"Cecil Moore" wrote
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


Cecil,
How do we apply (calculate char. imp.) the above to say, full wave (quad)
loop or vertical monopole?

Yuri, K3BU

Yuri Blanarovich wrote:
"Cecil Moore" wrote
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.

How do we apply (calculate char. imp.) the above to say, full wave (quad)
loop or vertical monopole?

That's a good question. For a horizontal wire, its obvious
that the forward wave reflects from the open-circuit at the
end of the wire. We know there are standing waves on a loop
but exactly where are the reflections originating? I
suspect they are originating at the feedpoint, i.e. the
forward wave travels all the way around the loop and is
reflected from the impedance discontinuity at the feedpoint.
Note that the feedpoint impedance of a full-wave loop is
in between the feedpoint impedances of a 1/2WL dipole and
a 1.5WL dipole indicating that the forward wave travels
about 1WL before being reflected in the loop.

Every segment of a monopole is a different
distance from ground and therefore has a slightly different
characteristic impedance which probably doesn't change very
fast as it is a log function. For instance, for the sake
of discussions, it seems reasonable to assume that the Z0
of a vertical stinger is in the neighborhood of a few
hundred ohms and would be easy to measure. At whatever
frequency causes the stinger to be 1/8WL, measure the
impedance. That will be fairly close to the characteristic
impedance of the stinger at the measurement point.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
At whatever
frequency causes the stinger to be 1/8WL, measure the
impedance. That will be fairly close to the characteristic
impedance of the stinger at the measurement point.

As a data point, using EZNEC's VERT1.EZ 40m vertical,
the feedpoint impedance at 3.6 MHz is 6 - j356 ohms.
That would make the Z0 at the feedpoint around 360
ohms and Z0 no doubt increases with distance above
ground.
--
73, Cecil http://www.w5dxp.com

Cecil, W5DXP wrote:
"I suapect they (reflections) are originating at the feedpoint, i.e. the
forward wave travels all the way around the loop and is reflected from
the impedance discontinuity at the feedpoint."

That would be a reflection from a virtual impedance bump wouldn`t it?

The wave travels both wires of a feedline simultaneously, and enters
both ends of the loop at the same time. The collision is at the midpoint
of the loop opposite the feedpoint.

Arnold B. Bailey says on page 399 of "TV and Other Receiving Antennas":
"Now, in the loop, the far-end reflection point is a short circuit, and
hence, the current is high at this far end."

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Cecil, W5DXP wrote:
"I suapect they (reflections) are originating at the feedpoint, i.e. the
forward wave travels all the way around the loop and is reflected from
the impedance discontinuity at the feedpoint."

That would be a reflection from a virtual impedance bump wouldn`t it?

No, the impedance bump is physical. The physical Z0 of the
feedline is no doubt different from the physical Z0 of the
loop.

The wave travels both wires of a feedline simultaneously, and enters
both ends of the loop at the same time. The collision is at the midpoint
of the loop opposite the feedpoint.

Waves traveling in opposite directions in a constant Z0
environment don't interact. If the Z0 doesn't change, they
pass each other "like ships in the night".

Arnold B. Bailey says on page 399 of "TV and Other Receiving Antennas":
"Now, in the loop, the far-end reflection point is a short circuit, and
hence, the current is high at this far end."

If there is no physical impedance discontinuity, there is
no reflection. Reflections occur only at physical impedance
discontinuities. That virtual short circuit is an effect,
not a cause.
--
73, Cecil http://www.w5dxp.com

On Nov 15, 2:41 pm, Cecil Moore wrote:

Reflections occur only at physical impedance
discontinuities.

Richard Feynman said there is a probability that reflection will occur
at any point within a partially reflecting media. He explained that
all the probabilities (including phase) sum in order to generate the
net, observed effect. Your observation about the full-wave loop is
probably a good example.

73, ac6xg

Jim Kelley wrote:
Richard Feynman said there is a probability that reflection will occur
at any point within a partially reflecting media.

Of course, there are always 2nd, 3rd, ... Nth order effects.
On this newsgroup, we are usually talking about first order
effects. I had a recent email exchange with someone talking
about the part of the ground wave that escapes absorption
because of the earth's curvature and is probably ignored
by NEC simulators but not by AM broadcasters.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:

Jim Kelley wrote:

Richard Feynman said there is a probability that reflection will occur
at any point within a partially reflecting media.


Of course, there are always 2nd, 3rd, ... Nth order effects.
On this newsgroup, we are usually talking about first order
effects.

And so was Dr. Feynman. You really ought to freshen up on your QED. ;-)

73, ac6xg