## 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

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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 :