Wireless Hacks

Microwave Cabling Guide

There are myriad antenna feed cables available. Which is the right one for the job?

Not all coaxial cable is appropriate for 2.4 GHz use. The same piece of cable that delivers high quality video and audio to your TV is nearly useless for connecting microwave antennas. Choosing the proper type and length of cable is just as important as choosing the right antenna for the job. A 12 db sector antenna is useless if you lose 18 db in the cable that connects it to the radio. While all cable introduces some loss as signal travels through it, some types of cable do better than others at 2.4 GHz.

LMR is a kind of coax cable made by Times Microwave, and is possibly the most popular type of cable used for extending 802.11b networks. LMR uses a braided outer shield and solid center conductor, and comes in various sizes.

Heliax is another kind of microwave cabling made by Andrew. It is made of a semi-rigid corrugated outer shell (a sort of flexible copper tubing), rather than the braided strands found in coax. The center conductor can either be solid or a corrugated tube inner conductor. It is designed to handle loads MUCH greater than (legal) 802.11b installations, is very expensive, and can be difficult to work with. It is also extremely low loss. The foam dielectric type part numbers start with LDF.

Warning

Don’t mess with air dielectric unless you enjoy the challenge of keeping your feed lines pressurized with nitrogen. Air dielectric cable at 802.11b power levels is like the proverbial elephant gun to kill the mosquito.

In addition to Times Microwave and Andrew’s offerings, Belden also makes a very common piece of cable that works okay in the 2.4 GHz range. You’ll frequently see references to “9913”; this is Belden 9913.

The properties of some common cables are provided in Table 4-1. Generally speaking, the thicker and better built the cable, the lower the loss (and the higher the cost). Cable in excess of half an inch or so in thickness is difficult to work with, and it can be hard to find connectors for it. Whenever possible, order the specific length you need, with the proper connectors preinstalled, rather than try to cut and crimp it yourself. A commercial outlet will usually have the tools and experience needed to make a well-built cable. The best cable in the world won’t help you if your connector isn’t properly installed.

Table 4-1. Attenuation, size, and approximate cost of microwave coax

Cable type	Diameter	Loss in db/100’ at 2,500 MHz	Approximate price per foot
LMR-200	0.195”	16.9	$0.37
LMR-400	0.405”	6.8	$0.64
LMR-600	0.509”	4.4	$1.30
LMR-900	0.870”	3.0	$3.70
LMR-1200	1.200”	2.3	$5.50
Belden 9913	0.405”	8.2	$0.97
LDF1-50	0.250”	6.1	$1.66
LDF4-50A	0.500”	3.9	$3.91
LDF5-50A	0.875”	2.3	$2.27
LDF6-50	1.250”	1.7	$10.94
LDF7-50A	1.625”	1.4	$15.76

To sum up: use the best quality cable you can afford, at the shortest length possible. A couple of dB here and there really adds up when dealing with the very low power levels of 802.11b. If you want to put an antenna on the roof, you might look into weatherproof enclosures for your AP, and mount it as close to the antenna as possible. Then run your Ethernet cable as long as you need (up to 100 meters!)