nec simulation - unexpected result ??
 

If the feed is 'changing the resonance' then there is a problem with the
feed!!

Richard Clark wrote:

On Fri, 24 Feb 2006 11:48:22 -0500, Amos Keag
wrote:


Resonance has NOTHING to do with impedance. Resonance is resonance; it
has a harmonic response.


Hi Amos,

Resonance is the absence of reactance, or more properly its term is 0.
As reactance is fully part of the specification to impedance,
resonance has a very unique relation: r ±j0.

You can take a dipole that exhibits this unique characteristic at
regular intervals of frequency - notably at harmonics (in a perfect
world, not so necessarily in life). You can also take that same
length of wire and shift the feedpoint such that its resonance (still
that same characteristic loss of X with some remaining R) changes in
frequency - as does the spectrum of other resonances which are
sometimes no longer related by harmonics.

Taking as an example, an 11 segment 3mm wire 37.9M long in free space
and feed it in the conventional way (in the middle) and its resonances
may be observed at:
3.8 MHz¹
7.95 MHz²
11.75 MHz¹
16.25 MHz²
19.65 MHz¹
24.65 MHz²
27.55 MHz¹

Or feed it at 68% along its length (or segment 8) and observe:
3.8 MHz¹ (with a Higher R as I had incorrectly argued with Dan)
5.65 MHz²
7.85 MHz¹
12.45 MHz²
15.65 MHz¹
17.55 MHz²
19.65 MHz¹
25.55 MHz²
27.55 MHz¹

where strictly speaking MHz¹ is resonance and MHz² is anti-resonance

A curious property has emerged, we now have 9 resonances (speaking
largely) where formerly we had 7 in exactly the same span of frequency
for the same piece of wire. Further, we also have the anti-resonance
of the standard dipole at 8 MHz replaced by a resonance in the OCF
dipole.

To roll back the calendar 10 years or so, this is also the hallmark of
fractal antennas in that they exhibit more resonances than found in
"conventional" dipoles.

There are certain lengths of wire, with certain offsets of feed that
offer fairly good overlaps with Ham Bands that are not otherwise found
in common dipoles. I am at a loss to specify those "certain"
characteristics, and it is arguable that feeding an offset dipole can
be successfully achieved without some effort in isolating (choking)
the feedpoint from the driveline - a distasteful reality conveniently
discarded in modeling.

73's
Richard Clark, KB7QHC

nec simulation - unexpected result ??
 

Dot wrote:
Looking at your definitions I would suggest that "resonance" is really the
point at which the antenna mimics a series resonant circuit, exhibiting a
low impedence and "anti-resonance" is the point at which it mimics a
parallel resonant circuit, exhibiting a high impedence.

True, but I cannot take credit for the definition which comes from
"Transmission Lines and Networks", by Walter C. Johnson, PhD, guru
and chairman of the Department of Electrical Engineering, Princeton
University during the 1940's and 1950's.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

Purely resistive load = resonance ............ PERIOD

Cecil Moore wrote:

Owen Duffy wrote:

On that basis, one would have to say that a full wave centre fed
dipole exhibits (at the feed point) resonance similar to a lossy
parallel tuned circuit and should be considered a resonant radiator.


I know that is what your gut feeling wishes were true. But a
large portion of the RF engineering community considers "anti-
resonance" to be the exact opposite of "resonance" and indeed
it is the exact opposite on a Smith Chart, being the opposite
side of the SWR circle. Semantics strikes again. I'm sure that
our Russian counterparts have a completely different word for
exactly the same effects.

nec simulation - unexpected result ??
 

Cecil Moore wrote:
In a transmission line with reflections, antiresonance is
indeed plus or minus 90 degrees from resonance and "never
the twain shall meet". Resonance and antiresonance cannot,
by definition, occur at the same point, i.e. if a point
is antiresonant, it cannot, by definition, be resonant.

One more thought: In a transmission line with reflections,
a voltage node is located at a point of resonance. A voltage
anti-node is located at a point of anti-resonance. Makes
perfect sense to me.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

Amos Keag wrote:
If the feed is 'changing the resonance' then there is a problem with the
feed!!

Not at all. As you can see at: http://www.qsl.net/w5dxp/notuner.htm,
I use "the feed" for the specific purpose of changing the resonant
frequency of the antenna system. The impedance transforming series-
section is really a series stub which resonants the entire antenna
system.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

On Fri, 24 Feb 2006 20:12:42 -0500, Amos Keag
wrote:

If the feed is 'changing the resonance' then there is a problem with the
feed!!


Hi Amos,

Where do you see that in the data? Or are you mis-interpreting the
distinction between the choice of the feed point with attaching a feed
line? The model distinctly lacks a feed line, or may be presumed to
have a feed line that is completely isolated from the antenna(s).

The wire retains its original fundamental resonance - within a couple
dozen KHz, a negligible difference. The remainder of its resonances
are strictly governed by the selection of the feed point's position
along the length of the wire. I incorrectly argued for a lower Z with
Dan earlier. By and large, moving away from the center raises the Z
(principally R at resonances above the fundamental).

73's
Richard Clark, KB7QHC

nec simulation - unexpected result ??
 

Dot wrote:
On Fri, 24 Feb 2006 22:41:54 GMT, Cecil Moore wrote:

Roy Lewallen wrote:
Resonance has everything to do with impedance. Resonance is defined as
any frequency at which the impedance is purely resistive; ...
In the distant past, when I had a dinosaur for a pet, resonance
was defined as the frequency at which the impedance is a purely
low impedance. The frequency at which the impedance was a
purely high resistance was known at the anti-resonant point,
the exact opposite of resonance, and indeed, it was the exact
other side of the SWR circle on a Smith Chart.

These days, resonance is described as either:

a) the point at which Inductive Reactance and Capacitive Reactance are equal
or
b) the point at which a load impedence is purely resistive.

The two points are exactly the same.

Looking at your definitions I would suggest that "resonance" is really the
point at which the antenna mimics a series resonant circuit, exhibiting a
low impedence and "anti-resonance" is the point at which it mimics a
parallel resonant circuit, exhibiting a high impedence.

The high-impedance full-wave resonant point (for a dipole; half-wave
resonant point for a monopole) is sometimes called "anti-resonance", but
not commonly, and mostly in older literature. It's a true point of
resonance, that is, where the reactance is zero. I don't believe I've
ever heard the term "anti-resonance" applied to other high-impedance
resonant circuits, such as a tank circuit.

It would then be reasonable for a given wire perpendicular to a good ground
plane to exhibit "resonance" at odd multiples of a quarter wavelength and
"anti-resonance" at even multiples of a quarter wavelength... Translating
gives low impedence at odds and high impedence at evens, which is where I
started out in this discussion....

If you choose to call the high-impedance resonant points
"anti-resonance", that's true. But again, they're points where the
reactance is zero, just like the points you're calling "resonant". The
only difference is that the impedance is high and the antenna acts more
like a parallel tuned circuit at nearby frequencies rather than a series
tuned circuit.

Your semantics is correct if you are looking to define an antenna as "a
current fed device", but that's not always the case. There are end fed half
waves out there... they are voltage fed, they are resonant and they do work.
(Ask anyone who owns a "Ringo Ranger".)

No, the definition of resonance has nothing to do with how an antenna is
fed. The impedance of the antenna doesn't change with the feed method
(assuming of course that it has a single feed point), and therefore its
resonant frequencies don't change with the feed method. (You can, of
course, alter the resonant frequencies of an antenna *system* by adding
reactance at the feedpoint or elsewhere.) And an antenna doesn't have to
be resonant (that is, have a non-reactive feedpoint impedance) to
"work". Resonance is only an indication of the reactance of the input
impedance, and has nothing to do with an antenna's gain, pattern,
bandwidth, or other performance characteristics.

Roy Lewallen, W7EL

nec simulation - unexpected result ??
 

Roy Lewallen wrote:
The impedance of the antenna doesn't change with the feed method ...

One feed method is center feed. Another feed method is off-center feed.
The feedpoint impedance of the antenna changes with position since
for 1/2WL, for instance, the net voltage is a sine wave referenced
to the center, and the net current is a cosine wave referenced to the
center. The feedpoint impedance is approximately sin(x)/cos(x)=tan(x)
where 'x' is the number of degrees away from center.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

Geez, Cecil, I don't have an argument with either of you. I'm just
telling you that the people I work with qualify resonance with
different terms than you do. You're welcome to use whatever terms you
want.

Cheers,
Tom

nec simulation - unexpected result ??
 

K7ITM wrote:
Geez, Cecil, I don't have an argument with either of you.

The Devil made me do it. :-)
--
73, Cecil http://www.qsl.net/w5dxp