5.1) Velocity of Propagation

      The velocity of a signal in free space or an air line is approximately 3 x 10⁸ m/s. The velocity of a signal in a coaxial cable is less than that in free space and is mainly dependent on the properties of the dielectric. A low loss coaxial cable working at high frequencies has a velocity of propagation that can be calculated from Formula 5.1.

         Formula 5.1

Where:    v is the velocity of propagation in m/s

                           C is the distributed capacitance per unit length

                         L is the distributed inductance per unit length

5.2) Velocity Factor of a Coaxial Cable

       It is possible to measure the velocity factor of a transmission line using the V/C Detector to detect phase cancellation. If a reflected signal in a coaxial cable is due to an OPEN termination, then at a point a quarter wave length (90°) from that OPEN termination the reflected sine wave will be 180° out of phase with the forward sine wave and a voltage cancellation will result. The Two Metre Cable is used to determine the velocity factor using this cancellation effect.

   For a 2 metre cable that is a quarter wavelength (λ/4)

   A full wavelength (λ) is 2 x 4 = 8 m.

   The frequency of a signal of wavelength λ can be calculated from:-

            Formula 5.2

Where λ is the wavelength in metres

       F is the frequency in MHz

     In free space the frequency of a signal of wavelength 8 metres would be

The test equipment is connected as per Figure 5.1

Figure 5.1

    The Signal Generator is set to give an output of +0 dBm at a frequency of 37.5 MHz. This gives an amplifier output of approximately +15 dBm (1.26 Volt rms. at 0.025 Amps into 50 Ohms). The OPEN Termination is connected to the output of the Two Metre Cable.

    While observing the DVM, the Signal Generator frequency is slowly reduced until a voltage null is reached. The frequency of the Signal Generator  and the indicated  voltage are recorded. The overall length of the coaxial cable is measured from the red line of the Voltage and Current Detector along the Two Metre Cable to the red line on the OPEN Termination, and the test assembly length is recorded. The Velocity Factor is calculated using Formula 5.3.

          Formula 5.3

Where          V_f is the velocity factor (there are no units)                              

Fg is the Signal Generator frequency for λ/4 in Hz

L is the length of the cable in metres                    

   The result obtained can be compared with the manufacturers quoted velocity factor.

     Note that the voltage null does not reach zero. This is because the reflected sine wave has a lower voltage than the forward sine wave due to the attenuation of the cable. The difference of these two values is therefore greater than zero. We record this voltage for use in the next measurement. This result also confirms that the Voltage and Current Detector measures the SUM voltage of the forward and reflected sine waves.

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