posted 11-23-2002 01:47 PM ET (US)
In a well-matched antenna-transmission line system, there are no antenna currents flowing on the outside of the outer conductor of a coaxial (unbalanced)transmission line. The transmission line can be arranged into loops or coils without affecting the transmission line currents flowing on the inside of the outer conductor.It is not unusual in high-power transmission lines for the outer conductor to be constructed of bare copper tubing. Even with power levels of 10,000 watts or more flowing in the (inside) of the transmission line, there is no current flowing on the outside of the outer conductor, and one can touch the bare copper surface without a problem. In fact these large transmission lines (6-inch diameter) are suspended from clamps and are not insulated from grounding.
These same laws of physics apply to 25-watt marine radios and the 0.375-inch transmission lines used in that application.
If the transmission line is of marginal quality and the shield coverage is low (as is often seen in expensive but poor quality "marine grade" transmission line) it may be possible that looping the line into a coil could cause it to be susceptible to pick up of stray fields. However, the antenna, usually a few inches away, is intentionally designed to pick up magnetic fields (that is how it receives radio signals), so the problem of noise induced from coiling of the transmission line seems to be not a significant one.
That said, I would not intentionally create a coil of transmission line and pass the spark plug leads through its center.
Because the matching of transmission line to antenna is often poorly accomplished in marine band antennas, they may be more suspectible to having some antenna currents flowing back toward the transmitter on the outside of the outer conductor. In this case it is possible to suppress these currents by creating a small coil of transmission line. To the currents flowing on the inside of the transmission line this coil is invisible. To the currents trying to flow on the outside of the transmission line the coil represents a small amount of reactance which tends to suppress the flow of such currents.
As a matter of fact, on my boat I have the excess transmission lined coiled up into a solenoidal winding for precisely this reason.