"Ken Bessler" <kg0wx@NOSPAMyahoo.com> hath wroth:
>I'm looking for a WiFi Yagi antenna and this one looks like a
>good choice:
>
>http://www.mfjenterprises.com/produc...rodid=MFJ-1800
Note that this antenna has very few specifications and no tests.
Having a ham license suggests that you're familiar with MFJ "quality".
The balun in bottom photo is not my idea of quality construction. It
really should be inside the center tube. The unprotected copper is
going to rot in any type weather exposure. Oxidized copper absolutely
sucks for surface conductivity. It should have been silver plated.
The offset folded dipole might create a boresight error (not sure). It
would be interesting to see the VSWR plot across the 2.4GHz band,
especially with the long exposed center pin on the N connector.
Note location of the balun:
|
http://www.alibaba.com/catalog/11477...i_Antenna.html
>I'm thinking about enclosing it in a white PVC pipe for protection
Al Dykes mentioned that you're going to have "effects" if you use a
PVC pipe. That's mostly true depending on the type, thickness, and
proximity of the pipe. The traditional radome material is fiberglass,
which has a dielectric constant of about 4.5. PVC is about 3.7 to
4.2. Get the tube too close to the antenna, or make the wall too
thick, and the antenna will move down in resonant frequency. I've
been playing with vinyl rain gutter material. It doesn't melt in my
microwave oven and doesn't ruin the resonance of several antennas I've
built. However, it might be a tight fit for the yagi. I'm not sure
what to recommend, but keep the walls thin and the diameter large.
>and mounting it on a 25' pole with a Radio Shack rotor.
I'm not sure of the horizontal -3dB beamwidth of a 15dBi yagi, but my
guess(tm) is about 30 degrees.
>The goal
>is to provide WiFi to my Thinkpad (equipped with hi-perf antennas also)
>while I'm at a park bench some 2200' from my house.
Doing the trigonometry, at 2200ft, a 30 degree beamwidth gives you 127
ft of perpendicular coverage. My cheapo Alliance rotator is not
repeatable to within 5 degrees. I have a small TV camera mounted on
the boom to help aim the beast. Dunno about your Radio Shack rotator.
My guess(tm) that your useable coverage will be about 60ft wide if you
can aim the antenna accurately.
>The AP is a WRT54g (v4 with DD-WRT) mounted just above the
>rotor.
I'm finding that the WRT54G is not the best RF performer on the
planet. No numbers available but definitely worse than some other
radios floating around the house and office.
>I have a 4' R-TNC to N pigtail to go from the router to the
>yagi antenna. The AP is mounted in a weatherproof enclosure.
>
>The Thinkpad has 2 Linksys 7dbi omni's mounted on the rear/top
>of the screen. It has an Atheros 5211 mini pci card.
Ok, we have enough to do the math. See:
|
http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
for the procedure. Anything over a 20dB fade margin (SOM) will work.
TX power = +15 dBm (starting at laptop end)
TX coax loss = 3 dB (u-FL pigtail and tiny coax)
TX ant gain = +7 dBi rubber ducky
Distance = 2200 ft (0.417 miles)
RX ant gain = +15 dBi yagi
RX coax loss = 2 dB (LMR-240 pigtail and mess of connectors)
RX sens = -84 dBm (at 12 Mbits/sec)
Fade margin = ???? dB
Plugging into:
http://www.terabeam.com/support/calculations/som.php
I get a fade margin of 19.4 dB. Good enough. It should work. Just
one warning. These calculations and numbers are best case. They only
get worse, never better. RX sensitivity tends to always be worse than
expected (and is difficult to measure). Antennas don't quite have the
advertised gain. Coax cable fill with water. Connector losses
increase when filthy. If you slow down the speed a bit, you'll pickup
some receiver sensitivity, which will give even more fade margin.
>I know, I know - the antenna is only $30 - not too much to worry
>about so why not just get it and see? I probably will but I wanted
>to get your opinions or suggestions if you have any....
Opinion? Sure. If you're only interested in one park bench, it's
probably acceptable. Forget the rotator and just point the antenna.
Alignment might be a bit of a challenge. I would use a panel antenna
instead of a yagi to avoid corrosion and birds.
If you wanna do it for a wider area, without a rotator, I suggest a
Franklin sector antenna. 10-15.5 dBi depending on size. Easy to
build and probably more mechanically stable than the MFJ yagi.
http://pe2er.nl/wifisector/ http://www.brest-wireless.net/wiki/materiel:amos http://yu1aw.ba-karlsruhe.de/vhf_ant.htm
--
Jeff Liebermann
jeffl@comix.santa-cruz.ca.us
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
WiFi Yagi antenna opinions 
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