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Characteristics of the  Loop Cable

The cable must not be shielded. A lot of communications cable come with an aluminum foil around the conductors to keep out stray signals. The shield also prevents radiation from leaving the cable, so this is not desirable for loop cable.
The conductors inside the cable must not be twisted. Wrapping two or more conductors together, twisting, is another technique to prevent radiation or pickup from stray fields. Cables that say "Twisted Pairs" are not  to be used.
For a portable system, it is highly desirable for the cable to have stranded conductors. This makes the cable more flexible and easier to handle.
The size of the conductors, the number of conductors, and the total length of the cable determines the impedance of the cable. Impedance is very important for a DIY Audio loop and it must match the impedance of the power amplifier.
If a loop system is intended to be portable, the cable needs a connector to join the two ends of the cable together. The connectors need to be heavy duty, reliable and easy to use. One such connector is found at automobile parts stores and it is intended for wiring lights to trailers. It is a four position connector, so four conductors in the cable work out fine.
20 Gauge speaker cable would be ideal to use, except that it only has two conductors. It may be necessary to go around the looped area twice to achieve enough impedance so the power amplifier is not overloaded. Example: 100 feet of 20 gauge speaker cable, with each conductor wired in series, has a resistance of about 2 ohms and a XL value of 1 ohm, resulting in only 2.23 ohms impedance. Therefore, at least  200 feet of speaker wire should be used, even for a 100 foot circumference, to bring the impedance value up to 4 ohms.