Can someone please confirm or deny the following arguments.

Let us have:

- a transmitting system operating at any given frequency
- and a metal bar, located far away from the transmitter, whose electrical
length is exactly half wavelength at the operating frequency.

An induced RF current will flow in the bar. Such RF current causes a re-radiated
field which adds up to the field generated by the trasmitter.

Two questions:

- which are the amplitude and phase shift of the re-radiated field with respect
to those of the field generated by the trasmitter? My instinctive answer would
be same amplitude (in absence of ohmic losses) and 180 degrees. The total field
(transmitted + re-radiated) at the metal bar would so be zero.

- how does the total field change moving away from the bar? I would say that
while the field generated by the transmitter varies very slowly with the
distance from the bar (the transmitter is assumed to be very far away), the
re-radiated field varies fast (also because one would initially be in the near
field). In conclusion, the more we move away from the bar, the lower is the
contribution of the re-radiated field to the total field. That should be the
reason why, in a Yagi antenna, a parasitic element cannot be put too far away
from the driven element.

Thanks and 73

Tony I0JX

"Antonio Vernucci" wrote in message
.. .
Can someone please confirm or deny the following arguments.

Let us have:

- a transmitting system operating at any given frequency
- and a metal bar, located far away from the transmitter, whose electrical
length is exactly half wavelength at the operating frequency.

An induced RF current will flow in the bar. Such RF current causes a
re-radiated field which adds up to the field generated by the trasmitter.

Two questions:

- which are the amplitude and phase shift of the re-radiated field with
respect to those of the field generated by the trasmitter? My instinctive
answer would be same amplitude (in absence of ohmic losses) and 180
degrees. The total field (transmitted + re-radiated) at the metal bar
would so be zero.

- how does the total field change moving away from the bar? I would say
that while the field generated by the transmitter varies very slowly with
the distance from the bar (the transmitter is assumed to be very far
away), the re-radiated field varies fast (also because one would initially
be in the near field). In conclusion, the more we move away from the bar,
the lower is the contribution of the re-radiated field to the total field.
That should be the reason why, in a Yagi antenna, a parasitic element
cannot be put too far away from the driven element.

Thanks and 73

Tony I0JX


Sounds about right. The electric field tangential to (i.e. parallel and
close to) the surface of a good conductor must be small, otherwise a current
would flow in the conductor which would tend to 'short out' the E-field, but
an electric field normal to a conducting surface can have any value ... of
course. A short conducting bar, rotating about an axis normal to its
length, can be used to measure the radiation pattern of a large antenna by
transmitting through the antenna and inspecting the signal reflected back
down its feeder.

Interestingly, even a matched dipole antenna re-radiates a signal - the
amount of power 'dissipated' in its radiation resistance. This is mentioned
in Kraus 'Antennas'.

Chris

"Antonio Vernucci" wrote in message
.. .
Can someone please confirm or deny the following arguments.

Let us have:

- a transmitting system operating at any given frequency
- and a metal bar, located far away from the transmitter, whose electrical
length is exactly half wavelength at the operating frequency.

An induced RF current will flow in the bar. Such RF current causes a
re-radiated field which adds up to the field generated by the trasmitter.

Two questions:

- which are the amplitude and phase shift of the re-radiated field with
respect to those of the field generated by the trasmitter? My instinctive
answer would be same amplitude (in absence of ohmic losses) and 180
degrees. The total field (transmitted + re-radiated) at the metal bar
would so be zero.

- how does the total field change moving away from the bar? I would say
that while the field generated by the transmitter varies very slowly with
the distance from the bar (the transmitter is assumed to be very far
away), the re-radiated field varies fast (also because one would initially
be in the near field). In conclusion, the more we move away from the bar,
the lower is the contribution of the re-radiated field to the total field.
That should be the reason why, in a Yagi antenna, a parasitic element
cannot be put too far away from the driven element.

Thanks and 73

Tony I0JX


sounds like you have the right instincts to me.