antenna voltage - between what two points?
 

In all the antenna books I have read there are copious diagrams of voltage
distributions along the various types of antenna, but as far as I can see,
none of them has explained where the two points are that the voltage is
being measured between. Can anyone enlighten me on this please?

DG

Don Gillies wrote:
"---none of these has explained where the two points are that the
voltage is being measured between."

Kraus calls the potentials near and far from antennas "retarded
potentials", involving t-r/c, where t is time, r is distance from the
origin, and c is the speed of light in free space.

On the conductor itself, the origin would seem logical as a reference
point for antenna voltage. Antennas are often considered as extensions
of the feedline,

Feedpoint rms voltage is simply the potential between the wires at that
point. Instantaneous volts depend on phase along the antenna, impedance,
reflection, mutual coupling, radiation, and loss.

Best regards, Richard Harrison, KB5WZI

Don Gillies wrote:
In all the antenna books I have read there are copious diagrams of voltage
distributions along the various types of antenna, but as far as I can see,
none of them has explained where the two points are that the voltage is
being measured between. Can anyone enlighten me on this please?

:-) You need a virtual ground to obtain those voltages. Actually,
they are deduced to exist because P=ExH and we can measure the
antenna current associated with a certain power level. Some purists
say that if a voltage cannot be measured, it doesn't exist. Some
detectives deduce its existence based on the laws of physics.
--
73, Cecil http://www.qsl.net/w5dxp


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Don Gillies wrote:
In all the antenna books I have read there are copious diagrams of voltage
distributions along the various types of antenna, but as far as I can see,
none of them has explained where the two points are that the voltage is
being measured between. Can anyone enlighten me on this please?

DG

I'm curious what antenna books those might be. I've seen plenty of
books that show current distributions on conductors in an antenna, and
plenty that show electric and magnetic field strengths, but darned few
that show "voltage distributions along..." antennas.

It makes plenty of sense to talk about electric field strengths, and if
you know them, you can calculate the potential between any two points
you wish _when_measured_along_a_particular_path_. But the potential
("voltage") between two points in general depends on the path along
which you measure it. In particular, if you're in the presence of a
time-varying magnetic field (which would be the case around an
antenna), the potential depends strongly on the path you take.

Cheers,
Tom

K7ITM wrote:
I'm curious what antenna books those might be. I've seen plenty of
books that show current distributions on conductors in an antenna, and
plenty that show electric and magnetic field strengths, but darned few
that show "voltage distributions along..." antennas.

ARRL Antenna Book, 15th edition, Fig. 1, page 2-2.
--
73, Cecil http://www.qsl.net/w5dxp


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Don Gillies wrote:
In all the antenna books I have read there are copious diagrams of voltage
distributions along the various types of antenna, but as far as I can see,
none of them has explained where the two points are that the voltage is
being measured between. Can anyone enlighten me on this please?

DG

Hi Don, If one is making a voltage reading, it is between a
reference node, 0.0 volts and a point which is higher or lower in
potential. The voltage distribution on the antennas you saw are
probably with respect to earth or the rest of the universe if you are
in space. This is the reference for the voltage along the antenna.
Volts and current on an antenna are periodic if the antenna is
radiating continously, and you can measure potential difference between
different points along the antenna. I don't think this is what the
diagrams you saw refer, it is with respect to earth, or ground.
73 Gary N4AST