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:

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?

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

"Cecil Moore" wrote

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
=====================================

If they don't cancel-out each other in the near field then they don't
cancel-out each other in the far field either.

A pair of radials behave as a continuous dipole fed at its center via
a single wire. And it radiates.

A circular disk, diameter = 1/2 wavelength, fed at its centre
radiates.

But don't ask me what its radiation resistance is. It must be very
low.
----
Reg.

Reg Edwards wrote:
A pair of radials behave as a continuous dipole fed at its center via
a single wire. And it radiates.

If the radials are horizontal and radiating, why is
there virtually no horizontally polarized radiation?
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
A pair of radials behave as a continuous dipole fed at its center
via
a single wire. And it radiates.

If the radials are horizontal and radiating, why is
there virtually no horizontally polarized radiation?
--
73, Cecil
======================================
Cec,
Your use of the word "virtually" indicates a weakness in your ideas on
the subject.

The radiation, as small as it may be, is vertically polarised.
----
Reg.

Cecil Moore wrote:
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.

Not true.

There is always an angle and direction where the fields do not fully
cancel. The problem is the spatial distance is different unless exactly
broadside to the pair.

Even 4 radials has this problem, but the more radials the less of an
issue it is.

73 Tom

wrote:

Cecil Moore wrote:

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.


Not true.

There is always an angle and direction where the fields do not fully
cancel. The problem is the spatial distance is different unless exactly
broadside to the pair.

Even 4 radials has this problem, but the more radials the less of an
issue it is.

73 Tom


How much not true? I expect you're right, but you could make the
same argument that a two wire transmission line radiates because the
wires don't occupy exactly the same physical space. Reg made the
same point, but didn't provide any numbers, so, according to his
own philosophy he doesn't understand the problem.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
wrote:

Cecil Moore wrote:
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.

Not true.

How much not true?

-45 DB, i.e. negligibly not true. :-)
--
73, Cecil http://www.qsl.net/w5dxp