When it comes to coax cable, most hams would agree that too much is better than not enough. Adding more cable on the fly is tough, especially when crawling around an attic or climbing a tower.
Now that you’re prepared with plenty of coaxes, what’s the best way to handle the leftovers after installation? Ask five hams and you’ll probably get five different answers—possibly six if someone gets long-winded. Myths persist about the correct procedures, and not following established cable rituals creates bad juju (otherwise known as high SWR).
Let’s cut through the pseudo-science and discover whether coiling, cutting, or creating free-range cable is the best solution Q & A style.
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Q: I bought a 100-foot coax assembly, complete with connectors. Once it reaches my radio, there are 20 feet left. Is it okay to crush it in a ball and stuff it behind my desk?
A: Your coax deserves a little respect. Kinks in the cable might damage the coax by crushing the center insulation, which can force the center conductor closer to the shield and change the cable’s impedance. Worst case, it can cause the center conductor to squeeze through the inner insulator and either directly short it or allow the signal to cross through to the shield more easily. Either scenario is bad.
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Q: Are there any rules for properly coiling coax cable?
A: Coil it up like you would a rope. The only thing you need to be concerned about is that the coax cable isn’t wound too tight, which might cause physical damage to the cable. One general rule is not to bend less than 10 times the diameter of the cable. A cable like DX Engineering’s DXE-213U (see DX Engineering’s Coaxial Cable Reference Chart) that is 0.5 inches in diameter should have a bending radius of no less than five inches. That translates into a minimum 10-inch diameter (2r = d), assuming the cable is secured in a fixed coil with tie wraps. I’d be generous and increase that figure by 50% to 15 inches.
Limiting the bend radius reduces signal degradation caused by conductor and dielectric stress. With a smaller bend radius, Ultraflex cables will have considerably more leeway.
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Q: I’ve read on some radio forums that coiling coax is a bad idea. They say it causes inductance and high SWR.
A: There shouldn’t be any measurable inductance just from coiling cable. Coiling cable doesn’t add inductance and will not affect SWR. The coax cable shield isolates each turn from the next one. The only inductance created is on the shield itself, which only happens if common-mode currents are present.
When you coil the coax, it makes an RF choke—a good thing. It reduces common-mode currents on the outside of the cable.
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Q: I don’t want to coil my RG-58 cable. I’d rather run it around the perimeter of the passenger compartment of my vehicle, under the carpet.
A: No problem. You can even let your free-range cable roam through your trunk.
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Q: I just bought a 2 meter mag mount antenna for my car with 16-1/2 feet of coax. I only need 11 feet. Is the 16-1/2 foot length better because it matches the antenna?
A: Mobile antenna mounts sold with coax (usually RG-58) for any frequency up to 900 MHz mostly all come with 12 to 18 feet of coax. That’s enough to reach just about any part of a vehicle without running too short or costing you extra for coax that probably won’t be used. It’s not a magic length. Cable loss is negligible for 5-1/2 feet, so don’t bother trimming it. Coil the excess and stow it under the seat.
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Q: Considering signal quality and losses, what’s really the best way to handle extra coax?
A: It’s always a good idea to cut it to the length you need. If you get rid of the extra, it won’t hurt. All coax cables have some amount of loss. The higher the frequency and the longer the cable, the more loss. But realize you can reach a point of diminishing returns and inconvenience.
Some hams (especially newbies) hesitate to cut the cable because of FCC (Fear of Coax Cutting). When you cut off the excess from a ready-made 100-foot cable, you now have two cables missing a connector—a scary thought. They’re uncomfortable with preparing coax for the various connectors needed and fear they may get burned with sloppy soldering connections that don’t work.
Fear not! Suppliers like DX Engineering can make cables the exact length you want without the hassle of doing it yourself. It’s easy—go to the Custom Cable Builder tool at DX Engineering.com, choose your cable, length, and connectors. Carefully measure the length needed from your antenna to your shack, including turns, pass-throughs, and bends. Use sturdy string or paracord to trace your coax’s exact path for better accuracy. Add an extra foot or two to easily pull out your transceiver to make connections on the rear panel.
