How can one couple this ?
A coaxcable with a dummyload: runningwaves everywhere and U and I are in phase.
Now the resonant dipole: the U peaks at the ends end I tops in the midle. So very reactive
for the driver. Should be. Now the dipole is coupled at the coax instead of the 'inphase' load
and, oh wonder, the coax cable doesn't notice the difference ?? The mind boggles.

Calltrex wrote:

How can one couple this ?
A coaxcable with a dummyload: runningwaves everywhere and U and I are in
phase.
Now the resonant dipole: the U peaks at the ends end I tops in the
midle. So very reactive
for the driver. Should be. Now the dipole is coupled at the coax instead
of the 'inphase' load
and, oh wonder, the coax cable doesn't notice the difference ?? The
mind boggles.

Indeed...

Calltrex wrote:

How can one couple this ?
A coaxcable with a dummyload: runningwaves everywhere and U and I are in
phase.
Now the resonant dipole: the U peaks at the ends end I tops in the
midle. So very reactive for the driver.

Nope, you are confused, at least about resonant standing
wave antennas like the 1/2WL dipole. Those peaks and nodes
of the voltage and current are *AMPLITUDES*. Amplitudes have
nothing to do with reactance. To detect the reactance, one
must look at the *PHASE*. You are not looking at the phase.

Instead of looking at the amplitudes of the voltage and
current, take a look at the phase of the voltage and current.
The phase angle which determines the reactance is the difference
between the voltage phase angle and the current phase angle.

Hint: The phase angles of the standing waves on a standing wave
antenna (like a 1/2WL dipole) don't change over the entire
length of the 1/2WL dipole. The standing wave is approximately
90% of the total wave on a 1/2WL dipole so the phase angle
of the total wave on the antenna changes very little from
end to end.

Should be. Now the dipole is coupled at the coax instead
of the 'inphase' load
and, oh wonder, the coax cable doesn't notice the difference ?? The
mind boggles.

At the antenna feedpoint, for a resonant antenna, the total
current and total voltage are in phase so the resulting
impedance is *purely resistive, not reactive*.

There is a forward wave at the feedpoint which, in a 1/2WL
dipole, travels to the end of the antenna and is reflected.
At the reflection point, the forward voltage and reflected
voltage do not undergo a phase shift but the forward current
and reflected current are 180 degrees out of phase at the
reflection point.

Bottom line is that the reflection phasor adds to the forward
phasor after a 180 degree round trip. In a 1/2WL dipole, Vfor
is 180 degrees out of phase with Vref and Ifor is in phase with
Iref. Assuming a zero degree reference, Vfor is at zero degrees
and Vref is at 180 degrees. Ifor and Iref are both at zero
degrees. Since everything is in phase or 180 degrees out of
phase, we don't need any trig. The feedpoint impedance of a
resonant 1/2WL dipole becomes a *magnitude only* calculation:

1/2WL Zfp = (Vfor-Vref)/(Ifor+Iref)

The negative sign on Vref takes care of the 180 degree phase
shift. The feedpoint impedance of a resonant one wavelength
dipole is also a *magnitude only* calculation but the signs
of the magnitudes change because of the extra 180 degree delay:

1WL Zfp = (Vfor+Vref)/(Ifor-Iref)

It's easy to see why the feedpoint impedance of a 1WL dipole
is so much higher than it is for a 1/2WL dipole. The reflected
voltage and current are delayed by an additional 180 degrees.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

"Cecil Moore" wrote
Calltrex wrote:

How can one couple this ?
A coaxcable with a dummyload: runningwaves everywhere and U and I are in phase.
Now the resonant dipole: the U peaks at the ends end I tops in the midle. So very reactive for the driver.

Nope, you are confused, at least about resonant standing
wave antennas like the 1/2WL dipole. Those peaks and nodes
of the voltage and current are *AMPLITUDES*. Amplitudes have
nothing to do with reactance. To detect the reactance, one
must look at the *PHASE*. You are not looking at the phase.


+ +
+
+
+
+
+
====================+============================ ½ dipole
+
+
+
+
+
+
+
+ +
voltage


If what you say is true then why draws every antennabook the voltages like above?
We all know that an amplitude can not be negative in value! So all books are wrong?
And could you keep the answer at amateur levels pls?

Calltrex wrote:
If what you say is true then why draws every antennabook the voltages
like above?
We all know that an amplitude can not be negative in value!

You would probably agree that the battery voltage
amplitude in your vehicle is +12 volts.

I once had a 1950 Dodge where the amplitude of the
battery voltage was -12 volts.

The instantaneous amplitude of the AC voltage out of
your wall socket at home goes negative every 60 Hz cycle.

So exactly why cannot voltage amplitudes be negative?

Changing the phase of an AC voltage by 180 degrees
changes the amplitude from positive to negative or
from negative to positive. That's what is happening
in the ASCII graphic that you drew.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

"Cecil Moore" wrote

Changing the phase of an AC voltage by 180 degrees
changes the amplitude from positive to negative or
from negative to positive. That's what is happening
in the ASCII graphic that you drew.
--
73, Cecil, IEEE

o.k. i was a bit to fast. But if we see the complete pictu
(hope it comes across in the original bits)




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We see here, as in all antennabooks, at the leftside the voltage and current are in phase,
But in the right side the voltage and current are in antiphase, hence my conclusion that
the antenna must be reactive ! From your explanation at 180 degrees, why doesn't
current at the right side flip over then ? Should be.

[Murphy at work ?] Here is another try of the basic grafic.



3
XM5;. ..:;S9A#@@@@@@@@@@@@@@@@@@@@#AXi;,H.
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Calltrex wrote:
We see here, as in all antennabooks, at the leftside the
voltage and current are in phase,

Those plots are for a *standing-wave antenna*. What you
are missing is that there is no phase shown in those plots.
Pure standing wave voltage and and pure standing wave current
have a constant phase. What you have plotted is a snapshot in
time of voltage and current *amplitude envelopes* in which the
phase is irrelevant. Those plots are not time domain plots.
They are plots of the voltage *envelope* and current *envelope*.

But in the right side the voltage and current are in
antiphase, hence my conclusion that

180 degree "antiphase" is still purely resistive with zero
reactance. All that has happened to the voltage is that
the sign of the voltage has changed. It is a snapshot in
time. 1/2 cycle later that same plot would be upside down.

the antenna must be reactive ! From your explanation at 180
degrees, why doesn't
current at the right side flip over then ? Should be.

When all the voltages and currents are either in phase or
180 degrees out of phase, the resulting impedance is purely
resistive. There is *no reactance* in an ideal resonant dipole!
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Calltrex wrote:

+ +
+
+
+
+
+
====================+============================ ½ dipole
+
+
+
+

If what you say is true then why draws every antennabook the voltages
like above?
We all know that an amplitude can not be negative in value! So all
books are wrong?
And could you keep the answer at amateur levels pls?

I can't answer for "every antennabook" except to say that any book
showing a graph like that and claiming it's a graph of antenna voltage
is wrong. As Tom K7ITM recently pointed out, you can't determine a
voltage at some point along the wire, as implied by the graph. A voltage
only exists *between* two points, and in the the presence of the fields
around an antenna, the voltage between two points also depends on the
path you take between them -- conceptually, it depends on how you
position your meter leads. You *can* find the strength of the E field
near various points along the antenna (and it looks pretty much like the
graph), but that's not the same as a voltage.

A resonant antenna is one having a feedpoint impedance that's purely
resistive, that is, it has no reactance. This impedance is the feedpoint
voltage divided by the feedpoint current; the feedpoint voltage is the
voltage between the two terminals. The reactance is zero only if the
feedpoint voltage and current are exactly in phase, and regardless of
their amplitudes.

If the terminals are very far apart in terms of wavelength, you have the
same problem in measuring or even defining voltage between them as you
do with points along the antenna. So the common definition makes the
assumption that the feedpoint terminals are very close together.

Roy Lewallen, W7EL

Roy Lewallen wrote:
You *can* find the strength of the E field
near various points along the antenna (and it looks pretty much like the
graph), but that's not the same as a voltage.

The assumption is that the voltage is proportional to
the E-field even when the voltage is difficult to measure.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com