METAL ANTENNA MAST - IS IT OK?

badspell68

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I'm planning on bolting a 40' metal mast to the side of my building, grounding it with 8' rod, adding lightning arrestors, then mounting two 10' tall antennas. Is this ok? I have been told that I am adding significant lightning risk and that I should be using a fiberglass rod.

Is this correct and should I re-thing my design?
 

prcguy

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Metal mast is fine, its not much different than the antenna and its coax running down an insulated mast. You might want to check into grounding per the National Electrical Code, do a search on NEC article 810 which is specific for antennas.

I'm planning on bolting a 40' metal mast to the side of my building, grounding it with 8' rod, adding lightning arrestors, then mounting two 10' tall antennas. Is this ok? I have been told that I am adding significant lightning risk and that I should be using a fiberglass rod.

Is this correct and should I re-thing my design?
 

badspell68

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Thanks and yup, got the ARRL grounding book. I am using a lighting arrestor (one of the gas ones), but should I also us a grounding block on the coax in addition to the arrestor?
 

mmckenna

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I have been told that I am adding significant lightning risk and that I should be using a fiberglass rod.
You are wise to question the validity of that statement. The metal antenna and conductive coax is going to be an equal path for energy as the mast.
A few fractions of an inch of cable jacket isn't going to protect you from a lightning strike that has traveled thousands of feet through the air.

Thanks and yup, got the ARRL grounding book. I am using a lighting arrestor (one of the gas ones), but should I also us a grounding block on the coax in addition to the arrestor?
The lightning arrestor will act as the grounding block. No need to add an additional connection to the coax.
What you should consider, especially if you are in a lightning prone area, is grounding the outer jacket of the coax at a few points along the mast/tower. Usually on the commercial side, installers will use "grounding kits" designed for the coax in use. The coax gets grounded near the top of the mast and at the bottom of the mast, and sometimes additional points in between. This helps get some of the energy onto the mast rather than the coax. What you want is to create the shortest possible path for the energy to reach the ground.
 

mmckenna

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mmckenna

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I have found it very stiff and once in the house I am not able to get it to where I need because of its stiffness.
OK. Might I suggest running standard LMR-400 down the mast to your lightning suppressor, then the LMR-400-UF to your radio? Not a huge impact, but it saves some money and a bit of loss.

It is difficult to run inside a structure, but can be done if you are careful. I've got a run of LMR-600 at home running from the roof into the kitchen.
 

chief21

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Taking mmckenna's suggestion one step further, a short jumper of RG8x from the LMR400 to your radio will better allow you to put the radio where YOU want it rather than where the coax wants it. It will also reduce any stress on your radios coax connection.
 

mmckenna

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RG8x

Is is flexible and what about 400 ultra-flex to do the same thing?
LMR-400 is heavier and a bit stiffer. The concern is that it'll put strain on the antenna jack on the back of the radio. Over time, that risks damaging it. It's not impossible to repair if you re good with a soldering iron, but breaking that connection and not realizing it can be an issue.

For a short run, the additional losses in the RG-8X are pretty small. The benefit of not busting off your antenna connector is well worth it.

I've used LMR-400 for making the connections to rack mounted radios where I can properly secure the cable and take the strain off the antenna jack. If that's your application, it'll probably be fine. If your radio is sitting on a desk, then you want to be careful and the smaller cable can prevent issues.
 
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