Antonio Vernucci wrote:
Win,

your explanation is very good and convincing. I believe that most of us
would read with interest some further remarks on what happens in the
case of a shorted element (e.g. a director or a reflector), instead of
the driven element.

In those cases all received energy must be re-radiated, but more details
on that mechanism would be welcome.

The impinging field induces a current on the element. The current
produces a field which radiates. An AC current flowing on a conductor
creates a field regardless of how that current is produced. It doesn't
matter whether the current is due to conduction from a source, by
induction from a field, or any combination of the two -- the field
resulting from the current is exactly the same. Yagis and other
"parasitic" arrays depend entirely on this phenomenon for their operation.

Roy Lewallen, W7EL

In article , Roy Lewallen
wrote:

Antonio Vernucci wrote:
Win,

your explanation is very good and convincing. I believe that most of us
would read with interest some further remarks on what happens in the
case of a shorted element (e.g. a director or a reflector), instead of
the driven element.

In those cases all received energy must be re-radiated, but more details
on that mechanism would be welcome.

The impinging field induces a current on the element. The current
produces a field which radiates. An AC current flowing on a conductor
creates a field regardless of how that current is produced. It doesn't
matter whether the current is due to conduction from a source, by
induction from a field, or any combination of the two -- the field
resulting from the current is exactly the same. Yagis and other
"parasitic" arrays depend entirely on this phenomenon for their operation.

Roy Lewallen, W7EL

Yes, and with the corollary that you can never extract more than half of
the power/energy transported by an EM wave. That maximum (available
power) is given by Voc^2/(4 * Ra) where Voc and Ra are the open circuit
voltage magnitude and real part of the extractor (receive antenna)
impedance, respectively, measured at the antenna terminals at the
frequency of interest. The remaining energy is scattered (reradiated)
from the antenna. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

Roy Lewallen wrote:

The impinging field induces a current on the element. The current
produces a field which radiates. An AC current flowing on a conductor
creates a field regardless of how that current is produced. It doesn't
matter whether the current is due to conduction from a source, by
induction from a field, or any combination of the two -- the field
resulting from the current is exactly the same. Yagis and other
"parasitic" arrays depend entirely on this phenomenon for their operation.

Roy Lewallen, W7EL

Andy asks:
Roy, is the field radiated by the parasitic element shifted in phase
relative to the incident field. If not, what determines the phase
shift ?

Andy W4OAH

AndyS wrote:

Andy asks:
Roy, is the field radiated by the parasitic element shifted in phase
relative to the incident field. If not, what determines the phase
shift ?

Yes, it can be different in both phase and magnitude, determined by the
length and diameter of the parasitic element. That's what you're doing
when you adjust the lengths of the elements of a Yagi -- adjusting the
phase and amplitude of the re-radiated fields from the elements.

Roy Lewallen, W7EL