Cecil wrote,


Tom Bruhns wrote:
There is no such thing as "the voltage" between the ends of your
excited dipole at an instant in time.

Please reference Fig 1, page 2-2, in the 15th edition of the ARRL
Antenna Book. "Current and voltage distribution on a 1/2WL wire.

The RMS (or peak) values of the voltages at the ends of the dipole
are maximum and 180 degrees out of phase. The ratio of net voltage to
net current is the impedance anywhere along the wire.
--
73, Cecil http://www.qsl.net/w5dxp




Cecil, that picture is a gross simplification. In order to show that there's
a unique voltage between the ends of a dipole, you first have to show that
the time-varying electric field between those ends is conservative.
Go ahead. (My money is on Tom, however.)
73,
Tom Donaly, KA6RUH

Tdonaly wrote:
Please reference Fig 1, page 2-2, in the 15th edition of the ARRL
Antenna Book. "Current and voltage distribution on a 1/2WL wire.

The RMS (or peak) values of the voltages at the ends of the dipole
are maximum and 180 degrees out of phase. The ratio of net voltage to
net current is the impedance anywhere along the wire.

Cecil, that picture is a gross simplification. In order to show that there's
a unique voltage between the ends of a dipole, you first have to show that
the time-varying electric field between those ends is conservative.

Each leg of a dipole is a one-wire transmission line with a Z0 around
600 ohms (according to Reg). These kinds of antennas are known as
"standing-wave" antennas because of (surprise) their standing waves as
depicted by the diagram in the ARRL Antenna Book. The center feedpoint
impedance is not 600 ohms because of the reflections from the ends.
Feedpoint impedance equals (Vfwd+Vref)/(Ifwd/Iref), just like a
transmission line.

The forward voltage wave hits an open circuit at the end of the dipole.
The reflected voltage wave possesses reversed direction and reversed phase,
just like an open circuit in a transmission line. The net voltage at the end
of a dipole is 2 times the forward voltage, just like an open circuit in
a transmission line. Thus, standing waves are created on the antenna wires.
The two ends of the dipole are also 180 degrees out of phase.

If you curve a dipole into a circle and measure the end-to-end voltage
with an RF voltmeter, you will get a voltage in the ballpark of four
times the forward voltage on the antenna.

You can use a fluorescent light bulb to locate the maximum electric
field. That will be at the ends of a 1/2WL dipole or at the top of
a 1/4WL monopole. I'm surprised you guys haven't ever done that.
--
73, Cecil, W5DXP

Cecil wrote,

You can use a fluorescent light bulb to locate the maximum electric
field. That will be at the ends of a 1/2WL dipole or at the top of
a 1/4WL monopole. I'm surprised you guys haven't ever done that.
--
73, Cecil, W5DXP

Do you actually read other people's posts? Or, do you just react to them.
Do you know the difference between an E field (a vector field) and a
V field (a scalar field)? Do you know what a conservative field is? Is the
E field surrounding the ends of a dipole conservative, or not? If it is, (it
isn't)
then the voltages are unique, and if it isn't (it isn't) then the voltages
aren't
unique and what you get depends on how you measure it. There is a good,
abeit challenging discussion of this in Vladimir Rojansky's book
_Electromagnetic Fields and Waves_ under the heading 99. A. C.
Voltmeters. He writes about a ring, but the difficulties, it seems to me,
would apply to measuring voltage at the ends of a dipole, as well.
Whaowncha read it, Cecil, and tell us what you think.
73,
Tom Donaly, KA6RUH

(P.S. I reserve the right to be wrong.)

Tdonaly wrote:
Do you know the difference between an E field (a vector field) and a
V field (a scalar field)? Do you know what a conservative field is? Is the
E field surrounding the ends of a dipole conservative, or not?

Heh, heh, reminds me of the [color] box on the job applications
in the 50's. The choice was []White, []Black, []Other________.
I checked 'Other' and wrote 'tan'.

Bend the ends of a resonant dipole around close to each other and
measure the voltage with a shielded differential RF voltmeter. For
100 watts input, you will get almost 1000 volts RMS between the ends,
a far cry from the ~70 volts RMS at the center feedpoint.
--
73, Cecil, W5DXP

Cecil wrote,


Bend the ends of a resonant dipole around close to each other and
measure the voltage with a shielded differential RF voltmeter. For
100 watts input, you will get almost 1000 volts RMS between the ends,
a far cry from the ~70 volts RMS at the center feedpoint.
--
73, Cecil, W5DXP


You missed the point, again, Cecil. Carry on.
73,
Tom Donaly, KA6RUH

Tdonaly wrote:
Cecil wrote,
Bend the ends of a resonant dipole around close to each other and
measure the voltage with a shielded differential RF voltmeter. For
100 watts input, you will get almost 1000 volts RMS between the ends,
a far cry from the ~70 volts RMS at the center feedpoint.

You missed the point, again, Cecil. Carry on.

Nope, you missed the point. This ain't rocket science.
--
73, Cecil http://www.qsl.net/w5dxp



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