.

George Brown was over precautious. Only one vertical radial is
needed. There is no loss in efficiency. The radiation pattern remains
sensibly the same.
----
Reg.

Tom Donaly wrote:
John Popelish wrote:

Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper
tape on a Duroid dielectric material, a lot like the window antenna
John described. An omnidirectional pattern was a requirement, and I
was concerned that either the flatness of the tape or the presence of
the dielectric might have some impact on the circularity of the
pattern. So I had it tested at a local lab. It was the most circular
pattern they'd ever seen, having about 1 dB maximum difference
between any two directions.

Did this antenna include any provision to prevent current on the
outside of the feed line?

Which direction did the feed line exit the antenna?

I don't know about Roy's antenna, but this subject has come up before,
and at the time I made a two meter vertical ground plane with only two
radials. No matter how I oriented the antenna, radially, I got the same
signal strength on my field-strength meter. And yes, I took precautions
to make sure the feedline wasn't radiating. (Many ferrite beads at
strategic places on the feedline to the point that feedline radiation
was undetectable.) If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.

I agree. The point is, that I wonder if Roy's antenna feed had this
precaution that reduces the radiation effect of the feed line, or if
feed line radiation was part of the antenna.