[TowerTalk] Tuning coax stubs w/ shack test equipment
km1h@juno.com
km1h@juno.com
Tue, 30 Mar 1999 11:14:11 -0500
On Mon, 29 Mar 1999 18:36:15 -0800 Steve Gehring / KL7DC
<gehring@alaska.net> writes:
>
>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.
Disagree.
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
>
>73
>
>Bill, W4AN
>
>
>
>
>
>
>
>
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>
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