[TowerTalk] Stub tuning

[Jim Lux]

At 09:35 PM 9/11/2005, Tom Osborne wrote:
>HI TT'ians
>
>I'm trying to get the stubs hooked up here.  I cut some and they are right
>where I want them to be, using the MFJ tuner option.
>
>I cut the 20 meter one right at the meter and wanted to hook them up on the
>outside of my antenna switch.  Going from the rig, there is about a 3 foot
>cable to the MFJ tuner/swr bridge.  Going from there is about a 3 foot cable
>to the homebrew antenna switch.
>
>Problem is, when I move the stub from the bridge to the output of the
>antenna switch, I would think it would be too long and would have to cut
>some off.  What is happening is that instead of the stub being on 14.100
>(where I want it), it moves up frequency to about 15.500 instead of going
>lower if frequency as they are now about 6' longer.  Can't figure what is
>going on.  There is no dip down below 14.100.  Any ideas?  73

So you're looking at the SWR of the system, looking out from your rig (thru 
the switch, etc.?)

You wouldn't expect that to be at the stub frequency, because you've hooked 
up another piece of TxLine.

The stub acts like a short at the desired frequency, and that short is 
transformed by whatever transmission line is between the short and where 
you're measuring.  At the "stub frequency", you'll see some sort of mismatch.

The way a stub "works" is by essentially creating a frequency dependent 
short across the feedline.  In theory, at the non-stub frequencies, the 
impedance in parallel is very large (either L or C) so it doesn't affect 
the overall match very much.

What really counts is the attenuation *through the entire system* at your 
operating frequency and at the frequency you're trying to suppress.  And 
that is something you can't measure with a SWR bridge (at least not 
easily.. you could do it by putting a series of known impedances at the 
antenna end, sweeping it with each one, and then doing some calculations)




>Tom W7WHY
>
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