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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
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Date: Tue, 30 Mar 1999 11:14:11 -0500

On Mon, 29 Mar 1999 18:36:15 -0800 Steve Gehring / KL7DC
<> writes:
>Hi Bill,
>I agree with low Q problem and the resulting broad notch with the use 
>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 
>RG-11) from the shack closet and connected it to my Autek RF-1.  For 
>open stub test, the impedance bottomed out at 15.05 MHz with 7 ohms, 
>only rose to 13 ohms at 14.05 MHz, or 1 MHz lower.  Also, the 
>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 
>impedance rose above 2300 ohms (the highest impedance my RF-1 will 
>around 14.8 Mhz and didn't start coming down to that value until 15.25 
>indicating a very broad passband.  Again, exhibiting a low Q.  I'd 
>that the results for cheaper RG-8 coax would fair about the same, 
>though they are 50 ohms.  Now, my Autek worked fine for this 
>and made finding the center of that low Q, broad notch and/or 
>rather uneventful.  I've never used an MFJ, but would guess the SWR 
>peak during the stub's electrical shorts and opens.  I feel they are a 
>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 
>very high.  Low Q is actually a problem.  Some guys make these stubs 
>of CATV hardline to increase the Q.  I am using some networking coax 
>has two shields.  Even so, it's not like you are going to need a stub 
>14Mhz SSB and another for 14Mhz CW.  Not even close.


With 3/4" hardline the stub will have to be changed.
K1EA found that out when the 87A amp wouldnt load properly.

I have used RG-11 CATV foam for stubs for over 12 years and find that I
do have to change on 80/75.

A noise bridge is used purely as a noise source and the RX is used to
tune, 15-25dB notches are easily obtainable.  Certainly not ideal but it
was all we needed here with a pair of TS-940's. 

73  Carl  KM1H

>Bill, W4AN
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