Mike Kaliski wrote:
and that the characteristic impedence will vary
along an antennas length.

Well, that's obviously false. The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Mike Kaliski wrote:
and that the characteristic impedence will vary along an antennas length.

Well, that's obviously false. The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.

Have you verified this experimentally, Cecil? If you did,
how did you do it?
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
Cecil Moore wrote:
The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.

Have you verified this experimentally, Cecil? If you did,
how did you do it?

Here's a quote from "Antennas Theory" by Balanis: "The current
and voltage distributions on open-ended wire antennas are similar
to the standing wave patterns on open-ended transmission lines.
.... Standing wave antennas, such as the dipole, can be analyzed
as traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by traveling
wave currents If and Ib ..."

As Balanis suggests, the body of technical knowledge
available for "open-ended transmission lines" is applicable
to "open-ended wire antennas", e.g. dipoles, which really
are nothing but lossy *single-wire* transmission lines.

That characteristic impedance equation for a single-wire
transmission lines can be found in numerous publications and
is close to a purely resistive value. A #14 horizontal wire
30 feet above ground is very close to a characteristic
impedance of 600 ohms. (One half of a 1/2 wavelength dipole
is simply a lossy 1/4 wavelength stub with Z0 = ~600 ohms.)

Before he passed, Reg Edwards had some earlier comments on
the characteristic impedance of a 1/2WL dipole above ground.

Like a normal transmission line open stub, a 1/2WL
dipole supports standing waves that can be analyzed. For the
purposes of a voltage and current analysis, I^2*R losses and
radiation losses can be lumped together into total losses
associated with some attenuation factor, similar to analyzing
a 1/4WL lossy normal stub.

In fact, the losses to radiation from
one half of a 1/2WL dipole can be simulated by EZNEC using
resistance wire in a 1/4WL open stub. Using EZNEC with a
resistivity of 2.3 uohm/m for a 1/4WL open stub gives a pretty
good model of what is happening with one half of a 1/2WL dipole
which is only a lossy single-wire transmission line above earth.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Tom Donaly wrote:
Cecil Moore wrote:
The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.

Have you verified this experimentally, Cecil? If you did,
how did you do it?

Here's a quote from "Antennas Theory" by Balanis: "The current
and voltage distributions on open-ended wire antennas are similar
to the standing wave patterns on open-ended transmission lines.
... Standing wave antennas, such as the dipole, can be analyzed
as traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by traveling
wave currents If and Ib ..."

As Balanis suggests, the body of technical knowledge
available for "open-ended transmission lines" is applicable
to "open-ended wire antennas", e.g. dipoles, which really
are nothing but lossy *single-wire* transmission lines.

That characteristic impedance equation for a single-wire
transmission lines can be found in numerous publications and
is close to a purely resistive value. A #14 horizontal wire
30 feet above ground is very close to a characteristic
impedance of 600 ohms. (One half of a 1/2 wavelength dipole
is simply a lossy 1/4 wavelength stub with Z0 = ~600 ohms.)

Before he passed, Reg Edwards had some earlier comments on
the characteristic impedance of a 1/2WL dipole above ground.

Like a normal transmission line open stub, a 1/2WL
dipole supports standing waves that can be analyzed. For the
purposes of a voltage and current analysis, I^2*R losses and
radiation losses can be lumped together into total losses
associated with some attenuation factor, similar to analyzing
a 1/4WL lossy normal stub.

In fact, the losses to radiation from
one half of a 1/2WL dipole can be simulated by EZNEC using
resistance wire in a 1/4WL open stub. Using EZNEC with a
resistivity of 2.3 uohm/m for a 1/4WL open stub gives a pretty
good model of what is happening with one half of a 1/2WL dipole
which is only a lossy single-wire transmission line above earth.

So you haven't verified it experimentally, and don't know how
to do so. Thanks for the answer.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
Cecil Moore wrote:
Here's a quote from "Antennas Theory" by Balanis: "The current
and voltage distributions on open-ended wire antennas are similar
to the standing wave patterns on open-ended transmission lines.
... Standing wave antennas, such as the dipole, can be analyzed
as traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by traveling
wave currents If and Ib ..."

So you haven't verified it experimentally, and don't know how
to do so. Thanks for the answer.

Do you distrust the theory of relatively because you
haven't verified it experimentally and don't know
how to do so?

I have simulated the configuration using EZNEC.

Tom, like you, I trust the great engineers and physicists
who came before me. I do not develop every concept from
first principles. If an analysis suggested by Balanis is
not good enough for you, that's your choice. Incidentally,
Kraus says essentially the same thing as Balanis about
analyzing standing-wave antennas.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Tom Donaly wrote:
Cecil Moore wrote:
Here's a quote from "Antennas Theory" by Balanis: "The current
and voltage distributions on open-ended wire antennas are similar
to the standing wave patterns on open-ended transmission lines.
... Standing wave antennas, such as the dipole, can be analyzed
as traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by traveling
wave currents If and Ib ..."

So you haven't verified it experimentally, and don't know how
to do so. Thanks for the answer.

Do you distrust the theory of relatively because you
haven't verified it experimentally and don't know
how to do so?

I have simulated the configuration using EZNEC.

Tom, like you, I trust the great engineers and physicists
who came before me. I do not develop every concept from
first principles. If an analysis suggested by Balanis is
not good enough for you, that's your choice. Incidentally,
Kraus says essentially the same thing as Balanis about
analyzing standing-wave antennas.

I actually do know how to verify Einstein's predictions because the
fellows who did it wrote detailed articles on how they did it.

Thinking of antennas as transmission lines is an old practice. It
doesn't mean it's very practical, or that it hasn't been superseded
by a better analogy. For that matter, a vibrating guitar string can
be analyzed as a transmission line, as can any woodwind instrument.
That doesn't mean it's worth doing, but it can be done. The problem is
when a gentleman, such as the late, lamented Reg Edwards, or the still
kicking, unlamented you, write that an antenna, or a clarinet _is_ a
transmission line.
73,
Tom Donaly, KA6RUH

Tom Donaly wrote:
The problem is
when a gentleman, such as ... unlamented you, write that
an antenna, or a clarinet _is_ a transmission line.

But Tom, page 18 of "Antenna Theory" by Balanis,
shows how a transmission line can be opened up
to cause it to radiate. A dipole is indeed a leaky
transmission line. During steady-state, it loses
about 20% of the power stored in the standing
waves to radiation. Maxwell's laws don't change
from transmission lines to wire antennas.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
Mike Kaliski wrote:
and that the characteristic impedence will vary along an antennas length.

Well, that's obviously false. The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.

Interesting point.

When I use a gamma match on a 1/2 wave vertical with counterpoise, I
have always wondered about the 50ohm impedance point where the gamma
taps the element.

To end feed the antenna, an impedance of thousands of ohms is
encountered, a, seemingly, "strange" distance up the element (in regards
to total element length) and a 50 ohm impedance point is encountered
(needing only a series capacitive reactance to correct for the gamma
rods' inductive reactance.)

It would be interesting if I had a formula which would predict what
impedance would be encountered for all points along the element--know of
any?

I have looked at setting up such a formula, but frankly have been
unsuccessful ... and of course, it is given that antenna length IS
resonate for the frequency in question.

Regards,
JS

"Cecil Moore" wrote in message
...
Mike Kaliski wrote:
and that the characteristic impedence will vary along an antennas length.

Well, that's obviously false. The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.
--
73, Cecil http://www.w5dxp.com

Cecil

Are you sure you are not confusing the characteristic impedance of a dipole
antenna with the characteristic impedence of an open feed line? One is
constant, the other appears to vary along its length. A dipole antenna has
low impedence at a centre feed point and high impedence at it's ends.

Reminds me of the tales of old ladies who used to tie knots in electric flex
to stop the electricity leaking out! I actually met one in real life many
years ago.

Mike G0ULI

Mike Kaliski wrote:
Are you sure you are not confusing the characteristic impedance of a
dipole antenna with the characteristic impedence of an open feed line?
One is constant, the other appears to vary along its length. A dipole
antenna has low impedence at a centre feed point and high impedence at
it's ends.

The feedpoint impedance of a stub is NOT the same thing
as the Z0 of the stub. The feedpoint impedance of an
antenna is NOT the same thing as the Z0 of the antenna.

The characteristic impedance of a #14 wire 30 feet above
the ground is close to constant at 600 ohms. The formula
for a single-wire transmission line is 138*log(4D/d).
A horizontal dipole is nothing more than a single-wire
transmission line which is known to be lossy.

The characteristic impedance of a transmission line is
constant. If the SWR 1, the voltage to current
ratio varies along its length. That varying impedance
(V/I) is NOT the characteristic impedance which is
constant.

The characteristic impedance of a horizontal dipole is
~constant. Since a dipole is a standing wave antenna,
the voltage to current ratio varies along its length.
That varying impedance (V/I) is NOT the characteristic
impedance which is relatively constant for a horizontal
wire.
--
73, Cecil http://www.w5dxp.com