But the free-space velocity is the velocity of light, c, and is given by
Provided that the medium is nonmagnetic (which is true), pr = 1, in which case Eq 4 reduces to:
So, reduced to the simplest terms, the velocity factor of a feeder cable is equal to the reciprocal of the square root of the effective dielectric constant of the material between the center conductor and the braid. Fig 1 shows the variation of velocity factor with dielectric constant (relative permittivity), with some common materials (from air to mica) marked on the curve. It is this velocity factor that makes the following technique possible.
Many readers will recognize Fig 2 as being a very simple relaxation oscillator based upon a Schmitt trigger inverter. The period of oscillation (ie, the time taken for one complete cycle) is determined by the time the capacitor, C, takes to charge and discharge through the resistor, R, between the two input voltage thresholds of the Schmitt trigger. As with all astable circuits, the period is the result of a simple delay circuit, in this case the resistor/capacitor combination.
Fig 3a shows the circuit of the delay-line oscillator used for measuring cable lengths. A friend called it "the
Fig 2—A simple relaxation oscillator.
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