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

"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.



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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

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

"Cecil Moore" wrote the message

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.

I agree that the antenna is a chain of different pure resistances. But that
doesn't explain the asymmetric 'snapshot' here.
At the left side we see '+' voltage and '+' current and
at the right side we see '-' voltage and '+' current ! Why should it be asymmetric?
When i put a car battery with the plus on a lamp i will see a minus current ??

On Apr 18, 3:58*am, "Calltrex" wrote:
"Cecil Moore" *wrote the message

* * * * * 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.

I agree that the antenna is a chain of different pure resistances. But that
doesn't explain the asymmetric 'snapshot' here.
At the left side we see '+' voltage and '+' current and
at the right side we see '-' voltage and '+' current ! Why should it be asymmetric?
When i put a car battery with the plus on a lamp i will see a minus current ??

Never the twain will meet !
What you are debating is the difference between resonance relative to
Earth and that relative to the Cosmos. Thus, according to Einstein, it
is a case of relativity or viewing point. Laws of Newton refer to
equilibrium where all forces are accounted for and balanced thus for
cosmic resonance the "period" or the full wavelength is the true
balance. Thus when dealing with science or the cosmos half a period or
a halfwave
the position is indeterminate. ( Simple relativity my dear Watson!)
Art

"Art Unwin" wrote in message
...
Never the twain will meet !

and never shall art understand or be able to explain them.

What you are debating is the difference between resonance relative to
Earth and that relative to the Cosmos.

time for your meditation art... ommmmmmmmmmmmmmmmmmmmmmm...

"Calltrex" wrote in message
...

"Cecil Moore" wrote the message

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.

I agree that the antenna is a chain of different pure resistances. But
that
doesn't explain the asymmetric 'snapshot' here.
At the left side we see '+' voltage and '+' current and
at the right side we see '-' voltage and '+' current ! Why should it be
asymmetric?
When i put a car battery with the plus on a lamp i will see a minus
current ??


Hi Calltrex

Now I see your situation. When you want to see the current meter read
plus instead of minus, reverse the leads on the current meter. Then, when
the voltmeter reads plus, the current meter will also read plus.

Jerry KD6JDJ

Calltrex wrote:

"Cecil Moore" wrote the message
There is *no reactance* in an ideal resonant dipole!

I agree that the antenna is a chain of different pure resistances. But that
doesn't explain the asymmetric 'snapshot' here.

My grandsons must have interrupted and distracted me on
that earlier posting. I should have said there is no
extra reactance associated with different signs on
the voltage and current than there is when they have
the same sign. Any reactance on the left side has its
mirror image on the right side.

At the left side we see '+' voltage and '+' current and
at the right side we see '-' voltage and '+' current ! Why should it be
asymmetric?

It is asymmetric because the voltage envelope and current
envelope are spatially displaced by 90 degrees. If you
plot the voltage and current envelopes for 3 wavelengths
you will get the following - pardon the ASCII. One is the
sinusoidal voltage envelope and the other is the sinusoidal
current envelope.

/ \ / \ / \ /
/ \ / \ / \ /
------------------------------------------------------
\ / \ / \ /
\ / \ / \ /

/ \ / \ / \ /
/ \ / \ / \ /
------------------------------------------------------
/ \ / \ / \ /
\ / \ / \ /

Sometimes voltage and current have the same signs and sometimes
they have opposite signs. Remember, this is a snapshot in
time. 1/2 cycle later the envelopes will be upside down.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com