How does a vertical monopole antenna with elevated quarterwave radials work
when receiving? The radials are in free space. Do the radials reflect or
emit a wave that cancels out? Is it possible to model this antenna in
receive mode with a NEC program? I am trying to work out whether the
electromagnetic principles that apply in transmit mode also apply in receive
mode.

David wrote:
How does a vertical monopole antenna with elevated quarterwave radials work
when receiving?

Exactly as it does when transmitting, in reverse.

The radials are in free space.

No, not really. If high enough, they behave like they are, but they're
not that high in typical HF applications.

Do the radials reflect or
emit a wave that cancels out?

No. The radials radiate very little. They certainly interact with the
vertical portion of the antenna, but not in a way that could be
described as "reflection" in any sense.

Is it possible to model this antenna in
receive mode with a NEC program?

Yes. Look up the explanation and examples of plane wave excitation in
the NEC manual. If done correctly, you should get identical but
reciprocal results as when modeled as a transmitting antenna. (EZNEC
doesn't implement plane wave excitation, but you can create an
essentially plane wave by adding a small "transmitting" antenna with
source a few wavelengths away.)

I am trying to work out whether the
electromagnetic principles that apply in transmit mode also apply in receive
mode.

Others have wondered the same thing, but somewhere around a hundred
years ago the problem was solved. They do. A google search on antenna
reciprocity should get you started in the learning process.

Roy Lewallen, W7EL

On 5 Mar, 14:39, Roy Lewallen wrote:
David wrote:
How does a vertical monopole antenna with elevated quarterwave radials work
when receiving?

Exactly as it does when transmitting, in reverse.

The radials are in free space.

No, not really. If high enough, they behave like they are, but they're
not that high in typical HF applications.

Do the radials reflect or
emit a wave that cancels out?

No. The radials radiate very little. They certainly interact with the
vertical portion of the antenna, but not in a way that could be
described as "reflection" in any sense.

Is it possible to model this antenna in
receive mode with a NEC program?

Yes. Look up the explanation and examples of plane wave excitation in
the NEC manual. If done correctly, you should get identical but
reciprocal results as when modeled as a transmitting antenna. (EZNEC
doesn't implement plane wave excitation, but you can create an
essentially plane wave by adding a small "transmitting" antenna with
source a few wavelengths away.)

I am trying to work out whether the
electromagnetic principles that apply in transmit mode also apply in receive
mode.

Others have wondered the same thing, but somewhere around a hundred
years ago the problem was solved. They do. A google search on antenna
reciprocity should get you started in the learning process.

Roy Lewallen, W7EL

David,
The word reprocity is often badly used when comparing transmit and
receive.
One cannot avoid the question of the envirovental or medium conditions
relative to the mode and direction of the radiation travel. Another
thing you might want to pursue is how a receiving antenna separates
radiation into different vectors. It is generaly stated that actions
are reciprocal but I believe that is an emperical response of which I
personaly have no scientific knoweledge that backs up that assertion.
Maybe some where it is all written down somewhere and everything is
really known about antennas
Art

But if the field of the radials cancels out, is there any benefit in having
radials in receive mode?

David wrote:
But if the field of the radials cancels out, is there any benefit in having
radials in receive mode?

Yes. If you eliminate the radials, you have a single conductor wire (the
"antenna") connected to a feedline or receiver. The feedline or receiver
has two terminals which comprise an electrical circuit. If you leave one
of those conductors open circuited, current induced by the passing
signal will not flow from the "antenna" to the receiver or feedline. In
practice, it's nearly impossible to really open circuit the other
conductor (that is, the one not connected to the "antenna") -- the
outside of the feedline, if coax, will become the other half of the
antenna, as will anything conductive connected to the receiver chassis.
This is often undesirable because those conductors are more likely to
receive noise from local sources than if the entire antenna is in the
clear. It will also distort the antenna pattern, possibly in a major way.

I've put "antenna" in quotes because the vertical wire is only half the
antenna. The other half is either radials or, if radials are missing,
the other conductors I mentioned.

Roy Lewallen, W7EL