[TowerTalk] Adjusting stubs w/ shack t*e*s*t equipment

[Steve Gehring / KL7DC]

Apparently my last try didn't get through, either.  Steve, K7LXC, indicates
that the word t*e*s*t won't let messages through.  So, here's my 3 or 4
try/rewrite.

Hi Bill,

I agree with low Q problem and the resulting broad notch with the use of
standard coax, although I believe these cheaper t*e*s*t 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 t*e*s*t, 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 t*e*s*t 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.  

How easy has it been for others to measure and cut stubs or 1/4 or 1/2 wave
lengths of line with their analyzers?

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.
>
> 73
>
> Bill, W4AN
>


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