The image "theory" is a simplification of a problem involving an antenna
over an infinite, perfect, ground plane.

Four radials behave nothing like an infinite, perfect ground plane, so
the premises upon which the image model is based do not apply. Trying to
extend it to such systems as four radials will inevitably lead to
seriously mistaken conclusions. The claim about stacked Yagis is just
one example.

Roy Lewallen, W7EL

David wrote:
Image theory is used to justify the idea that a ground plane reflects the
radio wave emitted by a vertical monopole antenna. The radio wave emitted by
the vertical induces currents in the metal ground plane. A charge on the
vertical induces an opposite charge in the ground plane. If charge on end of
vertical is +q, then it induces a charge of -q in the ground plane. The
effect is that the charge -q appears to be a distance below the ground, the
distance being equal to the distance between ground and charge +q above
ground. A distibution of charge is induced in the ground plane by the
radiating vertical.

With a dipole the electric field lines go from +q to -q. If a ground plane
is inserted at the zero or middle point, with charge below ground plane
removed, the electric field lines above the ground plane stay the same. The
metal ground plane is a pool of electrons that adjust so that the voltage at
the surface is zero i.e. tangent E = 0.

Four radials form a counterpoise. The counterpoise has currents induced.
These induced currents then re-radiate, altering the radiation pattern and
inducing currents back in the vertical. The induced currents in vertical
then affect antenna impedance.

A counterpoise is a metal conductor that has currents induced in it by the
radiating element. The currents re-radiate resulting in a field distribution
where the countrpoise is a mirror image or opposite version of the radiating
element.

What surprises me is the claim that the ground plane can mirror a 3D image
e.g. a stacked Yagi.

What are your views on above?

Why do some articles say that the ground plane needs to be connected to the
outer braid of coax, while others says this is not necessary?

But the academic books on Antenna theory written by Professors of
Electromagnetics all use image theory for vertical monopole antennas
including those with elevated radials. Can anyone quote an antenna theory
book that does not?

On Fri, 22 Dec 2006 17:44:41 -0000, "David" nospam@nospam wrote:

But the academic books on Antenna theory written by Professors of
Electromagnetics all use image theory for vertical monopole antennas
including those with elevated radials. Can anyone quote an antenna theory
book that does not?

Hi David,

The objection arises out of your commingling radials in the discussion
where the dons never asserted an image theory for them. Can you quote
any source that does? (Let's try proving a positive.)

73's
Richard Clark, KB7QHC

On Fri, 22 Dec 2006 10:08:03 -0800, Richard Clark
wrote:

On Fri, 22 Dec 2006 17:44:41 -0000, "David" nospam@nospam wrote:

But the academic books on Antenna theory written by Professors of
Electromagnetics all use image theory for vertical monopole antennas
including those with elevated radials. Can anyone quote an antenna theory
book that does not?

Hi David,

The objection arises out of your commingling radials in the discussion
where the dons never asserted an image theory for them. Can you quote
any source that does? (Let's try proving a positive.)

Let's just cut to the chase, vis-a-vis radials. These elements serve
to balance and match, not to propagate (in the sense of ground
reflections). In fact, when all the radiative contributions of a
ground plane (radials in a plane) are considered, they are self
negating.

If we were to consider the aspect of this image theory (reflection of
a wave); then those radials would have to consume both a lot of
distance out from the feed point, and a lot of real estate. By any
standards found in the market place, or in implementation, this is so
rare as to be exceedingly exceptional. Commercial AM antenna radial
fields DO NOT come close to this either.

73's
Richard Clark, KB7QHC

David wrote:
But the academic books on Antenna theory written by Professors of
Electromagnetics all use image theory for vertical monopole antennas
including those with elevated radials. Can anyone quote an antenna theory
book that does not?

Kraus, _Antennas_
Johnson, _Antenna Engineering Handbook_
Lo and Lee, _Antenna Handbook_
Balanis, _Antenna Theory_
King and Harrison, _Antennas and Waves_
Jordan & Balmain, _Elecromagnetic Waves and Radiating Systems_
King, Mimno, and Wing, _Transmission Lines, Antennas, and Wave Guides_
Brown, Sharpe, Hughes, and Post, _Lines, Waves, and Antennas_
Stutzman & Thiele, _Antenna Theory and Design_

to name a few. I'm sure there are many more, but those are the ones
currently on my shelf.

Several of these authors use an image to analyze a vertical antenna over
a perfectly conducting ground of infinite extent. None use it for any
other situation, including elevated radials. And for a very good reason
-- it's not valid for other situations.

Roy Lewallen, W7EL