Better Coaxial Cable Runs
Jun 1, 2013 8:30 AM, By Doug Irwin, CPBE DRB AMD
In last month's Tech Tips I finished by mentioning that antenna installations should use a flexible jumper (pigtail) to transition between a stiffer coax and the antenna connector itself. The reason I gave was that the transition, done with coax only, usually forms some sort of a loop that has a lot of leverage on the connector itself, making it susceptible to damage from the wind over a period of time.
That's not the only problem with outdoor coax installations though, when it comes to dealing with the elements. Let's look at a couple of others.
The coax itself always seems rather robust when one rolls it off a spool and runs it up a tower or wooden pole. However, in reality, apply the right amount of wind and rain, and failure of the coax is practically a given.
Figure 1 shows a 7/8" Heliax that comes out of the transmitter building, has a horizontal run, and then makes a 90-degree bend to run up a wooden pole.
In this case the actual bend is run through a piece of PVC piping, which is a good idea in this case especially, because it makes for a convenient way to secure the cable to a steel messenger cable. The PVC also helped in forming a reasonable bend in the Heliax. It should be noted, however, that PVC will not last long exposed to sunlight. I will become brittle and break. ABS and other materials are better suited for outdoor use.
The messenger cable is a device used to provide support and strain relief to cables that run across from the transmitter building to the pole. I also want to note that this wooden pole has been treated with creosote, which will make it stand up to the elements for much longer than those without creosote. (The railroads learned that lesson 150 years ago.)
Figure 2 shows a run of several coaxes across from the building to a different wooden pole.
Aside from the fact that this pole is not treated with creosote, it's also obvious that the cables are just hanging there, ready to be damaged in the wind. (The wind turbine in the background is a clue as to how windy this site can be.)
Fortunately, riggers we hired (ComPlus of Frazier Park, CA) were able to fix the original installation after the fact by adding a messenger cable. Figure 3 shows the results.
Finally, one point I want to make about running coaxes in to a building. It seems pretty obvious that you don't want water coming in on a cable run. Figure 4 shows that this wasn't considered very well.
This cable runs in to the building pretty much horizontally. Any water running down the cable will run right into the building. The preferred way is to provide a drip loop (still supported by the messenger cable of course) that would make the water try to run uphill to get in to the space. That's a much more logical way of keeping water out.
The finer details of coax installation are important to consider when it comes to long-term use and reliability.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at email@example.com.
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