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?

Hooo boy, big can of worms... Watching this oughtta be f u n...

denny / k8do

David wrote:

David wrote:
Four radials form a counterpoise. The counterpoise has currents induced.
These induced currents then re-radiate, ...

Let's look at two equal radials, 180 degrees apart,
horizontal in free space. The radial currents are equal
in magnitude and opposite in direction, i.e. differential.
Theoretically, this causes considerable cancellation
of the radiating fields of those two radials. EZNEC
confirms very little horizontal radiation from two
symmetrical horizontal radials in free space. The same
thing holds true for symmetrical top hats.
--
73, Cecil http://www.w5dxp.com

Hi David,

My advice would be to expand your reference library. One can't expect
to accurately describe an elephant after seeing only its tail.

ac6xg


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?

On Wed, 13 Dec 2006 13:18:21 -0000, "David" nospam@nospam wrote:

Image theory is used to justify the idea that a ground plane reflects the
radio wave emitted by a vertical monopole antenna.

Hi Dave,

Pretty crummy theory, isn't it? You should consider better sources of
information.

73's
Richard Clark, KB7QHC

The books also say that if the antenna is vertical to groundplane the iamge
is in phase and reinforces the wave emitted by vertical.
If antenna is horizontal and close to ground plane, the image cancels out
wave emitted by antenna.

Is this what happens in real life? It is known that if the 2 conductors of
twin feeder are close together, the fields cancel out and there is very
little radiation.

Are you saying that electromagnetic theory is wrong? What about fibre optic
cable? If ordinary glass strands were used, when the cable was bent, the
light would be blocked and not travel any further.

David wrote:
Are you saying that electromagnetic theory is wrong?

Without a quote, nobody can tell to whom you are posing
that question. My threaded newsreader says you are replying
to your original posting.
--
73, Cecil http://www.w5dxp.com

On Wed, 13 Dec 2006 22:44:38 -0000, "David" nospam@nospam wrote:

The books also say
If antenna is horizontal and close to ground plane, the image cancels out
wave emitted by antenna.

Hi David,

Your sources are pretty crummy. This information is quite in
contradiction with the operation of a halfwave horizontal a
quarterwave above earth, isn't it? A simple EZNEC model will reveal
this clearly.

Is this what happens in real life?

Umm, no.

It is known that if the 2 conductors of
twin feeder are close together, the fields cancel out and there is very
little radiation.

Not the same thing.

Are you saying that electromagnetic theory is wrong?

Who is "you?"

What about fibre optic
cable? If ordinary glass strands were used, when the cable was bent, the
light would be blocked and not travel any further.

I've worked with a lot of fiber optics, and that statement simply
doesn't make much sense. Ordinary and un-ordinary glass (as well as
plastic) strands work differently by degree, not differently by
theory.

73's
Richard Clark, KB7QHC

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?

"David" nospam@nospam wrote in message
news
The books also say that if the antenna is vertical to groundplane the iamge
is in phase and reinforces the wave emitted by vertical.
If antenna is horizontal and close to ground plane, the image cancels out
wave emitted by antenna.

Is this what happens in real life?

Yes, once you throw in the (sometimes negligible, sometimes significant)
complications that real "ground planes" are finite in extent and have non-zero
loss.

What about fibre optic cable? If ordinary glass strands were used, when the
cable was bent, the light would be blocked and not travel any further.

Fiber optics aren't antennas; fiber optic "cables" are low loss (ideally
lossless) dielectrics clad in another dielectric with a different enough
permittivity to create total internal reflection. Using traditional
terminology, fiber optics are actually just a particular type of waveguide.