nec simulation - unexpected result ??
 

Cecil, read your post closely. then Read my post.

Antenna and antenna system are two different [read different] things.

A physical length of wire of dimension 'L' is 1/2 wave resonant at it's
fundamental frequency, 3/2 waves resonant, 5/2 waves resonant, 7/2 waves
resonant, at harmonics of the fundamental etc.

An antenna system is what connects the antenna to your radio in a manner
where your radio can deliver energy efficiently to the antenna. it
includes feedline, stubs, tuners, etc. these things are not part of the
antenna but are part of the antenna system.

Once again, the basic resonance of a wire is the basic resonance of the
wire. You can fiddle all you want with stubs, tuners, or what not, and
still not change the resonance of the wire.

A 100 feet long wire is 1/2 wave resonant at 4.68 MHz [depending on
which formula you choose]. You can tune it to transmit where you want.
But the wire is still resonant at 4.68 MHz while the antenna system may
be tuned anywhere between dc and light [hyperbole].

AK



Cecil Moore wrote:

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.

nec simulation - unexpected result ??
 

Feedpoint impedance does change. But, what has that to do with resonance??

Cecil Moore wrote:

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.

nec simulation - unexpected result ??
 

Amos Keag wrote:
Feedpoint impedance does change. But, what has that to do with resonance??

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

I may have inferred from Roy's posting what he didn't mean
to imply. Here's what I assumed he said: "The *feedpoint*
impedance of the antenna doesn't change with the feed
method." I hope you agree that is a false statement but
may not be the meaning that Roy intended. I apparently
made the same mistake for the meaning of your posting.
If you guys would just post in Texan, I could understand
you. di-di-di-dit---di-dit

If Roy had said, "The radiation resistance of an antenna
doesn't change with the feed method", I wouldn't have
questioned it.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

I apologize for not being more clear. The context of my comment was in
reply to a statement about the resonance somehow being different for
"current fed" and "voltage fed" antennas. What I meant was that the
antenna impedance is independent of the source impedance or other
characteristics of the source. Of course, the feedpoint impedance will
be different if you choose another feedpoint.

I'm surprised that this topic has aroused so much commentary. It reveals
a general lack of understanding about some of the most basic and
fundamental characteristics of antennas and circuits in general.
Hopefully, the discussion is helping some of the readers to gain a
better grasp of the subject matter.

Roy Lewallen, W7EL

Amos Keag wrote:
Feedpoint impedance does change. But, what has that to do with resonance??

Cecil Moore wrote:

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.

nec simulation - unexpected result ??
 

Roy Lewallen wrote:
Of course, the feedpoint impedance will
be different if you choose another feedpoint.

Here is the confusing quotation:

The impedance of the antenna doesn't change with the feed method
(assuming of course that it has a single feed point)

A center-fed dipole has a "single feed point". An off-center-fed
dipole has a "single feed point". The feedpoint impedance of the
antenna does indeed change depending upon where that single
feed point is located.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

Agree! But, the radiation resistance ... ?

Cecil Moore wrote:

Roy Lewallen wrote:

Of course, the feedpoint impedance will be different if you choose
another feedpoint.


Here is the confusing quotation:

The impedance of the antenna doesn't change with the feed method

(assuming of course that it has a single feed point)

A center-fed dipole has a "single feed point". An off-center-fed
dipole has a "single feed point". The feedpoint impedance of the
antenna does indeed change depending upon where that single
feed point is located.

nec simulation - unexpected result ??
 

Amos Keag wrote:
Agree! But, the radiation resistance ... ?


The topic of radiation resistance has been pretty much beaten to death
on this newsgroup. A groups.google.com search should provide a good
evening's reading and entertainment.

In brief, "radiation resistance" is the part of an antenna's impedance
which "absorbs" the power actually being radiated. It can be referred to
any point on an antenna. At that point, wherever it is, the power
radiated is I^2 * Rrad, where I is the current at that point. Kraus
generally refers the radiation resistance to the feedpoint; that is, he
calls the resistive part of the feedpoint impedance the radiation
resistance. In AM broadcasting, where monopoles are often higher than a
quarter wavelength, it's common to refer the radiation resistance to a
point of maximum current. Either approach is perfectly valid. But it
means that you have to be careful to say what you mean when you use the
term, particularly if the antenna is longer than a quarter wavelength
(monopole) or half wavelength (dipole).

There's a common equation for efficiency, Eff = Rrad / (Rrad + Rloss).
When using it, you have to make sure you're referring Rrad and Rloss to
the same point. A common error when dealing with folded monopoles is to
refer the radiation resistance to the high-impedance, transformed
feedpoint, while neglecting to refer the loss resistance to the same
point by transforming it. This leads to the erroneous conclusion that
folding the element improves efficiency. It doesn't, except that the
additional wire surface area reduces the wire loss -- but this isn't
usually a significant contribution to the total loss.

So to address the comment --

Changing the feedpoint location can change the current distribution on
the antenna. This in turn will change the radiation resistance referred
to some fixed point along the antenna, although in most common cases I
can think of, it won't be a large change. You could contrive some cases
where it would. If you refer the radiation resistance to the feedpoint,
that is, define it as being the resistive part of the feedpoint
impedance, then changing the feedpoint location can have a major impact
on the radiation resistance.

Again, if you want more about the topic, read some of the threads where
it's been discussed in more detail.

Roy Lewallen, W7EL


Cecil Moore wrote:

Roy Lewallen wrote:

Of course, the feedpoint impedance will be different if you choose
another feedpoint.


Here is the confusing quotation:

The impedance of the antenna doesn't change with the feed method

(assuming of course that it has a single feed point)

A center-fed dipole has a "single feed point". An off-center-fed
dipole has a "single feed point". The feedpoint impedance of the
antenna does indeed change depending upon where that single
feed point is located.

nec simulation - unexpected result ??
 

Amos Keag wrote:
Agree! But, the radiation resistance ... ?

Nobody said anything about radiation resistance.
--
73, Cecil http://www.qsl.net/w5dxp

nec simulation - unexpected result ??
 

"Roy Lewallen" wrote
Hopefully, the discussion is helping some of the readers to gain a
better grasp of the subject matter.
======================================

What a forlorn hope!

Novices learn next to nothing. They are unable to sort out a very
small amount of simple wheat from a great amount of over-complicated
chaff.
----
Reg.

nec simulation - unexpected result ??
 

Rom, KC7ITM wrote:
"I don`t think I`ve EVER heard anyone call a parallel-tuned circuit
antiresonant."

There are many synonyms I am unfamiliar with also, but I`ve heard of
antiresonance and used the word. My "Dictionary of Electronics" says:
"Antiresonance - A type of resonance in which a system offers maximum
impedance at its resonant frequency."

An antiresonant circuit is defined:
"Antiresonant circuit - A parallel-resonant circuit offering maximum
impedance to the series passage of the resonant frequenccy."

A parallel-tuned resonant circuit fits the dictionary definition. It`s a
synonym for antiresonant circuit.

Best regards, Richard Harrison, KB5WZI