Nate Bargmann <n0nb.DO.NOT.SPAM@ME.networksplus.net> hath wroth:
>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. :-)
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:
for 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
I 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.
>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?
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: http://www.linksysdata.com/ui/ http://support.dlink.com/emulators/
Figure 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.
Jeff Liebermann firstname.lastname@example.org
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558