> Some are open and some are shorted. They are bandpass
stubs, as per K1TTT's
> site, for a multi-transmitter setup I'm trying to get
going.
....then the location of the stubs, other than what is
related to the notch depth, is not critical.
The stub should not be "retuned" with a change in location
because it only shunts the transmission line.
If you add a T connector to a transmission line and the stub
noticably changes SWR the stub is not tuned to the operating
frequency. In other words a 1/4 wl shorted stub that is
properly tuned has virtually no effect on the desired pass
frequency no matter where you place it in the transmission
line. Same answer as Dave gave, just different words.
A 1/2 wl open stub would follow the same rules.
> Trying to set up for 2 transmitters. I want to put the
stubs on the output
> of the antenna switch so I can switch the stub when I
switch antennas. I
> don't have any multi-band antennas here.
Makes sense. That's what I do because my 80 meter CW
harmonic is audible on 40 meters. That is illegal even if it
is 55 dB down, since the law says we are not allowed to have
any spurious emission that causes interference regardless of
level. I added a 1/4 wl shorted stub of LMR400 shunting the
80 meter antenna. Since my PA has a shunt C in the output I
installed my 80 meter stub about 20 feet from the PA. This
gained several dB additional suppression over the same stub
right at the output port of the PA.
The optimum distance from the transmitter (amplifier) varies
with the type of output network of the PA stage. Networks
used in transmitters have greater or lesser (largely 2nd
harmonic) suppression depending on the impedance presented
to the PA at the harmonic by the stub system.
The location of properly tuned resonant stubs in the
transmission line significantly affects attenuation level of
harmonics, even though it does not significantly affect pass
SWR on the desired band.
73 Tom
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