Shorted 1/4 wave stub ?
 

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?

--
73
Hank WD5JFR

"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?
73 Hank WD5JFR



Put a Tee on a coax line and connect the stub there. At the H2 there is
a short and reflectinonback up-stream.
Common filters you see everywhere are "reflective" by nature / design as
well. They have other than Zo off frequency and therefore reflect the
unwanted stuff back to the source. It has to be able to handle it.

Help any?
--
Steve N, K,9;d, c. i My email has no u's.

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
Shorts can't diisipate power and must reflect, so how does a
stub work?
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.

73 Gary N4AST

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.

73 Gary N4AST
============================

R is not zero. It can be several ohms. And lots of amps can flow.

Just to add a 1/2-wave shorted stub also makes a mess of the tuning of the
remainder of the antenna system at all other frequencies. It must be
designed "in".

R is not zero. It can be several ohms. And lots of amps can flow.


OK Reg, but in this ideal rf environment we Amateurs live in R is zero.

Just to add a 1/2-wave shorted stub also makes a mess of the tuning of the
remainder of the antenna system at all other frequencies. It must be
designed "in".

That should be fairly obvious to the most casual observer. :-).
In the receive applications that I have used a 1/2 wave shorted stub as a
notch filter, I can assure you I did not have lots of Amps flowing.


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?

Consider the following configuration:

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

If the stub is 1/4WL, the forward voltage and reflected voltage in the
stub is the same as on the feedline. 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.

Double the frequency. That makes the stub 1/2WL. The forward voltage and
reflected voltage are essentially the same as in the 1/4WL stub but this
time they are in phase and superpose to a maximum value which obeys Ohm's
law and delivers maximum power to the load.

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).

Consider this. If a stub really presented an infinite impedance, you could
simply remove it and nothing would change. Something inside the stub happens
to cause that infinite virtual impedance. Hint: Stubs have a near-infinite SWR.
To understand that assertion, consider how much voltage exists at a voltage
minimum inside a stub. It is nearly zero.
--
73, Cecil http://www.qsl.net/w5dxp



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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



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JGBOYLES wrote:
That should be fairly obvious to the most casual observer. :-).
In the receive applications that I have used a 1/2 wave shorted stub as a
notch filter, I can assure you I did not have lots of Amps flowing.

Have you ever measured it? Have you any idea of the magnitude of
current measurable at the shorted point in a stub during transmit?
--
73, Cecil http://www.qsl.net/w5dxp



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Have you ever measured it? Have you any idea of the magnitude of
current measurable at the shorted point in a stub during transmit?

No Cecil I don't, should I? The only experience I have with a shorted 1/2 wave
stub is in receive applications. I have swept them with a homebrew spectrum
analyzer and tracking generator and can say they look like series resonant
circuits. At the micro watt level will I still have several Amps in the stub?
How would you measure the current in the stub during transmit using equipment
the average ham (like me) has laying around the shack ?
73 Gary N4AST

JGBOYLES wrote:
Have you ever measured it? Have you any idea of the magnitude of
current measurable at the shorted point in a stub during transmit?

No Cecil I don't, should I? The only experience I have with a shorted 1/2 wave
stub is in receive applications. I have swept them with a homebrew spectrum
analyzer and tracking generator and can say they look like series resonant
circuits. At the micro watt level will I still have several Amps in the stub?

Of course not for receive. Here's the statement to which I was objecting:

"... they don't dissipate any power since Power=I**2*R and R equals zero."

R is not zero half-way into a shorted 1/2WL stub. For a lossless stub, R
is infinite at the half-way point. Stubs have considerable I^2*R losses
during transmit. Of course, if you are using them to notch filter out
unwanted receive signals, you want them to be lossy.

How would you measure the current in the stub during transmit using equipment
the average ham (like me) has laying around the shack ?

Wrap ten turns of pickup wire on a ferrite toroid. Use a dummy load
to calibrate it by running the dummy load wire through it. Then install
the shorted stub wire through the ferrite toroid and make the measurement.
Roy Lewallen describes how to measure RF current in his article in "The
ARRL Antenna Compendium", vol 1, in "Baluns: What They Do and How They
Do It."

Or install a one ohm resistor at the stub short and measure the voltage.
An oscilloscope will do. You don't need super high accuracy.

For the following stub, we can estimate the value of the current at
the short in the stub assuming a lossless line. The source is a 100W
Signal Generator equipped with a perfect Circulator and Load

100W SGCL----50 ohm feedline-----------+---50 ohm load
|
|1/2WL Stub
|50 ohms
|
short

No current reaches the load. The forward current is 1.414 amps and the
reflected current is 1.414 amps on the feedline and inside the stub.
100 watts of reflected power is being dissipated in the circulator load
resistor. The RMS current at the short is 2.828 amps, the in-phase sum
of the forward and reflected currents. The peak-to-peak current at the
short is 8 amps.

The only reflection point in the entire above system is at the short
at the end of the stub. No reflections occur at '+'.
--
73, Cecil http://www.qsl.net/w5dxp



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