[TowerTalk] TowerTalk Digest, Vol 131, Issue 31

XV4Y (Yan) xv4y at nature-mekong.com
Tue Nov 12 20:39:10 EST 2013


I agree that's the best way to measure Vf and have the right length of cable for a 1/4 or 1/2 line.
However, having a longer cable run is not always desirable as it may increase losses and and produce a broader measurement peak.
It all depends on the work frequency and lambda multiples...

About VNA and antenna analysers, the product below is not a VNA and seems to be limited to SWR measurements, however it is dirt cheap.
Two options : connected via a PC or via Bluetooth (reading with an Android tablet).

Yannick DEVOS - XV4Y
http://www.qscope.org/ 	(Free log analysis tools) 

Le 13 nov. 2013 à 06:02, towertalk-request at contesting.com a écrit :

> The most accurate method to determine Vf and/or electrical length of a 
> piece of transmission line is to measure it as a stub near the frequency 
> where you care about its length.  You can use TDR by the FFT method Jim 
> describes, but the number you get will be for the frequency range where 
> you have swept it, typically 100 MHz - 500 MHz. That's good enough for 
> cutting precisely equal lengths of coax cut from the same roll, and 
> maybe even of the same type if it's from a high quality manufacturer, 
> but it's not good enough for coax with different part numbers, or from 
> companies with poor manufacturing consistency.
> Simple analyzers like the MFJ-259 are plenty good enough for finding the 
> resonances in a stub if you read the null very carefully.  I've done 
> that by finding the zero reactance frequency, then finding the 
> frequencies just either side where the digit changes from zero 
> reactance, and take the average of the two, which should be quite close 
> to the first center reading. I then carefully measure the physical 
> length of the sample with a tape measure.
> BTW -- the best bargain, by far, in a VNA is the VNWA 3e designed by 
> DG8SAQ, built and sold by a UK ham. It's USB powered, runs via software 
> from the USB port, full specs to 500 MHz, reduced dynamic range to 1.5 
> GHz, delivered to my door by post for $700 with calibration load and 
> short. It does TDR by the FFT method Jim Lux described, and can be used 
> as a spectrum analyzer. It reads and writes Touchstone files for use by 
> Smith Chart and other design software. Control software is by DG8SAQ, 
> who a university prof, and is being updated regularly to add features.
> 73, Jim K9YC
> Jim, K9YC's comments on how to use an antenna analyzer are a valid and a
> useful alternative to determining the length of cables.  I would think for a
> long cable run more accuracy would be experienced if a spectrum well below
> the antenna resonance is chosen.  
> I have been accustomed at using the old school TDR approach from my days as
> a CATV engineer.  But I also have a MiniVNA which will accomplish the same
> thing as Jim describes.
> One would still have to calculate each leg separate from the other to
> determine the difference in branch cable lengths.  If the delta is <20
> degrees, I probably wouldn't make any changes on a two bay array spaced a
> 1/2 wave (not the best spacing for gain), or more apart (better).
> Ray, W4BYG 

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