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In order to route my antenna feedlines out of my house, I have to pass through a wall, into the garage and through another wall. All of this involves four 90 degree turns. I am wondering what the possible issues would be with using 2 inch electrical conduit with a couple of conduit bodies (small housing with a cover to help pull wire through an angle). My worry is that the tight turns that these would introduce are not good for coax, specifically LMR 400. Without a conduit though, I fear that the lines could be damaged being exposed in my garage.

I spoke with an elmer in my club and he recoomended I use Pl-259 90 degree elbows inside the conduit body. I had heard that these tend to attenuate signal rather greatly, but he said that was not his experience. I then saw this video https://www.youtube.com/watch?v=5AgFum5K7bw which also supports his claim.

What is a good solution?

ghendricks
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2 Answers2

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I am sure you will get many good suggestions for your situation.

The higher quality coax cables do have a specification for their minimum bend radius during install. For LMR-400 it is 1 inch (25.4 mm) [ref. https://www.timesmicrowave.com/documents/resources/LMR-400.pdf].

I talked with a Times Microwave technical sales specialist and here is his answer regarding bends:

You can bend the cable to a 1” radius and leave it. You just do not want to continue to bend it to such a small radius since it will start to work harden the conductors. If you are planning to get down to such a tight radius, we’d recommend first pulling the cable around a mandrel of roughly 2” diameter such as a piece of pipe. There is no problem with the 1” radius as long as it’s done in a controlled manner.

Without knowing your installation limitations, instead of pull boxes try to use sweep elbows where the pipe turns. The gradual radius is much easier on the cable and your installation will be much less stressful on you as well.

Don't forget to include moisture drains in the pipe if it is has exterior exposure. The diurnal cycles will cause condensation build up in the pipe that will eventually flood the pipe if a suitable drain is not provided.

Glenn W9IQ
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  • The specification for installation bend radius is 1 in / 25.4 mm, but the specification for "bend radius: repeated" is 4 in / 101.6 mm. Personally I take those figures to mean that it's OK to bend the coax to a 1 in / 25.4 mm radius during installation, but it's not OK to leave it like that; it should be left with a radius no less than 4 in / 101.6 mm. – rclocher3 May 08 '18 at 13:36
  • @rclocher3 I think you are correct. When left in a tight bend, the center conductor distorts, or even migrates through, the dialectric causing impedance bumps or worse. A 2 inch sweep has a >9 inch radius so it mitigates this effect. – Glenn W9IQ May 08 '18 at 13:45
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    I would think that "bend radius: repeated" means to bend and rebend the coax multiple times. – mike65535 May 08 '18 at 14:37
  • @mike65535 See my edit above. – Glenn W9IQ May 08 '18 at 15:41
  • @rclocher3 I got the details from TM. See the edit to my answer. – Glenn W9IQ May 08 '18 at 15:49
  • Also keep in mind the effort of actually pulling the cable through such a tight bend. – Duston May 08 '18 at 17:05
  • @GlennW9IQ interesting. That contradicts the "conventional wisdom" that I heard, which says that if a tight bend is left in the coax, then the center conductor might start to migrate through the foam dialectric when the coax gets hot. (I'll take the word of an engineer who has studied the problem over conventional wisdom any time.) – rclocher3 May 08 '18 at 17:40
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A 90 DEGREE BEND IN 1/2" EMT MUST HAVE AN INSIDE BEND RADIUS OF 6". SINCE YOU HAVE SEVERAL BENDS YOU MIGHT INSERT PULL POINTS [ A I/2" FITTING LIKE A HANDI-BOX OR SIMILAR] TO HELP MAKE THE PULL. A GOOD WAY TO PULL IS A PAIL OF WATER AND IVORY FLAKES. MIXED INTO A RUNNY PASTE TO BE SLOPPED ON THE CABLE AS IT IS PULLED FROM THE OTHER END. [PRESUMABLY YOU'LL BE USING A SNAKE OR TAPE.OR YOU CAN BUY WIRE - EZ. THE FLAKE MESS WILL DRY AND YOU'LL BE SET. GOING TO 3/4" WOULD SIMPLIFY THINGS. WITHOUT SEEING THE INSTALLATION, IT SOUNDS LIKE YOU HAVE YOUR WORK CUT OUT FOR YOU. BEST OF LUCK

Rich Morgan - KF9F
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