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Using an Old Satellite Dish as a WLAN Antenna
=m8s=Dark Underlord writes "I was browsing for wireless stuff and came across this link that shows how to use an old satellite dish as an 802.11 antenna." The directions tout the range as being 10 miles given line of sight. We've had other stories about building antennas, but I think these are cooler because of their focused nature, but a Primestar dish is a little tougher to locate than a Pringles can or a floppy disk.
The Original Slashdot Article
Looks like you can pick up a Primestar dish on eBay for about 50 bucks (current auctions here). Shipping looks to be about $20. Anybody know of other sources for acquiring these dishes?
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I tried this a while back, but at the time, even unused sattelite dishes were too expensive. I borrowed a neighbors' dish for the experiment. The hard part was focusing the dish - If you didn't have something sturfy to mount it to, then you would lose the signal pretty easily.
Why bother with the Primestar dish for $50 on Ebay when you can get a real 802.11b/g antenna with 24dB gain for about $65? If memory serves, every 3dB is double, so 24dB is 2^8 or 256 times the signal strength.
Primestar was purchased by Directv, who made the old primestar dishes "useless". Directv sent new satellite dishes and receivers to anyone who was subscribing to primestar.
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Hey, guys, this would violate FCC part 15 and other applicable rules. Specifically, we are referring to ERP (effective radiated power).
Have fun if you get nailed!
If you are interested in this, check out the Canteena solution which has a 12 db gain. You can get a *new* shiny antenna for much less than the cost of a used Primestar dish on EBAY. I know the dish in the article boasts 22 db gain with a 10 mile range but, keep in mind that setting up communication that is line of site 10 miles apart is nearly impossible. (Just try using a camera with an 8x telephoto lens to take a non blurry picture, you'll start to get the idea. ) Have fun!
Actually... with equipment that will use loose timing 30 miles would be no problem. You could even use an omni on the other end.
The main problem with these is getting them high enough for line of sight and also mounted sturdy enough. Mountain to mountain is the best for these kind of links.
With a beamwidth of 1degree or less those 6-10ft dishes have to be held dead still or you loose your signal at any distance.
Anyways.. the record is something like 120+km with two of these things and some VERY modified radios in the cards.
Telcos have alot of dark fibre in the States. Most people assume that's optical fibre...but it's actually moral fibre.
For what it's worth, two Linksys WAP11's in bridge mode, and two directional 24dBi gain antennae, and my link is rock solid at 1.1 miles.
I use Linksys WET11 bridges with 24dBi grid antennas made for 2.4GHz (also less than $70 each) and have a rock-solid 11Mbps link a bit over 6 miles away. The trick is to mount your WET11 in a weatherproof box right underneath the antenna, to keep the coax pigtail as short as possible, because signal loss in a long coax is the distance killer. I have only a 3 foot long coax pigtail and run the dc power up the unused pairs of the CAT5 cable to power the WET11. You may have a significant voltage drop at the end of a long CAT5 cable, so you may need to use a +6V or even a +7V DC power supply of adequate amperage to ensure that you still have +5V DC at the end of the CAT5 cable with the WET11 plugged in loading it down. I had to use a +6V, 1.5 amp DC power supply on mine to keep the voltage at +5 volts at the end of my CAT5 cables, or the WET11 wouldn't run.
Only the party obtaining the FCC certification is allowed to specify another antenna. You, the user, cannot alter the device even if it meets the requirements of 15.247. Modifying approved transmitters is generally not allowed, with the notable exception of the Amateur Radio Service. A new configuration with higher antenna gain requires a new certification. 47CFR2:
[emphasis added]So unless you have the money to spend on a complete recertification (it's neither cheap nor easy), leave your wifi alone!
How the heck do you sight something like this in? You have to be accurate within a few seconds, don't you? A search pattern would take forever with that narrow of a beam, wouldn't it?
The beam isn't that narrow. Let's do a quick mental calculation. A typical direct-to-home Ku-band dish is going to have a beamwidth of about 1 degree (2 degrees, whatever) in order to be able to isolate the right satellite on the arc. That's at Ku-down, which is about 11 GHz. A given antenna will get "wider" as you go down in frequency, so the beam width at 2.4 GHz for this same antenna is much wider. I believe the beam width and frequency scale inversely, so if you go down in frequency by about a factor of 5 (11/2.4) then your beamwidth (however you want to measure it) goes up by the same factor.
So your beamwidth at 2.4 GHz is going to be something like 5-10 degrees. And it's not a super sharp rolloff, so you'll find that signal easy.
Another way of expressing all of the above is to say that an antenna with a certain gain at a high frequency (like Ku band) is going to have a lower gain at a lower frequency, and the corollary of that gain reduction is lobe spreading.
Hey, maybe someone here can point us to a visualization tool for this -- looking at an antenna pattern for a given antenna, crank down the frequency and watch the lobes spread out and drop.
One simple rule for its versus it's
The FCC doesn't seem to care much about power levels and antennas in the ISM band. Remember, it's kind of a throwback "freebie" given to appease the anarchist crypto parasites and cheap low end consumer equipment for those unable to afford the "protected and scarce" high dollar bands that go up for auction. :-)
The only exception might be certain commercial product vendors who try and sell out of spec equipment to the masses. That is actually worth their time, but some guy with a primestar dish? no way. [ Like linksys getting pressured to take their 2.4Ghz amps off the market because they could interoperate with too many other "unapproved" equipment configurations. Supposedly they can sell them again after making them harder to use with anything but linksys/cisco. arg. ]
There are a number of smaller WISP's that I've come across in the northwest that run 1/2W and 1W amps on their directional point-to-point and point-to-multipoint configurations, but until someone complains that they are interfering, there is no way the FCC is going to proactively come out and bitch.
Wireless users groups across the nation post plans and site configurations using all sorts of unapproved antennas, radios, amplifiers, etc. There is no widespread FCC CRACKDOWN going on. In fact, I challenge anyone to name an incident where a WiFi user (not company) was pressured or forced by the FCC to alter their equipment back into spec. It doesn't happen.
Personally I think this is a good thing. The FCC has done more harm in the 802.11 space than good. Like antenna connectors. Do you know why there is a proliferation of SMA, RP-SMA, N-type, BNC, RP-BNC, MMCX, and any number of other bastardized formats for antennas and equipment? The FCC requires vendors to make their radio's use proprietary connectors to prevent people from easily and usefully extending the range of their equipment with generic antennas. Not that the vendors mind. Nothing like vendor only parts with the associated 400% markup to pad the profit line.
Let the FCC play with the Big Co's and handle licensed spectrum. The ISM bands are where its at.