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Audio should be sent to the SSTRAN transmitter via shielded audio cable. However twiested pair (like CAT5) can be used if your audio source has a relatively low output impedance such as is the case for the headphone output of a computer. Most audio line outputs of mixers, etc, actually have relatively low output impedance, so they can be good too.
With twisted pair, be very careful to look at the wire colors and be absolutely sure you are connecting ground-ground and signal-signal. Reversing the connections will cause problems.
Regardless of whether you use shielded audio cable or twisted pair, you can still experience ground loop problems. A ground loop isolator (like the Radio Shack unit) is simply a 1:1 transformer. Any other connection that includes a transformer in the path will do the same thing.
Ground loops are hard to visualize. The basic problem is that a small AC current can flow between your grounded audio source (computer or other 3-prong-plug grounded audio equipment) and the antenna ground because of the difference in ground potential between the house AC ground and your antenna ground.
The first line of defense (as has been said previously) is to connect a wire from your antenna ground system back to the ground rod at your home AC service entrance. This will eliminate the difference between AC power ground potential and the antenna ground potential. If this doesn’t work to your satisfaction, then try the RS ground loop isolator.
The antenna ground wire doesn’t actually need to be connected directly to the AC service entrance ground rod. You can run the wire into you breaker box and connect it to one of the neutral bus bar terminals. The neutral bus bar is connected to the ground rod, assuming your box is code compliant. You can easily check this by looking for the ground rod wire connection to the neutral bus bar in your breaker box.