Suggestions for 802.11x Broadcast on Farm

Hello all, I'm looking to setup an 802.11x network broadcasting a mile in all directions. The use is for a horse farm where notebooks will be used to take measurements and report back to a server on the farm.

I need help in finding an antenna (or more?), what its gain should be and if standard wireless NICs are powerful enough to transmit a mile away.

Any help would be great appreciated!

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If your client radios are all laptops or PDA's, you're going to have difficulties getting a reliable connection at 1 mile. The antennas supplied with most laptops are terrible. I'm having a difficult time visualizing how you're going to run the laptop or PDA on horseback, but that's your headache.

If your topography is flat and you can have some *UNOBSTRUCTED* elevation for central access point, it can be made to work with a rather high gain omni. The problem is that the vertical radiation angle is very narrow for a high gain omni. The antenna I'm thinking of is a 15.4 dBi omni with about a 6 degree -3dB beamwidth and 3 degree downtilt. At 1 mile, that 6 degree beamwidth is about 500ft of vertical coverage so you should be ok. Any bushes, trees, or hills in the way will probably be fatal. I don't think it will work but lets go through the exercise anyway.


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"Omnidirectional Antennas".

I'll assume that you're using a Linksys WRT54G or other common router or access point. The nice thing about this unit is that it has:

  1. Removable antennas.
  2. Alternative firmware (Sveasoft Alchemy) which I recommend installing.
  3. Very wide range power input. The unit can be powered by anything from about 4VDC to 18VDC. That means you can install it and the antenna on the roof of your barn, run just CAT5, and set it up for Power over Ethernet.

Let's run the numbers. The idea is to keep the fade margin over about

20dB. That's the minimum necessary for reliable communications.

The receiver sensitivity is related to connection speed. I'll assume that you can live with the slowest 6Mbits/sec OFDM connection, which will give you about half that in thruput. This is from the DI-624 spec sheet but is close enough to the WRT54G to be useable. * 54Mbps OFDM, 10% PER, -68dBm * 48Mbps OFDM, 10% PER, -68dBm * 36Mbps OFDM, 10% PER, -75dBm * 24Mbps OFDM, 10% PER, -79dBm * 18Mbps OFDM, 10% PER, -82dBm * 12Mbps OFDM, 10% PER, -84dBm * 11Mbps CCK, 8% PER, -82dBm * 9Mbps OFDM, 10% PER, -87dBm * 6Mbps OFDM, 10% PER, -88dBm * 5.5Mbps CCK, 8% PER, -85dBm * 2Mbps QPSK, 8% PER, -86dBm * 1Mbps BPSK, 8% PER, -89dBm

Plugging into:

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TX Power : +15dBm (at access point end) TX Coax Loss : -3dBm TX Antenna gain : +15dBi Range : 1 mile at 2.4GHz RX Antenna gain : 0dBm (really bad laptop antenna) RX Coax Loss : -3dB (mostly connector losses) RX sensitivity : -88dbm (at 12Mbits/sec) Min fade margin : 20dB

The resultant fade margin is 13.8dB which sucks and isn't going to work. You're short about 6dB of gain.

What needs to happen is either that:

  1. the antenna gain at the laptop end needs to increase drastically
  2. the center omni antenna needs to be replaced by 3 or 4 separate sector antennas and access points.
  3. the client radios (laptops or PDA's) need external gain antennas pointed at the central omni.
  4. The central omni needs to be replaced by access points distributed around the property to reduce the range.
  5. Install repeaters, "range extenders", or WDS repeaters which methinks are a lousy idea, but will probably help increase the coverage.

A good question would be "How far can I get with the above setup"? Well, if I tweak the range until I get 20dB fade margin, the range is about 0.5 miles. 6dB is half the range. 12dB is 1/4th the range.

I used rather optimistic values for antenna gains and coax losses, so some extra margin would be helpful.

Before anyone suggests "power amplifier", please note that a PA only solves the gain problem in one direction. The other direction is still short 6dB of gain.

Reply to
Jeff Liebermann

Wow, thanks for the very informative reply.

I'm new to achieving this kind of distance for a wireless network. I've set up plenty of houses with Netgear and Linksys wireless routers and WAPs, but I fall short in trying to push the signal beyond the size of a house.

The terrain is mostly flat. There are a few trees, unfortunately one right in the middle of the farm. The diameter of the farm is roughly 1 mile, so really we could get away with a half-mile signal. I stated 1 mile so that we overshoot the required distance.

With my above stated experience, should I not bother with this and hire someone else with more experience?

Thanks again!

Jeff Liebermann wrote:

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I don't think it will do a mile without some help at the client end with better antennas. Laptops and PDA's on horseback?

Dunno. It's easy enough to try by installing a battery operated wireless router in the highest place possible and driving around with a laptop or PDA. You can check out the local interference situation at the same time. Someone with experience will probably do the same thing. "Mostly flat" sounds like there will be areas with no coverage to due terrain blockage.

Personally, I don't like very high gain omni antennas. The slightest tilt on the antenna and the signal is gone. I prefer to use sector antennas and multiple access points.

Reply to
Jeff Liebermann

It's a _horse farm_, Jeff, not a farm that is worked on horseback :-) I expect the laptops will be in pickups or ATVs, so it would be workable to use external antennas mounted on the vehicle.

Reply to
Derek Broughton

Well, I'm possibily getting in over my head. I've set up multiple WAPs inside buildings before, but each WAP had a single CAT5 run to the switch. In this case there is a barn in the center of the farm on which I could place the primary anntena. I could then place repeaters at to other spots on the farm.

Any reason standard repeaters shouldn't work? I know that you can't see the farm, but assuming there's not much more land beyond each repeater would I really need three roof mounted anntenas?

Thanks for all the great > Jeff Liebermann wrote:

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