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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