These days light aircraft come with lots of instruments that require thin co-ax cable for GPS antennas, transponders, collision avoidance systems, tracking and navigation systems.Like this GPS antenna as an example.
Whats the best way (from the signal quality perspective) of handling extra co-ax?
Space behind an instrument panel is often at a premium, manufacturers typically supply antennas with 1-2 meters of cable but in a small aircraft (microlight or glider) the antenna might be only 12 inches from the instrument. With 4 or 5 such antennas the space behind an instrument panel can quickly become a cluttered mass of wires. The people who install these instruments are often the pilots or engineers with little knowledge of electronics. There seems to lots of 'psedo-science' around how to handle the extra meter or two of cable.
- 1) the simplest (and most common) option seems to be to leave the cable excess in a neat loop (like in the photo) secured with a cable tie. However, some say this will cause inductance which will effect signal quality (my view is that inductance is a current related phenomena, and the current flowing through the coax will be negligible)
- 2) the 2nd prevailing opinion is that the spare cable should be looped in a figure of eight fashion to prevent induction.
- 3) another suggestion I've heard is to loop the spare cable in the
biggest possible loop (i.e. a two meter cable looped twice into a
30cm diameter loop) the argument here is that fewer turns means less inductance.
- 4) the best solution is almost certainly to trim the cable to the exact length, however this isn't without it's own problems, the SMA/SMC cables are so small that they can be really hard to crimp, and antenna often need to be moved about when searching for optimum positioning, so there's always the danger of getting the length wrong
So back to the original question. From an electrical engineering perspective, with a view to getting the best signal quality without interfering with similar instrument nearby whats the best way of handling excess cable? Is it one of the options listed above? Or something else? Or maybe it won't really make any difference?