"Cecil Moore" wrote in message
...
JGBOYLES wrote:
Hi Hank, A 1/2 wave shorted stub looks like a series resonant circuit.
It
makes a good notch filter, which is one of the Amateur applications that
I have
used. You are right, they don't dissipate any power since Power=I**2*R
and R
equals zero.

Stubs dissipate the maximum amount of power possible since the SWR on a
stub is nearly infinite. If you don't believe it, hang a Bird Wattmeter
halfway down a stub. The ratio of Vmax/Vmin and Imax/Imin is very high.
If the stub were lossless, the SWR inside a stub would be infinite.
After all, a stub is merely a shorted or open piece of transmission line.
--
73, Cecil http://www.qsl.net/w5dxp

I would have to agree Cecil. The point is proven by the fairly shallow notch
afforded by a single stub ( approx -20dB at VHF). I can do better with a
quality L-C combination.

Dale W4OP

Uh.. I'm gonna do this anyway...


"Cecil Moore" wrote in message
...
Henry Kolesnik wrote:
...shorted 1/4 wave stub...
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low
impedance
or a short. There are claims that this can be used to filter the even
harmonics. Shorts can't diisipate power and must reflect, so how does
a
stub work?

Consider the following configuration:

Source----ideal 1 WL feedline-----------+-----matched load
|
|Stub
|
|
open

Oops! Time to wipe the CRT clean... Henry wants to discuss a SHORTED
stub...reset. end. stop.
My original comments hold. Harmonics are "reflected" or however you
want to look at the fact that they don't make it to the "matched load".
....... Unfortunately, I continue...


Assuming Cecil's comments will be valid if I swap the 1xF (1/4W) and the 2xF
(1/2W) comments, I proceed...


If the stub is 1/4WL,
[... RESPONDIND as though this was the 1/2W remark...]

At the mouth of the stub, they
are 180 degrees out of phase and superpose to zero volts which obeys
Ohm's law and delivers zero power to the load.

Yea, ok, so a little sneaks by since it ain't lossless.



Double the frequency. That makes the stub 1/2WL.
[...RESPONDING as though this was the 1/4W remark...]

The forward voltage and
reflected voltage are essentially the same ...
they are in phase and superpose to a maximum value which obeys Ohm's
law and delivers maximum power to the load.

In my mental model, a hi Z across the line.


Absolutely nothing except superposition and interference happens at the
mouth of a stub. All the reflected action happens at the physical open
circuit. Virtual impedances are only a V/I ratio and CANNOT cause
reflections. Absolutely no reflections are happening at the mouth of
the stub (unless a physical impedance discontinuity exists there).

\\ Nit alarm on\\ (Though I consider this a clarification in an attempt
to simplify the explanation and remove the extra complexity added by the
double terms) I consider superposition and interference to be the same
thing. Superposition is the addition of two quantities (voltages ot
currents). They can be of similar sign or different. If they are closer to
'in-phase', then they add to a greater value and if they are closer to 180
degree phase difference, then they add to a smaller value. You can call
that "algebraiclly add" if it makes you feel better.
That is, a +1 and +1 _add_ to +2. A -1 and a +1 _add_ to 0.0
\\ end nit alarm \\


This IS a correct way to look at it. Think of the "main" line" (the one
going to the "matched load" above) as being the thing which is "added to"
the shorted system. Since it is added to a low Z point (on this high VSWR
line), little goes down that path to the desired load. Any 50 ohm "thing"
put in parallel at the point of the lowest voltage of the standing wave,
sees a low/zero voltage, therefopre nothing goes there.



Consider this. If a stub really presented an infinite impedance, you could
simply remove it and nothing would change.

At the fundamental (_OPEN_ 1/4W stub, remember) this IS TRUE.
Are you saying that the 1/4W _shorted_ stub addition or removal makes a
BIG difference at the fundamental -I think not, or you still talking about
the 1/4W _open_ stub which kills the system at the fundamental when added.

Steve Nosko wrote:
Oops! Time to wipe the CRT clean... Henry wants to discuss a SHORTED
stub...reset. end. stop.

Sorry, a shorted 1/2WL stub exhibits a short at the fundamental frequency
and an open when it is 1/4WL at 1/2 the frequency of the first. Same
principles apply.

Absolutely nothing except superposition and interference happens at the
mouth of a stub. All the reflected action happens at the physical open
circuit. Virtual impedances are only a V/I ratio and CANNOT cause
reflections. Absolutely no reflections are happening at the mouth of
the stub (unless a physical impedance discontinuity exists there).

\\ Nit alarm on\\ (Though I consider this a clarification in an attempt
to simplify the explanation and remove the extra complexity added by the
double terms) I consider superposition and interference to be the same
thing.

So do I. I probably should have said superposition/interference, which logically
equals (superposition AND interference) since ONE AND ONE = ONE.

Consider this. If a stub really presented an infinite impedance, you could
simply remove it and nothing would change.

At the fundamental (_OPEN_ 1/4W stub, remember) this IS TRUE.
Are you saying that the 1/4W _shorted_ stub addition or removal makes a
BIG difference at the fundamental -I think not, or you still talking about
the 1/4W _open_ stub which kills the system at the fundamental when added.

Any stub that presents an infinite impedance should be able to be removed
and without anything changing. You have an infinite impedance before the removal
and you have an infinite impedance after the removal, so nothing changes when
you remove the stub. (Hint: Devil's advocate reasoning applied) Of course,
something changes when one removes the stub - that's the entire point. There
are forward components and reflected components flowing in and out of a stub.
That's why removing it changes things.
--
73, Cecil http://www.qsl.net/w5dxp



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"Cecil Moore" wrote in message
...
Steve Nosko wrote:
Oops! Time to wipe the CRT clean... Henry wants to discuss a SHORTED
stub...reset. end. stop.

Sorry, a shorted 1/2WL stub ... Same
principles apply.

That's why I went ahead and continued on the "complementary situation"
[...]

Any stub that presents an infinite impedance should be able to be removed
and without anything changing. You have an infinite impedance before the
removal
and you have an infinite impedance after the removal, so nothing changes
when
you remove the stub. (Hint: Devil's advocate reasoning applied)

Your "Devil's advocate "meaning escapes me...


Of course,
something changes when one removes the stub - that's the entire point.
There
are forward components and reflected components flowing in and out of a
stub.
That's why removing it changes things.

Yea, but in the practical sense, it does not change anything at the
fundamental (desired) frequency [[high Z stub, power still to the load]].
It's only when you consider the harmonics, if they are there, that anything
important changes, right. So I'm puzzled why you included that part in your
comment.
--
Steve N, K,9;d, c. i My email has no u's.

Any stub that presents an infinite impedance should be able to be removed
and without anything changing. You have an infinite impedance before the
removal and you have an infinite impedance after the removal, so nothing
changes. There are forward components and reflected components flowing in and
out of a stub. That's why removing it changes things.

Is it because energy is stored in the stub? If the impedance is infinite then
no power dissipation is removed from the system. Something is dynamic in
nature in this example because if you remove an infinite impedance, nothing
will happen. Could the change be due to the fact that the stub is in the near
field of the antenna and subject to induced rf currents? If this were the
case, then removal of the stub would change things.
If the stub presents an infinite impedance, with no external influences,
removing it should have no effect. If the stub were replaced with a quality
parallel resonant LC circuit in a shielded box, would removing it make any
difference? I don't know, I am asking.


73 Gary N4AST

Henry Kolesnik wrote:

I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low impedance
or a short. There are claims that this can be used to filter the even
harmonics. Shorts can't diisipate power and must reflect, so how does a
stub work?


It works exactly as you stated: "It reflects the second and all even
harmonic energy back to the source."

Steve Nosko wrote:
Yea, but in the practical sense, it does not change anything at the
fundamental (desired) frequency [[high Z stub, power still to the load]].
It's only when you consider the harmonics, if they are there, that anything
important changes, right. So I'm puzzled why you included that part in your
comment.

Some people believe that there is no current flowing into or out of a
1/4WL shorted stub. But all they have to do to change their minds is
measure the current through the short at the end of the stub. It is sky
high. The SWR inside a 1/4WL shorted stub is near infinite, i.e. the
square root of Pr/Pf inside a stub is near unity.
--
73, Cecil http://www.qsl.net/w5dxp



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JGBOYLES wrote:
If the stub presents an infinite impedance, with no external influences,
removing it should have no effect.

Exactly! Therefore, the infinite impedance at the mouth of a stub
is a virtual impedance, not a physical impedance. The physical
impedance exists at the end of the stub where all the reflections
are taking place. Reflections occur only at a physical impedance
discontinuity.
--
73, Cecil http://www.qsl.net/w5dxp



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Dave Shrader wrote:

Henry Kolesnik wrote:
I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low
impedance
or a short. There are claims that this can be used to filter the even
harmonics. Shorts can't diisipate power and must reflect, so how does a
stub work?

It works exactly as you stated: "It reflects the second and all even
harmonic energy back to the source."

Yes, but only after making a round trip to the short or open at
the bottom end of the stub. V/I ratios don't cause reflections.
Only physical impedance discontinuities cause reflections.
--
73, Cecil http://www.qsl.net/w5dxp





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"Henry Kolesnik" wrote in message
...
I know that a shorted 1/4 wave stub exhibits a very high impedance. But
for the 2nd harmonic it's a 1/2 wave stub and exhibits a very low
impedance
or a short. There are claims that this can be used to filter the even
harmonics. Shorts can't diisipate power and must reflect, so how does a
stub work?

stubs work very nicely. you can get practical stub information at my web
site, including how to build a 40m to 15m 3rd harmonic stub filter:
http://www.k1ttt.net/technote/techref.html#filters

as you may have noticed by now you have kicked the proverbial hornets nest.
reflections are a touchy word in this group, usually attracting the endless
argument that travels from thread to thread. in time this will deteriorate
into name calling and endless argument over reflections, interference,
virtual impedances, and a few other topics.