What to do with excess co-ax cable

Question

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?

Answer 1

• Best would be to shorten the cables. A shorter cable means less loss of signal in the cable, and a neater installation. Doing so requires proper tools, new connectors, and some practice in attaching the connectors to the cables. That's too much to expect pilots working on their own planes, or even mechanics who do the work for many pilots. There are simply too many differing connectors, cables, and tools. Doing it wrong is worse than just leaving the cable a bit too long.

• Leaving the cables at the stock length is the safest way. The cables were assembled at the factory by technicians (or machines) that can do the job properly and the cables can be tested for proper function.

Don't worry about induction losses in the cables. They are shielded. There shouldn't be any measurable inductance just from coiling them up. Whether you just coil them or make them into a figure eight shouldn't make any difference, either.

What you do need to be careful of is coiling them too tightly. The cables are made of several layers of materials. The proper function of the cable depends on the relationship of each layer to the other.

Here's a typical coaxial cable (from wikipedia.)

In the cables you are dealing with, the layers are thinner and more easily damaged. A kink in the cable can cause the center conductor to squeeze through the inner insulator and either directly short it or allow the signal to more easily cross through to the outer conductor. Either is bad.

Make your coils large, don't kink the cables, and you should be fine.

Answer 2

The best thing to do is leave the cable the original length, and to gather the excess in whatever is the physically tidiest way. Few people are equipped to shorten the cables and remake a connector properly, and it needs to be done properly. If you have to ask, don't do it.

There are two aspects to 'cable inductance', differential and common mode.

Differential mode. This is the inductance of the inner with respect to the return shield that the signal sees. The whole point of coaxial cable is that this inductance is completely screened by the coax outer, and is totally unaffected by whatever physical configuration the cable takes. Loop it small, loop it big, put it into a figure of 8, none of these will affect the signal handling at all.

Common mode. This is the inductance of the outer with respect to the instruments at each end. This is affected by the physical configuration. The main effect of this inductance is to modify slightly the electromagnetic compatibility performance of the box. Does RF leak out of the box onto the cables and get radiated, or is the box more or less susceptible to strong RF fields around the box getting picked up on the outer and leaking into the box? With a good design of the box, all of this shielding is provided where the coax enters the box. However, if this aspect has been 'value engineered' to only just meet requirements, then cable looping might make a difference. You often see cylinders of ferrite clamped onto computer video and USB cables, these make an order of magnitude more difference to the emissions performance than looping the cable around.

If you do have strange interactions between, for instance, your transmitter and other instruments, then moving the cables around might be a way to diagnose whether there might be any interaction from leaky boxes, but it would not be the way to get a cure. A reliable cure would only be effected by buying higher quality instruments, or putting clamp-on ferrites on the cables close to the offending boxes.

Answer 3

1. Leaving the excess coax in a loop as shown in your picture is perfectly OK. Doing so does not cause or increase inductance as the coax is basically a shielded conductor. Its inductance is determined by the geometry (conductor diameter, insulation thickness, etc) and not by how it is arranged physically. Just make sure you follow manufacturer's or best-industry practice and don't bend the coax too much by trying to make a really small coil. Our standard is to keep the bend radius at least 10 times the diameter of the cable.

2. The figure 8 arrangement will neither hurt nor harm the inductance of the cable, for the reason described above.

3. No need for the larger diameter loop.

4. This would be the best solution. But you need to make sure the new connectors are properly attached to the coax. And you also want to allow for a service loop on each end to aid in attaching and removing the coax from its jack on the equipment.

Answer 4

Inductance (of the whole cable) is unrelated to most usage scenarios of the coaxial cable. The currents flowing in the core and the screen should exactly cancel each other so no magnetic field is expected outside the cable. Thus, all inductance-related questions are pointless.

Related to signal quality, the best is to shorten the cable to the needed size AND crimp it adequately. People usually fail on the crimping part and even if they don't, the signal level increase is usually minor, because manufacturers generally use adequate cables for the signals used.

Creating bigger loop is good up to the point of radius, say, 20x of the cable width. Making smaller loop risks damaging the cable, making bigger loop is not any better. Making 8's in the loop is not good - curvatures get sharper, increasing the possibility of damaging the cable.