Cecil Moore wrote:

Jim Kelley wrote:
That angle is the 'phase' of the current standing wave as a function of
position, not to be confused with the phase of the current with respect
to voltage. Roger?

Since any reference to voltage on an antenna seems to be verboten, I
have avoided any such reference.

I can understand how at DC, references to current in a dipole might be
verboten. :-)

I wonder if voltage on a dipole could be roughly likened to transverse
velocity at various points along a whip.

Don't you just love the phrase, "It is generally assumed ..."?

I guess it allows: 'I'm not responsible should it turn out not to be a
good assumption'. :-)

For that general assumption to be true, the reflected current would have
to equal the forward current on a standing-wave antenna. But we know it
doesn't. However, this implies that the reflected current arriving back
at the feedpoint is not extremely/severely attenuated.

Seems to me it just implies that current at the end of a dipole isn't
really zero.

Assume the Z0 of a traveling-wave dipole is 600 ohms. The ratio of forward
voltage to forward current is 600 ohms. The ratio of reflected voltage to
reflected current is 600 ohms. Assume the feedpoint current is one amp and
the feedpoint voltage is 50 volts. Assume the forward current and the reflected
current are in phase at the feedpoint. Assume the forward voltage and reflected
voltage are 180 degrees out of phase at the feedpoint. This is enough information
to solve for the ratio of forward current to reflected current at the feedpoint.
Assuming the net current is one amp at the feedpoint, I get 0.542 amps for
the forward current and 0.458 amps for the reflected current, i.e. the reflected
current is 85% of the value of the forward current. Remember, that is a ballpark
estimate.

50 volts difference across a 600 ohm impedance, over 2, plus and minus
half an amp.

That means the current only decreases by 15% in its round trip to the
end of the antenna and back.

I think it may actually make many round trips. There may be multiple
reflections.

The argument seems to occur due to the ignoring of the component waves
on a standing wave antenna. Such is the steady-state model seduction attended
by its sacred cows.

Well, you'll either have to write out everything as a series, which is a
lot of busy work, or use the steady state equivalent.

So what does all this say about reflectivity at the end of the dipole?
;-)

73, Jim AC6XG

Jim Kelley wrote:
Seems to me it just implies that current at the end of a dipole isn't
really zero.

The net current is very close to zero because the forward and reflected
currents are very nearly equal and 180 degrees out of phase.

I think it may actually make many round trips. There may be multiple
reflections.

Of course, but like a transmission line, there is only one forward wave
and one reflected wave. All the multiple reflections are contained in
those two waves.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

Jim Kelley wrote:
Seems to me it just implies that current at the end of a dipole isn't
really zero.

The net current is very close to zero because the forward and reflected
currents are very nearly equal and 180 degrees out of phase.

Is that what you meant by:

"For that general assumption to be true, the reflected current would
have
to equal the forward current on a standing-wave antenna. But we know it
doesn't"?

It's not clear whether you're making this point in recognition of the
fact that wires are not lossless, or whether you're claiming it's
somehow fundamental to the performance of a radiator.

I think it may actually make many round trips. There may be multiple
reflections.

Of course, but like a transmission line, there is only one forward wave
and one reflected wave. All the multiple reflections are contained in
those two waves.

Then apparently you've decided not to completely eschew making at least
some steady state assumptions. Seductive indeed. ;-)

73, Jim AC6XG

Jim Kelley wrote:
Cecil Moore wrote:
The net current is very close to zero because the forward and reflected
currents are very nearly equal and 180 degrees out of phase.

This is at the tip end of a dipole.

Is that what you meant by:

"For that general assumption to be true, the reflected current would
have
to equal the forward current on a standing-wave antenna. But we know it
doesn't"?

This is apparently not at the tip end of a dipole.

It's not clear whether you're making this point in recognition of the
fact that wires are not lossless, or whether you're claiming it's
somehow fundamental to the performance of a radiator.

Both, radiation is a "loss".

Of course, but like a transmission line, there is only one forward wave
and one reflected wave. All the multiple reflections are contained in
those two waves.

Then apparently you've decided not to completely eschew making at least
some steady state assumptions. Seductive indeed. ;-)

There are only two possible directions, forward and reverse, in which
energy can flow. Multiple reflections do not create any more directions.
I am not opposed to the steady-state solution and use it all the time. I am
opposed to people being seduced by the steady-state solution into believing
there is not such thing as forward and reflected waves even though standing
waves require forward and reflected waves.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
I am
opposed to people being seduced by the steady-state solution into believing
there is not such thing as forward and reflected waves even though standing
waves require forward and reflected waves.

I also apologize if this particular dose of reality brought this thread to
a screeching halt.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
I am
opposed to people being seduced by the steady-state solution into believing
there is not such thing as forward and reflected waves even though standing
waves require forward and reflected waves.

Say that in Sacramento, and some knucklehead legislator will pass a law
against it.

73, Jim AC6XG

Cecil, W5DXP wrote:
"There are only two possible directions, forward and reverse in which
energy can flow. Multiple reflections do not create any more
directions."

True. Further, all the same-frequency, same-direction signals merge. So,
as Cecil said, there are only two same-frequency signals on a
transmission line, forward and reverse. The interference pattern these
signals produce does not represent another signal.

Trying to use ordinary circuit analysis on standing-wave antennas is
problematic, but it`s been tried in this thread. Here is what R.W.P.
King wrote in "Transmission Lines, Antennas, and Wave Guides", King,
Mimno, and Wing, 1945, on page 86:
"Inductance and capacitance as used in near-zone circuits with uniform
current cannot be defined, and ordinary circuit analysis does not
apply." This has not stopped efforts in this thread to analyze LC
circuits as if we were dealing with low frequencies.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Cecil, W5DXP wrote:
"There are only two possible directions, forward and reverse in which
energy can flow. Multiple reflections do not create any more
directions."

True. Further, all the same-frequency, same-direction signals merge. So,
as Cecil said, there are only two same-frequency signals on a
transmission line, forward and reverse. The interference pattern these
signals produce does not represent another signal.

So the claim is that the amplitude of the reflection being bandied about
is the sum of multiple reflections? I haven't seen any indication of
that. It appears to be simply the amplitude of the first reflection.
At least, that's the way it appears to me, the uninitiated.

73, Jim AC6XG

Jim Kelley wrote:
So the claim is that the amplitude of the reflection being bandied about
is the sum of multiple reflections? I haven't seen any indication of
that. It appears to be simply the amplitude of the first reflection.
At least, that's the way it appears to me, the uninitiated.

Shirley, you jest. Reference pages 17-20 in _T-Lines & Networks_ by Johnson.
Consider a T-line with an SWR of 5.8284:1 and a constant 100W Z0-matched
source. The forward power will be 100W, 150W, 175W, 187.5W, 193.75W,
196.875W, 198.4375W, ..., 200W. After steady-state is reached, the
reflected power is a constant 100W (sans modulation and noise).
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Harrison wrote:
Trying to use ordinary circuit analysis on standing-wave antennas is
problematic, but it`s been tried in this thread. Here is what R.W.P.
King wrote in "Transmission Lines, Antennas, and Wave Guides", King,
Mimno, and Wing, 1945, on page 86:
"Inductance and capacitance as used in near-zone circuits with uniform
current cannot be defined, and ordinary circuit analysis does not
apply." This has not stopped efforts in this thread to analyze LC
circuits as if we were dealing with low frequencies.

Very true. The basic problem, as I see it, is in assuming that the standing-
wave current only has one component. For standing-wave antennas, the
standing-wave current must necessarily have two components, If and Ir, as
explained by Balanis in _Antenna_Theory_, (page 489, 2nd edition).

In general, when forward waves and reflected waves exist in the circuit,
lumped circuit analysis fails and distributed network analysis is the only
method that yields the correct result. So even if one allows that the forward
current magnitude is constant through a coil and the reflected current
magnitude is constant through a coil, the net current magnitude will not
be constant because of the phase differences in the two superposed currents
at each end of the coil.

I've been told by the gurus that I can ignore the forward and reflected
waves and still obtain the correct steady-state solution. A mobile bugcatcher
coil on 75m seems to disprove that assertion. But I have been called a
"Grasshopper" and thus apparently have a lot yet to learn.
--
73, Cecil http://www.qsl.net/w5dxp



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