The text below applies to vertical quarterwave antenna in free space with
four quarterwave radials.

In transmit mode, the field from the radials cancel out.

When the antenna is receiving, the incoming wave interacts with the radials
and is reflected into the vertical element.

How does this fit in with the antenna reciprocity theorem? The antenna field
arrangement is working in different ways between transmit and receive. In
transmit mode, the radials cancel out, and in receive mode the radials
reflect the wave to develop a signal in the vertical element.

In receive mode, the radials strengthen the signal in the vertical element
by reflecting RF energy into the vertical element and also providing the
other half of the antenna by providing a RF ground side. The signal exists
as a potential difference between two conductors.

In transmit mode, the radials have a standing wave on them and provide the
RF ground side by 'converting' the high voltage at open circuit end to a low
voltage (RF ground with high current) at base of antenna.

Do the radials have a standing wave on them in receive mode? Does antenna
reciprocity theorem agree that the antenna can form same feedpoint impedance
and field pattern in both transmit mode and receive mode, even though the
electromagnetic field works differently? Are the electromagnetic field
patterns noticeably different according to whether antenna is receiving or
transmitting?

The vertical element is always known as the radiating element.

David wrote:
The text below applies to vertical quarterwave antenna in free space with
four quarterwave radials.

In transmit mode, the field from the radials cancel out.

When the antenna is receiving, the incoming wave interacts with the radials
and is reflected into the vertical element.

No. The wave strikes the radials. This induces currents in the radials.
Those currents cause radiation, and also the currents are conducted to
the feedpoint. When the signal is coming from directions in which the
radial radiation was zero, the induced currents will add to zero at the
feedpoint, and the re-radiation will be zero in those directions.

How does this fit in with the antenna reciprocity theorem? The antenna field
arrangement is working in different ways between transmit and receive. . .

No, your concept of how it works when receiving is incorrect. That's the
cause of the apparent contradiction.

. .
In transmit mode, the radials have a standing wave on them and provide the
RF ground side by 'converting' the high voltage at open circuit end to a low
voltage (RF ground with high current) at base of antenna.

Unfortunately, your concept of how the antenna works when transmitting
is also incorrect. The radials are just as much a part of the antenna
system as the "antenna". Their symmetry results in their total radiated
field being much smaller than that from the "antenna". The notion of an
"RF ground" in space is simply a matter of your imagination; no such
thing exists.

Do the radials have a standing wave on them in receive mode?

Yes. So does the vertical. And both do in transmit mode also.

Does antenna
reciprocity theorem agree that the antenna can form same feedpoint impedance
and field pattern in both transmit mode and receive mode, even though the
electromagnetic field works differently?

The impedance is the same for transmitting and receiving. The
electromagnetic field does not work differently.

Are the electromagnetic field
patterns noticeably different according to whether antenna is receiving or
transmitting?

No. They are the same. This is part of what is known as reciprocity.

The vertical element is always known as the radiating element.

The antenna doesn't know or care what you call the parts. If the radials
lie in the same plane, they radiate very little. If bent down, say, as
is often done with a ground plane, they radiate a very substantial
amount. What you call or "know" the various parts of an antenna as
doesn't make any difference in how they operate. All conductors behave
the same with respect to currents and fields, regardless of how you've
positioned them or what you call them.

Roy Lewallen, W7EL

Roy Lewallen wrote:

David wrote:

Are the electromagnetic field patterns noticeably different according
to whether antenna is receiving or transmitting?

No. They are the same. This is part of what is known as reciprocity.

I think you would agree the field pattern the antenna sees from a
distant source is different than the field pattern the antenna
generates on transmit. The field is uniform about the vertical axis
on transmit and the phase of the signal in each radial is identical.
But on receive the phase of the induced signal is unique into each
radial.

This difference has nothing to do with reciprocity of course, but
noting the differences between a unidirectional field pattern versus
an omnidirectional one might help to clarify the apparent assymetry
between receive and transmit.

73, Jim AC6XG

On Thu, 26 Apr 2007 14:27:38 -0700, Jim Kelley
wrote:

Roy Lewallen wrote:

David wrote:

Are the electromagnetic field patterns noticeably different according
to whether antenna is receiving or transmitting?

No. They are the same. This is part of what is known as reciprocity.

I think you would agree the field pattern the antenna sees from a
distant source is different than the field pattern the antenna
generates on transmit. The field is uniform about the vertical axis
on transmit and the phase of the signal in each radial is identical.
But on receive the phase of the induced signal is unique into each
radial.

Hi Jim,

What you describe about the asymmetry of response to a distant source
is all perfectly true (and offers a similar argument in regards to
placing a 3D antenna into 2D null space).

However, your conclusion that there is a difference in field patterns
does not follow. Such patterns infer a multiplicity of loads for
transmit; or, in this case, a multiplicity of sources for receive.
These loads/sources populate a spherical constellation of constant
radius to exhibit the patterns we see in models. Two points do not
render a pattern.

For a one transmit source, one receive antenna, the receive antenna
current asymmetry will ALWAYS be oriented towards the source
regardless of where it is. As long as either moves in a constant
radius, the asymmetry will always follow the shared the axis. Add a
second source, and the receive antenna will similarly develop the same
axial/asymmetry relationship. The net contribution of a constellation
of sources will simple average out the asymmetry back into the rather
pedestrian distribution pattern.

73's
Richard Clark, KB7QHC

Hi Richard,

You're discussing the reciprocity of the antenna. I was not.

Thanks,

ac6xg


Richard Clark wrote:
On Thu, 26 Apr 2007 14:27:38 -0700, Jim Kelley
wrote:


Roy Lewallen wrote:


David wrote:

Are the electromagnetic field patterns noticeably different according
to whether antenna is receiving or transmitting?

No. They are the same. This is part of what is known as reciprocity.

I think you would agree the field pattern the antenna sees from a
distant source is different than the field pattern the antenna
generates on transmit. The field is uniform about the vertical axis
on transmit and the phase of the signal in each radial is identical.
But on receive the phase of the induced signal is unique into each
radial.


Hi Jim,

What you describe about the asymmetry of response to a distant source
is all perfectly true (and offers a similar argument in regards to
placing a 3D antenna into 2D null space).

However, your conclusion that there is a difference in field patterns
does not follow. Such patterns infer a multiplicity of loads for
transmit; or, in this case, a multiplicity of sources for receive.
These loads/sources populate a spherical constellation of constant
radius to exhibit the patterns we see in models. Two points do not
render a pattern.

For a one transmit source, one receive antenna, the receive antenna
current asymmetry will ALWAYS be oriented towards the source
regardless of where it is. As long as either moves in a constant
radius, the asymmetry will always follow the shared the axis. Add a
second source, and the receive antenna will similarly develop the same
axial/asymmetry relationship. The net contribution of a constellation
of sources will simple average out the asymmetry back into the rather
pedestrian distribution pattern.

73's
Richard Clark, KB7QHC

If the radials are horizontal, the field of the radials cancels out. If the
radials are drooping, the radials radiate considerably. Is there any
advantage in either configuration? Perhaps for feedline decoupling?

The field pattern will be different between transmit and receive, because in
receive mode the radio wave can be coming from the source in any direction.
Does antenna reciprocity theorem cover this, as usually people state that
the field pattern is the same for transmit and receive?