Finding velocity factor of coaxial transmission line – a challenge

An upcoming article works through an approach to finding the velocity factor of a sample of coaxial cable using an antenna analyser.

As a precursor, this article poses a challenge that will identify the issues relevant to the problem.

Case 1:

A Rigexpert has been used to measure the first quarter wave resonance of a length of ‘unknown’ semi air dielectric RG6.

The length of RG6 Dual Shield is terminated in an F connectors at one end, the other end cut cleanly square. It is connected via  N(M)-BNC(F) and BNC(M)-F(F) adapters to a Rigexpert AA-600 antenna analyser and the quarter wave resonance noted (ie the lowest frequency at which measured X passes through zero).

Above, the line section is connected to the Rigexpert via adapters, and the length overall is measured from the case of the AA-600 to the of the cable. The measured length is 1.077m, make any adjustment to that length that you think is justified on the information presented here.

Above, the quarter wave resonance measurement of the cable. The analyser is adjusted to the mid point of the interval between X=-0.1Ω and X=0.1Ω.

So, the key data apart from the pic of the analyser connection is the cable length (measured from the instrument case as shown) and resonance are:

  • 1.077m: 57.352MHz.

What is the velocity factor of the cable itself (ie absent couplers adapters etc)?

Case 2:

A 1.917m length of cable (connector end to connector end) is connected to a Youkits FG-01 analyser and the first resonance found at 25.025MHz.

What is the velocity factor of the cable?

The classic approach is to calculate the length of the cable in degrees at 25.025MHz, it is 57.61° so the VF=57.61/90=0.640… or is it?