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Re: [TowerTalk] Hardline Fault

Subject: Re: [TowerTalk] Hardline Fault
From: Jim Brown <>
Date: Wed, 22 Aug 2012 09:53:09 -0700
List-post: <">>
On 8/22/2012 6:49 AM, Roger Parsons wrote:
> VNWA. I'm not very keen on taking the latter, along with a laptop, out into 
> the
> depths of the bush, but it would certainly be more accurate, at least before 
> I drop
> it:-)


Don't forget that you can calibrate the VNWA to ANY length of cables, 
and it takes only a few minutes to do that.  As it happens, I took mine 
out "into the bush" yesterday and set it up at the base of my second 
tower. I calibrated it to a length of coax long enough to reach up the 
tower to where Glen, W6GJB, connected it to a length of coax that fed a 
15M dipole.  Made a couple of sweeps, saved them as S1P files, and used 
one of them to design a stub matching network.  As our British friends 
would say, "it worked a treat."

It's also possible to do a TDR anywhere along a cable run.  My 
experience, though, is that anything "funky" in the path close to the 
analyzer will obscure cable detail.  I was using the very nice Ten Tec 
antenna tuners for my wire antennas, and also using them as manual 
antenna switches for other antennas. The RF path uses single wires for 
antenna switching rather than coax, so there's lots of stray inductance 
in the path.. That's good enough for HF, but it's terrible for VHF, so a 
500 MHz TDR sweep saw only the mess in the tuner and the antenna about 
200 ft up the line.

BTW -- I've found that the Hamming window often works best, and switch 
between the bandpass and lowpass filter settings to see which gives the 
most detail.  Note that you can change the time settings, including the 
windowing settings, after you've made a sweep and you'll simply be 
viewing that data differently.  The only changes that affect the 
MEASUREMENT are the frequency limits and the number of data points. The 
window settings are simply changing the VIEW of the data.

73, Jim K9YC


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