nec simulation - unexpected result ??
 

dansawyeror wrote:
I have been experimenting with various loaded antennas to use in my
relatively limited space. For this I assumed the two arms of a dipole
must be identical to support resonance, this assumption has not been
supported by modeling. Is there an explanation for this?

An electrical 1/2 wavelength conductor is resonant
no matter where you feed it. Even if you don't feed
it anywhere, it is still resonant.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

"dansawyeror" wrote in message
...
All,

I have been experimenting with various loaded antennas to use in my
relatively limited space. For this I assumed the two arms of a dipole must
be identical to support resonance, this assumption has not been supported
by modeling.

Actual model runs show that if the two arms of a dipole are close then
there is sufficient interaction that they will combine to form a single
resonance. The model below shows a simple example of this. The loads and
length of the arms are not equal, however nec predicts a single resonance
at about 3.5 MHz. Changes of 10 to 20 percent around resonance seem to
create one resonance.

Is there an explanation for this?

Thanks - Dan kb0qil

No matter what Dan, you should see only one resonance at the overall length
of a half wave. Not that resonance has any bearing on antenna efficiency.

Also; your NEC model has uneven segmentation, which does produce significant
errors. Interesting to note that your antenna is also resonant at 7 MHz.

73,

Frank

nec simulation - unexpected result ??
 

"a 1/2 wave segment is resonant no matter where you feed it." That allows for a
large single coil to 'tune' one arm of an antenna and for the other to be
adjustable.

Simulation predicts the impedance will change when it is not feed at the center,
it appears to go up as the feed point is moved.

I will play with the segmentation and see what happens.

Thanks - Dan

Frank wrote:
"dansawyeror" wrote in message
...

All,

I have been experimenting with various loaded antennas to use in my
relatively limited space. For this I assumed the two arms of a dipole must
be identical to support resonance, this assumption has not been supported
by modeling.

Actual model runs show that if the two arms of a dipole are close then
there is sufficient interaction that they will combine to form a single
resonance. The model below shows a simple example of this. The loads and
length of the arms are not equal, however nec predicts a single resonance
at about 3.5 MHz. Changes of 10 to 20 percent around resonance seem to
create one resonance.

Is there an explanation for this?

Thanks - Dan kb0qil


No matter what Dan, you should see only one resonance at the overall length
of a half wave. Not that resonance has any bearing on antenna efficiency.

Also; your NEC model has uneven segmentation, which does produce significant
errors. Interesting to note that your antenna is also resonant at 7 MHz.

73,

Frank

nec simulation - unexpected result ??
 

On Thu, 23 Feb 2006 21:40:31 -0800, dansawyeror
wrote:

"a 1/2 wave segment is resonant no matter where you feed it."

Hi Dan,

I don't know where to start on that one. 1/2 wave "segment?" And
then to partition (into what? it is already describe as A segment) for
a feed - that is resonant irrespective of where it is fed? Any wire
is resonant, further elaboration does nothing to change that one
obscure characteristic - and in fact, any wire is multi-resonant.

That allows for a large single coil to 'tune' one arm of an antenna
and for the other to be adjustable.

Then it ceases to be "a 1/2 wave segment" unless the frequency is
adjusting with the length - this would seem to be obvious, but what
end is served in saying it? What distinguishes an arm from a segment?

Simulation predicts the impedance will change when it is not feed at the center,

Simulation should.

it appears to go up as the feed point is moved.

In distinct contradiction to most OCF dipoles - odd. In fact one of
the hallmarks of the OCF is being resonant in many ham bands where the
standard dipole does not.

73's
Richard Clark, KB7QHC

nec simulation - unexpected result ??
 

dansawyeror wrote:

"a 1/2 wave segment is resonant no matter where you feed it." That
allows for a large single coil to 'tune' one arm of an antenna and for
the other to be adjustable.

Simulation predicts the impedance will change when it is not feed at the
center, it appears to go up as the feed point is moved.

I will play with the segmentation and see what happens.

Absolutely true! But, what does feedpoint impedance have to do with
resonance? ... NUTTIN!

nec simulation - unexpected result ??
 

Amos Keag wrote:
"But, what does feedpoint impedance have to do with resonance?"

Imagine a whip worked against ground. It is resonant at 1/4-wavelength
where it presents a low impedance. It is resonant again at
1/2-wavelength where it presents a high impedance.

Best regards, Richard Harrison, KB5WZI

nec simulation - unexpected result ??
 

dansawyeror wrote:
Simulation predicts the impedance will change when it is not feed at the
center, it appears to go up as the feed point is moved.

An off-center-fed dipole will match 300 ohm twin lead if
fed at the correct point. This is covered in my 1957
ARRL Handbook.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

"dansawyeror" wrote in message
...
"a 1/2 wave segment is resonant no matter where you feed it." That allows
for a large single coil to 'tune' one arm of an antenna and for the other
to be adjustable.

Simulation predicts the impedance will change when it is not feed at the
center, it appears to go up as the feed point is moved.

I will play with the segmentation and see what happens.

Thanks - Dan

Dan,

As for NEC segmentation. "0.05 wavelengths per segment is preferred, but
can be as long as 0.1 wavelengths. Segments shorter than 0.001 wavelengths
should be avoided". From L. B. Cebik's "Basic Antenna Modeling: .....". In
most cases all segments within a structure should have equal length
segmentation. Where antenna models become very large, with 1000 segments or
more, there are work-arounds which allow for uneven segmentation without
introducing errors. The problem with large numbers of segments is that
processor time increases dramatically.

I see nothing wrong with using one loading coil in a dipole. The effect is
simply the same as an off-center-fed dipole.

73,

Frank

nec simulation - unexpected result ??
 

Richard Harrison wrote:

Amos Keag wrote:
"But, what does feedpoint impedance have to do with resonance?"

Imagine a whip worked against ground. It is resonant at 1/4-wavelength
where it presents a low impedance. It is resonant again at
1/2-wavelength where it presents a high impedance.

Best regards, Richard Harrison, KB5WZI


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

Feed point impedance is the load presented to a transmission line when
you want to make a wire, any wire, resonant or non resonant, into an
antenna. I can feed any antenna with a single wire against ground, I can
feed the same antenna with 50 ohm coax, 70 ohm coax, 90 ohm coax, 72 ohm
balance line, 300 ohm balanced line 450 ohm balanced line 600 ohm
balanced line. None of these transmission lines changes to resonance or
non resonance of the antenna.

Resonance is determined by the physical characteristics of the antenna.
Generally these include the antenna length and the length to diameter
ratio. PERIOD.

nec simulation - unexpected result ??
 

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