Can I run my antenna coax near a gas line?

Question

I just moved to a new place and would like to put an antenna up in the attic. My shack location is in the basement with the furnace and water heater. In my explorations, I've discovered that there is a 2ft by 2ft shaft in the wall going up past one of the main-floor bedrooms that contains the chimney for the water heater/furnace. I realized this shaft would be perfect for running my coax feedline from my basement shack to the attic. The shaft is large enough that I'm not worried about any heat from the chimney damaging the coax; I can place the coax far enough away.

The concern I do have is that one of the natural gas lines going into the furnace runs fairly close to the bottom opening of this shaft. If I did run coax up it, the furthest I could get it from the gas line would be 2-3 feet.

My questions then:

1. How significant is the danger in placing a coax feedline near a natural gas line? Right now, I'm only running QRP, though I'd like to get a higher-power radio some time in the future.
2. Do I need to worry about lightning if the antenna is inside the attic?
3. Does installing a current balun at the feedline-antenna connection make any difference? I was planning on a simple dipole/inverted-V.

FWIW this house was built in 1991-1992, and I believe everything is up to code.

Answer 1

The common sense way, followed by most radio hams, is not always strictly according to the rules. In my town for example, permits and inspections are required for any wiring beyond changing a lightbulb, including low voltage like loudspeakers or coax. This is my common-sense take, you could also ask a licensed electrician.

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1: Gas pipe etc: If the chimney is hot metal, not cold plastic, then you should worry about it somehow igniting the coax. Coax might burn, and in a well hidden vertical shaft deep in the house that's not good. At minimum, find a way to secure the coax away from the hot chimney. Also test the chimney temperature with your hand. My ancient furnace exhaust gas is about 200 C / 400 F and the pipe runs against some wooden framing with a little half-inch metal spacer thing.

Is the shaft wood or brick? Is there already something non-metal in there, or just a metal gas pipe and a metal chimney? If it's not a general-purpose cable-and-pipe duct, you may be breaking some rule by adding a flammable cable to it.

FEP/PTFE cables are good for up to 200 C and can't be ignited. RG400 would be fine for HF.

I wouldn't worry at all about the gas pipe being near the coax. It will be a heavy metal pipe, tested for air-tightness. Gas pipes run all over the house near to regular electrical cables carrying 120 V and many amps. Just don't damage it mechanically.

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2: You always need to worry about lightning.

Keeping the antenna indoors will make it more according to code, so if something bad happens they won't be able to blame it on your antenna, fairly or unfairly. Any cable that runs from outside to inside needs to be protected in specific ways - at least grounding the shield to an earth rod, like a cable TV coax is. This is also good practice, will reduce the chance of huge sparks flying across your shack.

Apart from that I don't see much difference between the antenna hanging just under the plywood, to one draped on top. It's still the highest metal object in the house. Some suggestions for protecting your indoor-antenna setup:

1. Ground the radio chassis to the mains earth with a separate solid wire
2. Better, have the coax go through a barrel connector on an earth plate, and ground that.
3. ground the plate to an outdoor earth rod
4. pass the mains power for the shack through a surge protector mounted on the same earth plate
5. use a coax surge arrestor suitable for your frequency and power level, install that through the ground plate instead of the barrel
6. run a separate #0 / 10 mm^2 ground wire from the antenna directly to its own ground rod by the shortest path

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3: A balun is a good idea - it will improve the antenna performance and reduce the RF on the coax. At QRP you can even get them from Amazon etc.

With 5 W you won't cause much trouble to your computers and TVs in the house, but by reciprocity if the coax is part of the antenna, it'll also conduct all sorts of noise up to the antenna. The balun will eliminate some of this common-mode noise.

For 100 W, you should think about what happens if the balun or antenna overheats and catches fire during a long digital QSO. It's unlikely, but it's also lying against the wood where you can't see it. Consider a balun in a metal box. Mount the antenna feedpoint away from flammable materials.

More than 100 W you start to have the risk of arcing too, it might be safer to take it outside.

Answer 2

Q1. How significant is the danger in placing a coax feedline near a natural gas line? Right now, I'm only running QRP, though I'd like to get a higher-power radio sometime in the future.

A1. It's insignificant and nothing to worry about. Just do not use it for any cable support.

Q2. Do I need to worry about Lightning if the antenna is inside the attic?

A2. No, sir, not in this case. NEC requires you to use a surge protector and bond the coax before it enters your home to discharge any harmful voltages generated by Lightning or electric utility striking the antenna. You would only do more harm by trying to ground it because if you do, it requires that ground coming from your utility meter. In a nutshell, NEC Articles 250 and 810 require you to route all outside services like CATV, SATV, Telco, and any outdoor antenna lines to the utility AC Service Ground, bonded to the ground, before entering the house. Otherwise, you will place yourself in a Ground Loop, inviting Lightning inside for lunch and a good look around, burning up your house wiring and radio. So, hiding in the basement or attic, no worries about Lightning finding your antenna.

Q3. Does installing a current balun at the feedline-antenna connection make any difference? I was planning on a simple dipole/inverted-V.

A3. I assume you are referring to talking about a 1:1 Common Mode Choke at the antenna feed point It makes a huge difference but has little to do with your question. Hams use poor, severely compromised antennas, and when you cram one up in the attic, you are doubling down on poor performance. Consequently, you will have significant common mode current riding on the coax shield, bringing RF into the shack, and will find you when you key up the mic. So yes, you will need the best CMC money you can buy.

One thing to look out for is having RF issues when you key up. For example, open/close your garage door, turn on your oven, wipe out your CATV modem, or eject grandma from her power lift chair. Best of all, RF burns when you touch your radio while keying up. You will have a lot of fun chasing it all down. It might not be an issue with QRP, but it's something to look forward to when you get a real radio.

Good luck and 73's