On 16 Oct 2003 09:59:07 -0700, (Tom Bruhns) wrote:

Roy Lewallen wrote in message ...
It's not clear to me what's meant by the voltage (presumably relative to
ground) at the tip of a dipole. Suppose it's a quarter wavelength above
ground. How would you measure it? Or, how would you measure the voltage
at the top of a quarter wavelength vertical?

Roy Lewallen, W7EL

Indeed... Or putting it another way, the potential between two points
does not have a unique value in the presence of a time-varying
magnetic field, which certainly is the case for a radiating dipole.
If you measure the voltage drop along the wire, it's essentially zero,
so along the wire the voltage between the end points of the dipole is
essentially the same as the voltage across the feedpoint. The only
difference between the ends and the feedpoint is due to I*R drop in
the wire. The voltage at the top of Roy's vertical, made out of

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.

The end of the antenna is high impedance while the center (in the case
of a half wave dipole) is low. Low voltage and high current at the
center with high voltage and low current at the ends.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

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:
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

Cecil Moore wrote in message ...
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.

It's TEM line, right? The voltages are practically all ACROSS the
line, between the conductors. There is very little voltage ALONG the
conductors, just I*R (and note the directions for _that_). Go look up
Faraday's Law of Magnetic Induction, and refresh your understanding of
Kirchoff's Voltage Law and Ohm's Law as well. Those three pretty much
let you figure it all out.

Cheers,
Tom

Tom Bruhns wrote:
There is very little voltage ALONG the conductors, ...

Depends upon how long the conductors are. The difference along a
1/4WL conductor is known to be minimum VS maximum assuming a
minimum at one end. Are you saying that EZNEC doesn't display
the current distribution on an antenna when I press the 'i' key?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil, I'm sorry you don't understand that in the presence of
time-varying fields, the potential between two points depends on the
path you take. Grasping that concept can be very empowering in
understanding what's going on in antennas, and in transmission lines,
and in inductors and transformers. I can only hope that some lurkers
have benefitted from the discussion.

Cheers,
Tom

Cecil Moore wrote in message ...
Tom Bruhns wrote:
There is very little voltage ALONG the conductors, ...

Depends upon how long the conductors are. The difference along a
1/4WL conductor is known to be minimum VS maximum assuming a
minimum at one end. Are you saying that EZNEC doesn't display
the current distribution on an antenna when I press the 'i' key?

Tom Bruhns wrote:
Cecil, I'm sorry you don't understand that in the presence of
time-varying fields, the potential between two points depends on the
path you take.

I know that, Tom, but we are talking about measuring the RF voltage
between two copper wires one inch apart. The path is well defined.
It is a no brainer. There is no need for obfuscation. The measurement
proves the voltages at the ends of a dipole to be at least a magnitude
higher than the voltage at the feedpoint. Are you not aware of how
the ratio of voltage to current varies over 1/4WL of a wire antenna?
--
73, Cecil http://www.qsl.net/w5dxp



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Really???


K9CUN