coax across steel roof

[VK4WDM]

I need to run some RG213 across my steel roof. One cable is for an 80-10m wire, the other is for a 6m, 2m, & 70cm vertical. The lenght is about 10m.

I have two options: 1. To simply lay them on the roof. 2. To elevate them about 25cm above the roof on PVC spacers. Will option 2 reduce losses?

73

Wayne VK4WDM

[VK2KRR]

Wayne,

I would elevate it for sure. I'd be more concerned with heat. The only losses it may reduce is losses from when the cable melts if its laying on the tin itself.

[VK2JDS]

run in conduit, electrical supplier will have lots. elevate and clamp down. should be ok. seal the ends with silicone
73 Dave

[VK6TU]

Wayne,
I have got 3 runs of 213 across my roof, the coax runs from the back to the front of my house and is just sitting on the roof, most on some agle iron that is used to hold the roof down, have had no problem with heat and it gets hot here

I should put it in some conduit just to protect the coax from the sun, but after 6 years I think it is a bit to late to worry

73 from MIchael
Karratha NWWA

[VK5ZD]

Hi

To answer the original question, being in contact with the steel roof will make no difference to the losses in the coax.
I also agree the reasons given by others for putting it in conduit.

73
Iain

[VK4WDM]

The question was the result of a discussion I had with another ham who told me that laying the coax on the roof would result in "inductive losses" - which I don't agree with. I do have concerns about the heat so conduit or similar makes sense.

73

Wayne VK4WDM

[VK3HJ]

I was told a quite believable story that coaxial cable was invented for use in ships where the feeder had to run near and through metal bulkheads. The EM fields *should* be entirely contained within the braid, and be unaffected by other coaxial feedlines and metal objects nearby.
A conduit would be a good idea, but I've not had any problem with coax laying across a colorbond roof.
A PVC pipe large enough to run as many cables as you want, directly on the roof should do the job just fine, and keep it all neat and tidy too. You don't want the Roof Clutter Police knocking at your door!
73,
Luke VK3HJ

[VK3AIF]

My unconsidered thoughts are that there should be no effect whatsoever unless there is a feedline radiation problem and if there was such an undetected problem, as I know you would fix it if you were aware of it, the effect could go either way i.e. beneficial or detrimental depending on which way the cookie crumbled. Either way, not an ideal situation and results a bit unpredictable to say the least.

I think Luke 3HJ and others have hit the nail squarely on the head so no need to repeat their comments. As for melting, I don't know, but it would no doubt soften up appreciatively and subsequently be more prone to damage and I reckon it would always be good to protect it from the UV and other possible damage but have I done that? No!

[VK2JH]

Hi Wayne,

I agree with the others re no electrical affect. However, if you have some friendly cockatoos and/or you want to protect it somewhat from UV and the weather, then 25mm (or 1" Imperial) agrictural poly pipe is great to fit one run. Larger sizes may fit more than one run, using a wire or rope pull-through. Unfortunately, it is a bit expensive to use new, but if you know someone on the land with some old stuff. I would not seal the ends, rather angle it to prevent direct ingress of water and allow it all to breathe.

Cheers,

John
VK4JHC/VK2JH
Mount Hallen Qld
"Forum Novice, but Diehard Lurker"

[VK4UH]

In my opinion it wont make a dot of difference either way in term of loss

There with be negligible field outside the coax even with a less than perfect SWR

The only difference will be mechanical, in terms of protection of the outer sleeve from chaffing or UV damage

Kevin VK4UH

[VK2AAH]

Remember some of that pretty ordinary RG213 coax Dick Smith's used to sell? I think the screen was so sparse that the metallic roof may have made a teeny difference. That was horrible stuff... but great if you just wanted to slide the inner out...

I loved John's comments about the cockies and the PVC! Not sure it will stop them but it will give them variety in their diet & slow down how quickly they finish their "meal". I saw what Deniliquin cockies can do- chewed straight through the stainless braid protection & chewed a SMD in two! They make 'em tough in the bush.

Cheers,

Richard
VK2AAH

[VK2JH]

Who's Jack and that should read "agricultural", NOT whatever I typed before!!

John
VK4JHC/VK2JH
Mount Hallen Qld.

[VK2XSO]

The question was the result of a discussion I had with another ham who told me that laying the coax on the roof would result in "inductive losses"

I was going to ask where this had come from, but you've answered that.
Urban Myth.

Remember some of that pretty ordinary RG213 coax Dick Smith's used to sell? I think the screen was so sparse that the metallic roof may have made a teeny difference. That was horrible stuff...

I haven't seen RG213 that bad, but then I never bought it from DSE.
But this practice is still going on. Jaycar are selling coax which is pretending to be RG59 and RG58.
Horrible isn't the word I use. Embarrassing is the word I use.