Measuring velocity factor

Connections:  T adapter to analyzer.  50 ^ load to one end of tee and the other end of  tee to the cable under test (CUT). Short the far end of the coax under test.

Estimate the frequency  where the line is 1/4 wave:

F = 984/ length in feet *4 =  246 / length in feet

VF = freq * physical length of cable / 246
example:   Radio shack RG6 QS , l = 50′

starting below 4.0 and increasing freq,  swr goes to 1:1  at  3.6  mhz

3.6  mhz @ 50′ -> .732

Example 2RG58 10′
F = 24.6 mhz ,  measured F = 15.1 mhz
vf == .61


Error in the latest ARRL Antenna Book , page 27-22, Measuring Velocity Factor.

The example is confusing using the 86 foot coax.  The example should have used a cable length of 30 feet or so.  It provided a measured frequency of 7.28 mhz and the math works OK but 86 feet is 1/4 wave of something like 2.8 Mhz so the example doesn’t pass a simple sanity test.   If you measured 7.28 mhz … something is wrong with the analyzer.

Start at a frequency  less than where the 86 foot line is 1/4 wave.

An 86 foot line x 4 =  344 feet == 104 meters.  The approximate frequency where 86 feet is  1/4 wave is the frequency corresponding to a wavelength of 344 feet or 104 meters.

freq (mhz) = 984 / length (ft)       approximate freq… ==  984/(86*4) == 2.8 MhZ

In this example, the antenna analyzer is used to find the frequency where the SWR reaches a minimum or 1:1 whichever is lower.

Also, the formula for calculation is not provided but is  :

VF = freq * physical length of cable / 246
The example asserts that an 86 foot cable (~26M) is a 1/4 wave at 7.58 MhZ.
Elsewhere, in “Measuring Characteristic Impedance” ,  the text does not explain or provide an example of Eq 13 and the reader will have no idea of how to compute the Characteristic Impedance  Z sub zero which is the whole point of providing the equations in the first place.   In the example, the characteristic impedance is never given.
If these quantities cannot be computed then that should be stated up front.
From the text,  the calculation of the antenna impedance is left to the TLW program on the CD which is fine.
Unfortunately, when I broke out the CD to look at this program, I find the CD is blank, which I’m sure is just one of those things – I’ve written separately requesting the non-blank CD that is supposed to go with the book …
But for a few flaws the 27.5.2 section can save the reader a many Internet/google hours – this is an important section and it would be great to see it improved.

~ by marksun on January 7, 2013.

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