RadioBanter - Antenna Reactance Question

RadioBanter (https://www.radiobanter.com/)

- Antenna (https://www.radiobanter.com/antenna/)

- - Antenna Reactance Question (https://www.radiobanter.com/antenna/1470-antenna-reactance-question.html)

Antenna Reactance Question

In my dipole/vertical modeling and analyzer measurements, as frequency
is increased past quarter-wave resonance, I've noticed with interest
that both reactance and resistance peak at different times, as they
increase with frequency toward the half-wave point.

Resistance peaks at approximately the half-wave point as expected, but
inductive reactance always peaks a little earlier (lower frequency).
This is also indicated in the ARRL Antenna Book in Figures 3 through 5
on pages 2-3 and 2-4 (it's the bulging on the right side of each of
the curves).

For a given antenna of particular length, adding inductive or
capacitive reactance changes the magnitude of the reactance peak, but
not the frequency at which it occurs.

Changing the thickness of the radiating elements changes (lowers) the
frequency at which the reactance peak occurs, but it also changes
(lowers) the frequency at which resistance peaks, and the difference
in these two freqencies stays approximately the same.

Why does the reactance peak occur slightly earlier than
half-wavelength? Can it be mathmatically predicted/explained? Any help
would be appreciated.

Al, WA4GKQ

alhearn wrote:
Why does the reactance peak occur slightly earlier than
half-wavelength?

Since the monopole is purely resistive around 1/4WL and
around 1/2WL, i.e. the reactance is zero at those two
points, it is simply impossible for it to be be any
other way.
--
73, Cecil http://www.qsl.net/w5dxp

-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

Al, WA4GKO wrote:
"Why does the reactance peak earlier than half-wavelength? Can it be
mathematically prtedicted / explained?"

At resonance, phase shifts abruptly from leading to lagging or vice
versa as resonance is crossed.

The resonant length depends on wave velocity along the antenna wire and
is a function of how thin the wire is. Resonant length is also shortened
by increased capacitive effect caused by the voltage pile up at the
open-circuit wire end. There are formulas involving wire length to
periphery ratio but these values have been already calculated and
plotted on convenient charts.

My 19th edition of the ARRL Antenna Book has Fig 8 on page 2-5 which
gives values from 94% to 98% of free-space wavelength for a range of
1/2-wavelength to conductor diameter ratios. Often the antenna is
shortened to 95% for "end effects" to avoid formulas and charts and in
most cases this is good enough.

Best regards, Richard Harrison, KB5WZI

Alhearn

If it is "understanding" that you want, I'd suggest you plot the antenna's
impedance on a Smith Chart. A plot of an antenna's impedance with varying
frequency will describe a continuous curve. Beyond that, any additional
comments would probably just make this post more confusing. I'd bet you can
answer your own question after looking at a Smith Chart.
Note the chart identifies R+/-jX. So the reactance added in series will
move an impedance along lines of constant R.
The path of the impedance plot resulting from shunt X can also be
identified, but maybe thats for another time for discussion

Jerry

"alhearn" wrote in message
om...
In my dipole/vertical modeling and analyzer measurements, as frequency
is increased past quarter-wave resonance, I've noticed with interest
that both reactance and resistance peak at different times, as they
increase with frequency toward the half-wave point.

Resistance peaks at approximately the half-wave point as expected, but
inductive reactance always peaks a little earlier (lower frequency).
This is also indicated in the ARRL Antenna Book in Figures 3 through 5
on pages 2-3 and 2-4 (it's the bulging on the right side of each of
the curves).

For a given antenna of particular length, adding inductive or
capacitive reactance changes the magnitude of the reactance peak, but
not the frequency at which it occurs.

Changing the thickness of the radiating elements changes (lowers) the
frequency at which the reactance peak occurs, but it also changes
(lowers) the frequency at which resistance peaks, and the difference
in these two freqencies stays approximately the same.

Why does the reactance peak occur slightly earlier than
half-wavelength? Can it be mathmatically predicted/explained? Any help
would be appreciated.

Al, WA4GKQ

Cecil Moore wrote ..
Since the monopole is purely resistive around 1/4WL and
around 1/2WL, i.e. the reactance is zero at those two
points, it is simply impossible for it to be be any
other way.

That's very true, but between the 1/4WL and 1/2WL zero-reactance
points, reactance increases with frequency, peaks, and then begins to
decrease to cross zero again at the 1/2WL point. This peak is NOT
half-way between 1/4WL and 1/2WL, but skewed heavily toward the 1/2WL
point. My question is what determines where that peak occurs?

Can the reactance peak be somehow adjusted (frequency position, not
magnitude) for tuning purposes? Magnitude is easily adjusted by simply
adding inductive or capacitive reactance, which doesn't change the
shape or peak position of the reactance curve, but simply moves the
curve up or down to change the points at which it crosses zero
(resonance frequency). Controlling the reactance peak position could
do the same.

The answer may be simple and common knowledge, but I haven't found it.

Al

alhearn wrote:
My question is what determines where that peak occurs?

Mathematically, it will be where the SWR circle is tangent
to the reactance arc. The higher the SWR, the higher the
maximum possible reactance and the closer it is to the
anti-resonance point.
--
73, Cecil http://www.qsl.net/w5dxp

-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

Cecil Moore wrote in message ...
alhearn wrote:
My question is what determines where that peak occurs?

Mathematically, it will be where the SWR circle is tangent
to the reactance arc.

That is not in general true; do you have reason to believe it's true
for the impedance of a dipole? Consider what happens when you change
the reference impedance for the SWR measurement.

The higher the SWR, the higher the
maximum possible reactance and the closer it is to the
anti-resonance point.

What's the reactance at the anti-resonance point? Is highest SWR at
anti-resonance, or at maximum reactance, or at some point between?

Tom Bruhns wrote:
Cecil Moore wrote:

alhearn wrote:
My question is what determines where that peak occurs?

Mathematically, it will be where the SWR circle is tangent
to the reactance arc.

That is not in general true; do you have reason to believe it's true
for the impedance of a dipole?

When the dipole is at maximum feedpoint reactance, can the SWR be
calculated? Of course. Will that point, when plotted on a Smith Chart
lie on the SWR circle? Of course. Will it also lie on a reactance arc?
of course. Will the SWR circle be tangent to that reactance arc?
Of course. Will the un-normalized value of the maximum feedpoint
reactance be constant? Of course, assuming nothing changes except Z0.

Consider what happens when you change
the reference impedance for the SWR measurement.

It doesn't matter. The Smith Chart reference changes and therefore
the SWR changes but *the value of the antenna feedpoint impedance
stays the same*. The new SWR circle is still tangent to the reactance
arc at the same value of un-normalized reactance even though the
normalized reactance value has changed. Xmax/Z01 is different from
Xmax/Z02 but Xmax has not changed (assuming Z0 is the only change).

The higher the SWR, the higher the
maximum possible reactance and the closer it is to the
anti-resonance point.

What's the reactance at the anti-resonance point?

Always zero, by definition. Examples: A full-wave center-fed dipole
or a half-wave end-fed monopole. ("Anti-resonant" is what we called
such antennas at Texas A&M 50 years ago.)

Is highest SWR at
anti-resonance, or at maximum reactance, or at some point between?

If the Z0 of your transmission line is equal to the feedpoint impedance
at anti-resonance, the lowest SWR will occur at anti-resonance. If the
Z0 of your transmission line is equal to the feedpoint impedance at
maximum reactance, the lowest SWR will occur at maximum reactance. :-)

Depends upon the Z0 of the transmission line. If you choose a Z0 that
is the square root of the resonant impedance times the anti-resonant
impedance, the SWR at resonance and anti-resonance will be the same.
For a dipole that Z0 value is usually between 450 ohms and 600 ohms.
That's why anti-resonance is no problem for ladder-line/open-wire.
--
73, Cecil http://www.qsl.net/w5dxp

-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

Cec, & Co.

Zo of an antenna wire seems to be an important parameter in your
discussions. Knowledge of its value would appear to be essential before
continuing with calculations. Otherwise nobody will get nowhere.

So how is the value of Zo obtained (without bringing Terman et al into it)?
----
Reg, G4FGQ