Best of the ISP-Lists

RF on Board

Members of the ISP-Wireless list discuss building a wireless LAN for a large merchant vessel, and come up with some creative ideas for networking spaces separated by waterproof steel walls.

[June 1, 2001]

On the ISP-Wireless list in May, RB inquired,

"Has anybody tried implementing wireless LANs below decks on a large merchant vessel? We're talking about ships made of steel here, not wood. The goal is for a hand to be able to take a wireless-equipped laptop from bow to stern below decks, through all the bulkheads, and maintain a network connection to the ship's wired infrastructure."

AD suggested it might be difficult:

"I used to do a lot of work testing combat frequency radios inside rooms with steel floors, walls, and ceilings. We called them 'RF sealed' because the loss was on the order of -120db. We did not want any receiver nearby to be able to pick up the frequencies and modulation. We were also told that the Navy did not have to use these special rooms because they had big ones called 'ships'. That should be a hint."

A number of respondents recommended one way to get around the problem:

[DB noted] "Mining companies and some railroads use a 'leaky coax' cable that they run through the mining shafts so that cell phones can work. It's basically a cable that has cuts in the shielding enabling signal to leak out. You could run this cable throughout the boat: it will act as one long low gain antenna."

[DM agreed] "Also check out Rubytron's leaky coax alternative, at www.rubytron.com."

JD challenged the Rubytron idea:

"If my understanding of ships and bulkheads is correct, these have to be able to be sealed completely. Drilling a hole and running cable probably isn't going to work."

Others offered different suggestions:

[JL advised] "My testing at VHF marine (156 MHz) frequencies indicated that a shipboard repeater was justified to get decent coverage. There's even a marine channel pair that's designated specifically for the purpose. UHF seems to go through the cracks and windows a bit better, and I suspect that 2.4 GHz will have the same characteristics. We found that going through the outside windows (where present) worked fairly well, but was useless for any interior communications. Fortunately, most of the interior doors were wood, so some RF did manage to penetrate."

[AM added] "It sounds like putting at least one AP per deck per compartment would do the trick. That might take 20-30 APs, but everyone would have a really strong link."

—End

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