I'm looking for a WiFi Yagi antenna and this one looks like a good choice:
I'm thinking about enclosing it in a white PVC pipe for protection and mounting it on a 25' pole with a Radio Shack rotor. 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.
The AP is a WRT54g (v4 with DD-WRT) mounted just above the rotor. 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.
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....
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: |
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.
I'm not sure of the horizontal -3dB beamwidth of a 15dBi yagi, but my guess(tm) is about 30 degrees.
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.
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.
Ok, we have enough to do the math. See: |
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:
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.
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.
Why bother enclosing it? Most yagi are designed for outdoor use. If this isn't, then get one that is. As for putting it on the rotor, why bother? If you're only using it in the park, at a known location then just fix it at that angle. But getting it aligned at 2200 feet will be a bit of a trick. You'll of course need to have CLEAR and UNOBSTRUCTED line-of-sight between the two. No trees and nothing 'near' the path. Even then I wouldn't hold out hope for very high performance. But hey, 1mb is better than zero so give it a shot.
Otherwise, if you're serious about using it wirelessly and the wifi doesn't work then get a cellular data card, (ev-do, gprs, etc). Or get someone else that's closer to the park to let you leech their wifi, giving them a suitable antenna or even an access point for the purpose.