I am thinking about linking two farmsteads via a wireless point-to-point link. Using Splat! (an RF path analysis tool for Linux written by John, KD2BD), I have determined that with antennas at 30 and 25 feet, I have a path clearance of 20 feet and a Longley-Rice path loss of 109 dB @ 2400 MHz. Assuming an RX sensitivity of -95 dBm (are they that good?), a TX power of +20 dBm, antenna gain of 17 dBi,throw in 2 dB for cable/connector loss (approximately 0 dB for an integrated unit) and I figure the signal at the far end would be about -74 to -76 dBm which should give about 20 dB of fade margin. We might have to trim a tree or two at one end. :-)
I've used Google to try and find some off the shelf equipment and I either wind up finding low grade consumer items meant to link rooms or high end commercial stuff intended for backbone use. So, what is out there, either integrated units or something I can put together, for about $500 or less for this link?
Nicely done. However, methinks you missed on some assumptions and didn't supply the parameters you used to generate your numbers.
See the FAQ at: |
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a sample link calculations. The receiver sensitivity should be at whatever connection rate you're expecting. My guess(tm) is that you're sharing a cable or DSL line and would like to see at least
6Mbit/sec thruput. That means you'll need at least a 12Mbit/sec wireless connection. Methinks some safety factor would be helpful, so I'll use 18Mbits/sec. The corresponding sensitivity for a Dlink DI-624 is -82dBm. Other hardware may be different, but this is in the ballpark. Also, everyone lies about rx sensitivity because it's so expensive and difficult to measure.
You didn't bother to mention the point to point distance. Reverse engineering your -109dB path loss, I'll guess it's 9500 ft (about 1.8 miles). Is this correct?
You also neglected coax cable losses. Unless you buy an integrated antenna and radio device, there will be some coax connector and cable loss. I'll assume you have an external radio, a short piece of coax, and a pair of dish antennas.
20dB of fade margin is a good target value. Anything less than about
15dB will be flakey.
+20dBm tx power is optimistic. There are some 100mw radios out there that claim this power level. Most commodity devices are rated at
+17dBm and actually deliver about +15dBm xmit power.
Now, we have enough to run the numbers. I'll do a template and you change the value to resemble your reality.
TX power = +17dBm TX coax loss = 2 dB (3 ft LMR-240 plus connectors) TX ant gain = 17dBi Distance = 1.8 miles RX ant gain = 17dBi RX coax loss = 2 dB (3 ft LMR-240 plus connectors) RX sens = -82dBm Fade margin = unknown
Plugging into: |
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get a fade margin of 19.7dB. Close enough. It will work.
Please note that this is the BEST case calculation. It always gets worse. You didn't mention if you were impacting the Fresnel Zone on your path. There may also be interference and reflection issues. Of course much of the hardware isn't up to specifications.
Sure. I'll assume a symmetrical transparent bridge. Just look for any of the access points that have a "bridge" mode. For example, Linksys WAP11, DLink DWL-2100AP, etc. Most access points will also play bridge. Check the feature list of web setup:
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on about $100 per radio. You'll probably need a PoE adapter and some kind of weatherproof package. Add about $50 per radio. Antennas, coax cables, connectors, and mounts will probably cost about $50. Yeah, it can be done.
I would probably be sharing a 384 kbps connection from a wireless ISP which is the only choice other than satellite out there. So, if the link does no poorer than 1 Mbs, I think we'll be fine. This is just an idea I'm working on if my brother would like something a bit faster than his
26.4 kbs dialup and I could tie the folks place in too. There will only be one PC at either end of the link and rarely will both be in operation at the same time.
I did neglect that. Per the coordinates, Splat! says it's 1.54 miles.
I did toss in 2 dB unless I had an integrated unit. I did forget to subtract 2dB at the far end and add in the 17 dBi antenna gain. OOPS!
There is one other farmstead in the path, but I think we have pretty good clearance. Of course, one won't know until it's tried. Like I said, a tree or two may need to be trimmed.
I do appreciate that, Jeff. It helps to get a bit of direction and a bit of refinement of my rough numbers. I spent about 10 minutes this morning guessing all of that up. :-)
Thanks to a post elsewhere, I found the 2.4GHz 19dBi RooTennaTM Compartment Antenna with CB3 Client Bridge Option at:
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for about $177 each. This looks like it would fit my situation very well as a pair can be configured for a point-to-point bridge link (they claim one is in operation covering a five mile link and I have just over 1.5 miles to cover).
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