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[TowerTalk] Tuning coax stubs w/ shack test equipment

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Subject: [TowerTalk] Tuning coax stubs w/ shack test equipment
From: (Steve Gehring / KL7DC)
Date: Mon, 29 Mar 1999 18:36:15 -0800
Hi Bill,

I agree with low Q problem and the resulting broad notch with the use of
standard coax, although I believe these cheaper test equipment items can be
used for easy pruning of an open or shorted stub.  For example, after
reading your comment, I grabbed an "odd" piece of coax (128 inches of cheap
RG-11) from the shack closet and connected it to my Autek RF-1.  For the
open stub test, the impedance bottomed out at 15.05 MHz with 7 ohms, and
only rose to 13 ohms at 14.05 MHz, or 1 MHz lower.  Also, the impedance
rose to only 84 ohms as I swept down to 7.05 MHz and up to 22.52 Mhz,
respectively!  It was low Q!  Now, the shorted stub test yielded that the
impedance rose above 2300 ohms (the highest impedance my RF-1 will read)
around 14.8 Mhz and didn't start coming down to that value until 15.25 MHz,
indicating a very broad passband.  Again, exhibiting a low Q.  I'd imagine
that the results for cheaper RG-8 coax would fair about the same, even
though they are 50 ohms.  Now, my Autek worked fine for this application,
and made finding the center of that low Q, broad notch and/or passband
rather uneventful.  I've never used an MFJ, but would guess the SWR would
peak during the stub's electrical shorts and opens.  I feel they are a good
way to check your work, especially with possibility of length
miscalculations and varying velocity factors, etc.  

Fun hobby, eh?

73 de Steve, KL7DC    

You have obviously never swept a stub with a SWR analyzer.  The Q is not 
very high.  Low Q is actually a problem.  Some guys make these stubs out 
of CATV hardline to increase the Q.  I am using some networking coax that 
has two shields.  Even so, it's not like you are going to need a stub for 
14Mhz SSB and another for 14Mhz CW.  Not even close.


Bill, W4AN

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