Roger Halstead wrote in message . ..
....
Ahhh...with the impedance variation I beg to differ.
Otherwise how could I draw a 3 or 4 inch arc off the end of a 160
meter dipole.

You're welcome to differ, but indeed you may not be differing at all.
I did NOT say there was a low electrical field strength near the
antenna. Rather, the field _must_be_ essentially perpendicular to the
conductor, at the conductor's surface. So the potential between the
antenna and a point a short distance away, along a line parallel to
the electric field (that is, perpendicular to the antenna wire) may be
quite high. If the electric field exceeds the breakdown voltage of
air, you'll get corona. But if you see corona streamers, are they
_parallel_to_ the antenna wire? I doubt it...they will almost
certainly be perpendicular to the wire, where they meet the wire's
surface.

As I've suggested in other posts in this thread, I'll be happy to
listen to explanations about fields around an antenna, but if you're
going to talk about voltages between two points, be sure you specify
the path along which you will measure those voltages. If you tell me
there is a large voltage along a good conductor, then I know there is
a very large heat dissipation in that wire. But if your meter and its
leads have enclosed an area outside the wire, you have not measured
the voltage along the wire, but rather around the loop composed of the
wire and the meter's leads.

Cheers,
Tom

Tom Bruhns wrote:
I'm also saying that the voltage (potential) between two points
depends, in general, on the path you take between the two points. You
should be _especially_ aware of that fact when you're in the presence
of time-varying magnetic fields, such as you have around a powered
antenna.


In practice that will means that the voltage you measure between say the
end of a whip and ground will depend on how you choose to route the
connecting leads to the voltmeter, and how you connect to ground... and
above (below?) all on what you define "ground" to be.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

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

Tom Bruhns wrote:
I'm saying that if you measure the voltage between two points on a
good conductor, in a path along that conductor, it will be very small.

True for DC and RF traveling waves. Not true for standing waves. A
1/2WL dipole is a *standing-wave* antenna. What do you get when you
measure the voltage between the voltage maximum and voltage minimum
on a feedline with a 10:1 SWR? Exactly the same principle applies
to *standing-wave* antennas.
--
73, Cecil, W5DXP

Tom Bruhns wrote:
If you tell me
there is a large voltage along a good conductor, then I know there is
a very large heat dissipation in that wire.

There are large voltages along my open-wire feedline when
the SWR is high, but very low heat dissipation in that wire.
Hint: think standing waves on the antenna wire.
--
73, Cecil, W5DXP

Ian White, G3SEK wrote:
In practice that will means that the voltage you measure between say the
end of a whip and ground will depend on how you choose to route the
connecting leads to the voltmeter, and how you connect to ground... and
above (below?) all on what you define "ground" to be.

How about using an artificial ground at the measurement point?
--
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 Moore wrote:
Ian White, G3SEK wrote:
In practice that will means that the voltage you measure between say
the end of a whip and ground will depend on how you choose to route
the connecting leads to the voltmeter, and how you connect to
ground... and above (below?) all on what you define "ground" to be.

How about using an artificial ground at the measurement point?

Define one, if you can! What are its properties, and how would you
achieve them?

It's rather like the early wireless users who "earthed" their receivers
to the aspidistra pot in the corner of the room - after all, it
contained earth, so why didn't it work?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

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