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.