Tech filter-Do you know which coax type is used to extend this antenna?
May 22, 2013 8:35 AM   Subscribe

I am currently installing a HP Indoor-Outdoor Point-to-Point Dual Band 10/13dBi MIMO 3 Element Antenna (J9170A). It comes with three pigtails with N type connectors. I need to extend the pigtails 100ft but am unsure of which coax type to use. Thanks!
posted by cheechman85 to Technology (7 answers total)
 
N-connectors come in 50 and 75 ohm versions. You just need to match the impedance.
posted by three blind mice at 8:50 AM on May 22, 2013


Best answer: You want to extend the antenna pigtails by one hundred feet? You do realize that is quite a lot of distance at the frequencies used by wifi, no?

You are going to be much better off putting the endpoint (whatever the antenna is connected to, i.e. the router) in a weatherproof enclosure near the antenna mounting point, and running Ethernet and AC power out to it, than extending the coax that distance. Also it will be much cheaper, because good coax is stunningly expensive.

But to answer your question ... if you really want to extend the coax that far, in order to not have ridiculous line losses, you are going to need some really good coax.

Using this coax loss calculator and assuming a 100ft run at 2.4GHz with 500mW input power, if you used something good like TMS LMR-900 (high-end 50ohm microwave coax) you'd lose about 2.9dB. If you are doing a point-to-point connection that might be acceptable; you'll lose half your output power and your input/receive ability will be similarly diminished, but it might work depending on the distances in question. If that won't work there is a better version, LMR-1200, which is 7/8" instead of 5/8", but it's much more expensive, not just for the cable but also for the connectors.

LMR-900 isn't the sort of thing you just pick up at RadioShack, though. My guess is that if you can find it for under $5/foot you'd be getting a good deal. And then you'd need connectors, and if you don't know how to terminate coax properly you'll want to get someone who knows what they're doing to handle it, because if water enters the line it'll basically be ruined (or at least a pain in the ass to fix). I'd probably run it up to a weatherproof enclosure near the antenna mast base and put the termination inside the box just for additional weather protection, and then go from there to the actual antenna with a separate pigtail of lighter coax that can be easily replaced if damaged. A little extra insertion loss, but worth it to protect the feedline, IMO.

If you are okay with something less flexible, then you could also use Heliax. Heliax is different from regular braided coax like the LMR-900 because it's solid corrugated copper (think flexible gas line), so it achieves low losses while being significantly cheaper, but it's much more difficult to work with. You can easily ruin it during installation if you bend it too far, and making tight turns like you'd need to go through walls or form drip loops is hard. Heliax LDF4/5/6 would all work depending on how low you really need your losses to be, at increasing cost. I think LDF5 is no longer manufactured but you might be able to find it for sale still. Anyway, you are still talking about at least $2.50-3.00 per foot for LDF4 so it's not a cheap proposition.

tl;dr: This is a bad idea and you should mount the digital endpoint device near the antenna to keep the RF run short, making the digital and power cables long instead.
posted by Kadin2048 at 9:07 AM on May 22, 2013 [3 favorites]


To add my (amateur, uninformed) thought to what kadin2048 said, I'd say that rather than run power and Ethernet to the router that kadin2048 recommends you use instead of an aerial, run an Ethernet cable with power over Ethernet in it, then you only have one cable to run to the remote router. And you should find an adequately weatherproof power over Ethernet router to stick on the remote end of the cable fairly easily.
posted by ambrosen at 9:15 AM on May 22, 2013


ambrosen is right; a PoE router would really be the most elegant solution.

Cisco, Aruba, and other manufacturers make hardened, weatherproof, PoE-powered APs designed to be mounted to an antenna mast or tower just below an antenna. You might be able to get a used Cisco Aironet for not too much money (less than coax).
posted by Kadin2048 at 9:27 AM on May 22, 2013


I dunno. It seems to me that a cable loss of 3dB should not really affect the performance of an otherwise decent 802.11n set-up. MIMO - multiple input/multiple output combines the signals received on different antennas (diversity reception) which provides for a lot more signal processing gain that the antennas themselves contribute. WiFi systems are almost always interference limited - noise is not really a problem. The type-N connector is pretty decent at these frequencies (2,4 and 5,47 Ghz) and really any cheap-ass coax cable (of the right impedance) will do.

My guess is that unless this is a marginal installation to begin with, putting 3-10 db of cable loss on the backside of the antenna isn't going to hurt the overall performance.
posted by three blind mice at 11:37 AM on May 22, 2013


Best answer: tbm: Even to get to 10dB loss (90% attenuation!) you have to use LMR400 which isn't cheap, it's probably ~$120 for the coax plus more for connectors.

And OP has already spent ~$500 (NewEgg price) for an antenna with 10dB of forward gain that they'd just be throwing away...basically $600+ sunk for unity gain. Just seems like a bit of a waste to me: at that point they would be better off returning the antenna, buying slightly better coax, and putting a trimmed coat hanger or something on the end of it as an isotropic radiator.

I dunno, it just seems like buying a Porsche and then filling the trunk up with bricks or something; why spend the money for the antenna gain if you couple it to a crap feedline?
posted by Kadin2048 at 5:53 PM on May 22, 2013


Response by poster: You guys really gave me some good insight. LMR400 is the coax type we went with.
posted by cheechman85 at 5:21 AM on May 23, 2013


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