RigExpert AA-230 Zoom Feedline Resonance?

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WD0AKZ

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Hi...

I posted this a few days ago on the RigExpert Yahoo forum but there doesn't seem to be much activity there so I thought I'd repost it here in the hopes of understanding the measuring of feedline/stub resonance.

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Ok, Here’s the deal…. I’m now a Faraday Cave Dweller, living in a aluminum clad co-op apartment. I figure I need to tweak the most out of any antenna system I put together for use here, so after years of using my MFJ-259B successfully on a variety of antennas, traps, baluns, I decided to upgrade into a AA-230 Zoom and enter the world of colorful graphs and charts.

All these years I have been comfortable in my little world measuring cables/stubs as open ended if needing a 1/4wl and shorted for 1/2wl. I noticed that the AA-230Z has a Stub Tuner tool so I grabbed a 3ft Belden 9907 RG-58 THINNET CL2 patch cable and plugged it into the analyzer. First I left the far end open as if it was a stub I was pruning and it reported 1/4wl @ 62.435mhz and 1/2wl @ 129.164. Then I shorted the far end and it reported 1/4wl @ 60.514mhz and 1/2wl @ 119.751. All I ever learned (although there is a chance that I was napping in class the day this was explained) was that 1/4wl transforms, a short at the far end will appear as a open at the other and a 1/2wl will mirror the impedance one end to the other. When I needed a 1/4wl matching section to match a antenna to a feed line I pruned it open, when I needed a 1/2wl feed line to properly tune a antenna I pruned it and then checked it shorted. How does what I have been doing relate to what I should be doing with the two choices/frequencies I have above?

I’ve been working on a 6 meter antenna centered on for 50.200mhz and have a 29.042’ of Belden 8219 I had previously cut/pruned for 2wl long using my MFJ with the far end shorted. It tests out to be 50.193mhz Z5.89-j0.03 on the AA-230Z R,X chart and viewed on the Data (#0) screen with the far end shorted. At the same frequency and the far end open I get Z447.4-j79.9 which makes sense because now the 2wl is open on the far end. When the AA-230Z frequency is adjusted for resonance it is found at 50.113 Z460.4-j0.90. Again there appears to be a choice between end open/shorted. Is my cable resonant at 50.193mhz or 50.113mhz (not that it probably makes much of a difference)?

This was printed further back in this thread:
1. Connect the coax to the test port on the analyzer. Leave the other end of the coax open.
2. Set up a sweep with a center freq of ~15MHz +- 30MHz (i.e. zero to 30MHz)
3. Plot R+JX. Reading the plot from left to right, find the lowest freq where the R flat-tops (off-scale), and J goes from minus to plus. That is the freq at which the unknown coax coil is 1/2 wavelength long.

I set a 30mhz +- 60mhz sweep in AntScope. When I plot my 50.193mhz cable open ended, the first resonance is the 1/4wl and the R & Z are very low and the X is rising – to + as would be expected because it’s transforming the open into a short. The lowest freq where the R flat-tops is the 1/2wl, but the X is going down (rapidly) from + to – and actual resonance is a bit tough to find even on a very narrow scan range in AntScope. Am I missing something here?

Also even further back was:
“IF your 170 can read R+-JX, connect one end of the coax to the 170, leave the other end of the coax open, and then find the lowest frequency at which JX changes sign. At that freq, the coax is 1/2 wavelength long, considering the velocity factor.”
Again, wouldn’t this lowest frequency be a 1/4wl not 1/2wl, shouldn’t X be changing sign at each 1/4wl?

I think I felt a tremor in the Force!

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Thanks for any help/suggestions.

George
 

lmrtek

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Joined
Feb 11, 2009
Messages
535
1\2 wave is 7.16ft for belden 8219

your analyzer has a 10% accuracy

28.64ft would be 7.16x4

the meter could show anything from 25.776ft and 31594ft and still be within its accuracy error.
 

WD0AKZ

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Mar 9, 2015
Messages
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1\2 wave is 7.16ft for belden 8219
Yup, I figured that out without the use of my new $475 analyzer [492 x 0.73 / 50.2].

I apologize for not making my questions more obvious, I have a tendency to get a bit long winded.

My questions from above were:
"How does what I have been doing relate to what I should be doing with the two choices/frequencies I have above?"
"Again there appears to be a choice between end open/shorted. Is my cable resonant at 50.193mhz or 50.113mhz (not that it probably makes much of a difference)?"
"Am I missing something here?"
"Again, wouldn’t this lowest frequency be a 1/4wl not 1/2wl, shouldn’t X be changing sign at each 1/4wl?"

Thanks for the reply
George
 
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