Archived from groups: alt.internet.wireless (
More info?)
On 21 Nov 2004 20:28:06 -0800, sdr32wfe@yahoo.com (Consultant) wrote:
>I'm looking to connect three farm houses using Buffalo .11g gear
>(using WDS) to share data and share a cable internet connection.
You realize that WDS is half duplex and therefore cuts your thruput in
half.
>The distances of the paths are 1.25 miles each. I have LOS for the paths.
At 1.25 miles and minimal antennas, my guess(tm) is that you'll get a
9Mbit/sec association with 802.11g. Thruput should be less than half
at about 4Mbits/sec. Add WDS and it gets cut in half again to
2Mbits/sec.
>I plan to use relatively short (<15') lengths of LMR-400 for the
>outdoor antennas. I want the best signal (who doesn't?) so I can get
>the most from the 11g, realizing that WDS will cut my bandwidth in
>half.
Yep. Your actually thruput will be highly dependent on the connection
speed, which is dependent on signal strength (and quality), which is
primarily dependent on antenna gain.
>I figured I'd use a router with a directional antenna at the
>house with the cable available, an AP with an omni at the middle house
>(it would also have clients connecting wirelessly), and either an AP
>(for multiple computers) or an ethernet converter (for a single
>computer) with a directional antenna at the third house:
>
>cable/router/directional->-------<AP/omni>-
> -
> about 125 -
> degrees -
> -<AP/directional
There's another method that *MIGHT* be worth considering instead of
the omni in the middle. Use a power splitter (lose -4dB) and two
directional antennas pointed at each end of the link. If your antenna
gain is more than the power splitter loss, you have a net improvement.
Of course it costs quite a bit more and tends to be ugly. It's major
benifit is that an omni picks up interference from all directions.
Having a directional antenna removes much of the interference. You
can start with an omni (because it's cheaper) but be prepared to
switch to directional antennas if interference becomes an issue.
Another possibility is to use TWO radios at the midpoint, on different
channels, each with a different directional antenna. Also more
expensive, but it eliminates the problem where WDS store-n-forward
cuts the bandwidth in half.
Note that an omni in the middle limits you to using vertical
polarization. However, two pairs of directional antennas can be any
polarization. In general, most of the interference arrives vertically
polarized, so horizontal polarization is a good idea if possible.
>What is the proper gain needed for each antenna?
Do the math.
http://www.ydi.com/calculation/som.php
What you're looking for is an absolute minimum of 10dB fade margin.
The system will work with 0dB of fade margin, but not be particularly
stable. A passing bird would cause the signal to disappear. At 10dB,
you'll 802.11g, if set to 802.11g only (ignore 802.11b), will probably
be running at the slowest speeds. Any interference will make it
worse. Let's see what 10 times as much (20dB) fade margin yields.
Use these for receiver sensitivity at various connection speeds.
They're from the D-Link DI-624 data sheet and appear to be rather
typical.
* 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)
Using rather conservative guesswork (better safe than suprised):
Distance = 1.25 miles
TX power = +15dBm (my measurements)
RX sens = -87dBm (at 9Mbits/sec OFDM)
TX ant gain = 15dBi (large panel or small dish)
RX ant gain = 8dBi (cheapo omni)
TX coax loss = -3dB (15ft LMR-400 plus connectors and pigtail)
RX coax loss = -3dB (assumed similar installation)
Fade margin is 12.9dB. That's way too close to be comfortable. You
need to squeeze some decibels out of something. Getting rid of the
omni and using a higher gain antenna is one way. Increasing the size
of the TX antenna from a 15dBi dish to a 24dBi dish is another.
Whatever it takes to get closer to 20dB fade margin.
>Can the gain be too great?
No, but there are side effects. Higher gain antennas have a much
narrower beamwidth. As the gain gets high (over 15dBi), alignment
becomes criticial. In the case of omnis, it's the vertical radiation
angle that gets narrow. If there's an elevation difference, a
rediculously high gain omni may have such a narrow beam width that the
signal goes over the head of the intended target. Watch the beamwidth
and downtilt specs carefully.
>Do the antenna gains need to be "balanced" some way?
No.
>Should I consider a sector antenna for the middle AP?
No. The 125 degree angle is at the limit of sector antennas. Sector
antennas have a different purpose. They're made of "area" coverage
and not for point to point.
>What grounding, if any, is needed? The antennas will be at or below
>the roof lines of the houses.
None. If you live in lightning infested areas, you might consider a
lightning arrestor. The coax cable usually provides all the grounding
that's necessary to prevent static electricity buildup (from the
wind), but even that's minor.
>What length of LMR-400 could I use without losing too much signal?
Do the math (and use the SOM calculator). LMR-400 is about 0.07dB/ft.
I use -1dB loss for each connector pair. Lightning arrestors lose
about 1dB plus two connector pairs. If you want to use more coax,
you'll just need to get a bigger antenna.
>LOS between the last two houses may be encrouched upon by a rotating
>irrigation boom (4" aluminum pipe plus some smaller pipes). Will this
>completely destroy the LOS for this segment?
Probably not. The beamwidth at 1.25 miles is quite wide. Assuming a
24dBi antenna with a -3dB beamwidth of 5 degrees, the signal is about
570 ft wide. There's no way a 4" pipe is going to block all the
signal. There may be some weird multipath effects, but I doubt if
there will be any blockage.
--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
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