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

In article ,
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.

I believe you are mistaken here.

A resonant antenna is *defined* by the fact that the voltage and
current are in-phase at each point along the antenna, and thus the
impedance at each point is purely resistive. A resonant antenna does
*not* present a reactive impedance to the driver.

Should be.

Isn't.

In a resonant dipole, it's true that the *ratio* of the voltage to
current varies as you move along the antenna from one point to the
next. However, at every point you look, the current (at that point)
and the voltage (at that point) are in phase with one another, and so
the impedance (at that point) is purely resistive.

The fact that the ratio changes, simply means that the impedance is
different (but still resistive).

For example, a resonant dipole (typically just a hair shorter than 1/2
wavelength) in free space has an impedance at the center of around 70
ohms. As you move out towards one end or the other, the impedance
rises... but remains entirely resistive. If you want, you can (for
example) feed such a dipole off-center, with a 300-ohm twinlead or a
450-ohm window line... if you move out far enough from the center
you'll find a point at which the dipole presents an exact match to
such a feedline.

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.

Only because you have an incorrect assumption here. The fact that the
*amplitudes* of the voltage and current vary in different directions
as you move out from the center (voltage increases and current
decreases) doesn't mean that the voltage and current waveforms at any
given point are out of phase... they aren't.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Apr 13, 12:40*pm, (Dave Platt) wrote:
In article ,

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.

I believe you are mistaken here.

A resonant antenna is *defined* by the fact that the voltage and
current are in-phase at each point along the antenna, and thus the
impedance at each point is purely resistive. *A resonant antenna does
*not* present a reactive impedance to the driver.

* * * * * * Should be.

Isn't.

In a resonant dipole, it's true that the *ratio* of the voltage to
current varies as you move along the antenna from one point to the
next. *However, at every point you look, the current (at that point)
and the voltage (at that point) are in phase with one another, and so
the impedance (at that point) is purely resistive.

The fact that the ratio changes, simply means that the impedance is
different (but still resistive).

For example, a resonant dipole (typically just a hair shorter than 1/2
wavelength) in free space has an impedance at the center of around 70
ohms. *As you move out towards one end or the other, the impedance
rises... but remains entirely resistive. *If you want, you can (for
example) feed such a dipole off-center, with a 300-ohm twinlead or a
450-ohm window line... if you move out far enough from the center
you'll find a point at which the dipole presents an exact match to
such a feedline.

* * * * * * * * 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.

Only because you have an incorrect assumption here. *The fact that the
*amplitudes* of the voltage and current vary in different directions
as you move out from the center (voltage increases and current
decreases) doesn't mean that the voltage and current waveforms at any
given point are out of phase... they aren't.

--
Dave Platt * * * * * * * * * * * * * * * * * AE6EO
Friends of Jade Warrior home page: *http://www.radagast.org/jade-warrior
* I do _not_ wish to receive unsolicited commercial email, and I will
* * *boycott any company which has the gall to send me such ads!

I understand "electric field strength" at a point, but I need a little
help with "voltage at a point (along the antenna conductor,
presumably). Voltage relative to what? Measured along what path?
It's usually pretty easy to get agreement about voltage at a
feedpoint, but things are much less clear away from that.

In any event, the only thing that matters with respect to the load on
a feedline is the impedance (that is, voltage divided by current) at
the feedpoint; the current at other points in the antenna does not
need to be in phase with that feedpoint current to yield a purely
resistive feedpoint impedance.

Cheers,
Tom

K7ITM wrote:
... the current at other points in the antenna does not
need to be in phase with that feedpoint current to yield a purely
resistive feedpoint impedance.

It indeed "does not need to be in phase with that
feedpoint current" but EZNEC says it is pretty
close to being in phase.

Here are the currents in 10 segments of a 1/4WL
monopole. The current phase varies by only 2.62
degrees in 90 degrees of antenna.

EZNEC+ ver. 4.0
Vertical over real ground 4/13/2009 9:17:57 PM
--------------- CURRENT DATA ---------------
Frequency = 7.2 MHz

Wire No. 1:
Segment Conn Magnitude (A.) Phase (Deg.)
1 Ground 1 0.00
2 .97418 -0.40
3 .92577 -0.80
4 .85611 -1.14
5 .76657 -1.44
6 .65894 -1.71
7 .53532 -1.96
8 .39796 -2.20
9 .24889 -2.42
10 Open .08785 -2.62

--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

On Apr 13, 4:58*pm, K7ITM wrote:
On Apr 13, 12:40*pm, (Dave Platt) wrote:



In article ,

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.

I believe you are mistaken here.

A resonant antenna is *defined* by the fact that the voltage and
current are in-phase at each point along the antenna, and thus the
impedance at each point is purely resistive. *A resonant antenna does
*not* present a reactive impedance to the driver.

* * * * * * Should be.

Isn't.

In a resonant dipole, it's true that the *ratio* of the voltage to
current varies as you move along the antenna from one point to the
next. *However, at every point you look, the current (at that point)
and the voltage (at that point) are in phase with one another, and so
the impedance (at that point) is purely resistive.

The fact that the ratio changes, simply means that the impedance is
different (but still resistive).

For example, a resonant dipole (typically just a hair shorter than 1/2
wavelength) in free space has an impedance at the center of around 70
ohms. *As you move out towards one end or the other, the impedance
rises... but remains entirely resistive. *If you want, you can (for
example) feed such a dipole off-center, with a 300-ohm twinlead or a
450-ohm window line... if you move out far enough from the center
you'll find a point at which the dipole presents an exact match to
such a feedline.

* * * * * * * * 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.

Only because you have an incorrect assumption here. *The fact that the
*amplitudes* of the voltage and current vary in different directions
as you move out from the center (voltage increases and current
decreases) doesn't mean that the voltage and current waveforms at any
given point are out of phase... they aren't.

--
Dave Platt * * * * * * * * * * * * * * * * * AE6EO
Friends of Jade Warrior home page: *http://www.radagast.org/jade-warrior
* I do _not_ wish to receive unsolicited commercial email, and I will
* * *boycott any company which has the gall to send me such ads!

I understand "electric field strength" at a point, but I need a little
help with "voltage at a point (along the antenna conductor,
presumably). *Voltage relative to what? *Measured along what path?
It's usually pretty easy to get agreement about voltage at a
feedpoint, but things are much less clear away from that.

In any event, the only thing that matters with respect to the load on
a feedline is the impedance (that is, voltage divided by current) at
the feedpoint; the current at other points in the antenna does not
need to be in phase with that feedpoint current to yield a purely
resistive feedpoint impedance.

Cheers,
Tom

A bit more about this...

If you model a center-fed dipole made of thin wire (say wire diameter
about 1/1000 of a wavelength) at half-wave resonance, you should see
that the current toward the ends of the wire lags the current at the
center by a few degrees. If you increase the wire diameter (to say
1/100 of a wavelength) and model at (the new) resonance, you should
see that the current toward the ends of the wire lags the current at
the center by considerably more than with the thin wire, perhaps about
double the lag. In both cases, the feedpoint impedance is by
definition nonreactive.

Now if you model each of these antennas at an operating frequency 90%
of the half-wave-resonant frequency, you should see that the phase at
the ends of the wire lags by much less than it did at resonance, but
now the feedpoint impedance is quite reactive: the reactance should
be considerably more than the resistance in the case of the thin wire
antenna, and the resistance and reactance should be similar in the
case of the thicker wire.

Enlighten yourself further by modelling these two antennas fed at
frequencies near full wave resonance and near the resonance associated
with 1.5 waves antenna length. You will see that the phase along the
wire changes considerably more than when the antenna is operated at or
near half-wave resonance, but that the frequency can be adjusted so
that the feedpoint impedance is non-reactive. You'll also notice, if
your model has divided the wire into enough segments, that the phase
along the wire does not change abruptly as you pass through a current
node or antinode.

The conclusion I draw is that it's fruitless to worry about the phase
of the current along the wire when you're considering the feedpoint
impedance. It may be very interesting to consider the phase of the
current on the wire for other reasons, but not for the reason of
adjusting the feedpoint impedance.

Cheers,
Tom

K7ITM wrote:
It may be very interesting to consider the phase of the
current on the wire for other reasons, but not for the reason of
adjusting the feedpoint impedance.

Any good antenna book will present the equations for voltage
and current on a lossless thin-wire dipole. Those equations
disagree with your conclusions.
--
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