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