Coaxial Lines

Author: N.H. Crowhurst

Sometimes a connection of considerable length will be required between a microphone and its amplifier. Because the signal is at a very low level, we have to take precautions against possible unwanted pickup due to induction from magnetic or electric fields. In addition, we must transfer to the amplifier as much as possible of the audio voltage picked up by the microphone, to maintain the signal well above the noise level.

As a protection against electric pickup, a high-impedance connection starting from a 50000-ohm or higher-impedance microphone would have to use a concentric or coaxial arrangement of the wires. The outside conductor (sleeve) would be connected to ground, so that any electric field reaching it would be conducted along this outer sleeve and go to ground at the amplifier without inducing any voltage on the inside wire. This effectively protects against electric pickup.

This type of line also protects against magnetic pickup, because any magnetic field will induce exactly the same voltages in both the inside wire and the outer sleeve. As the complete circuit from ground to the live side of the input at the amplifier consists of the entire line out to the microphone and back, the total voltage induced by the magnetic field will cancel out giving zero resultant induction.

Shielding a line against electric and magnetic fields

The reason why this type of line is not satisfactory is that the capacitance between the center wire and the sleeve is very considerable - usually between 30 and 100 micromicrofarads per foot. (This capacitance shunts the source and load impedances between which the line is usually connected.) A 1000-foot length of coaxial line will have a capacitance of 30,000 to 100,000 micromicrofarads, which on a 50,000 ohm circuit would have a reactance equal to the source impedance at a frequency between 32 and 100 cycles. This would cut the signal voltage fed to the lead by 70% (3 db). At 1000 cycles, the reactance would be about 1600 to 5400 ohms, resulting in a loss of 20 or 30 db, and the loss gets progressively more severe with higher frequencies. Even a 10-foot length of coaxial line, with a capacitance of from 300 to 1000 micromicrofarads, will have a reactance of 50,000 ohms at from 3200 to 10,000 cycles, which will cause an appreciable loss of signal at frequencies above this point.

A reactance chart used for calculations of high frequency losses

Low-impedance circuits, as already mentioned, are more susceptible to the pickup of magnetic fields than electric fields. Use of a concentric line helps to shield them, and a twisted line improves matters by making the field induce voltages opposite directions in successive twists. The twisted line may also be shielded by a separate ground sleeve to minimize any electric pickup.

Reducing the effects of magnetic induction