NetStumbler.org Forums - View Single Post - Antenna, Amp, Orinoco, and phone question.

03-20-2002

Hey Joe, don't be too hastly... You'll just get flamed if you start in with the attitude on your second post. Maybe DonChicago did not answer to your satisfaction, but it doesn't mean it was devoid of information.

To answer your questions, and add some detail:

1)The base setup that you describe is illegal. Here's why:

The FCC regs are in Part 15, which is rather long and involved. If you want the full details, the FCC does make it available on their website. I can quickly summerize the section which you are interested in however. The maximum gain for the 2.4GHz ISM band (which 802.11b this stuff is in) is 36db for a "Point To Multi-Point" design. This is what you are describing. A PTMP is a base-to-mobile or base-to-multiple-bases setup. A "Point To Point" design can radiate an u to 48db gain. A PTP is two dishes or similar antennae pointed directly at each other, with no other systems receiving signals between them. This is mainly for long distance links.

Now, your next logical question is "What is my gain with the setup I described?" Here's the equation: 1 watt = 30db, so 30db + 15db (gain of antenna) = 45db. Clearly over the limit for a PTP setup. To be legal with a 15db antenna, you could have no more than a 21db amp, or about .7 watts. (The db is a logirithmic scale, so that's not quite right, but close enough for explanation purposes.)

Stick with a 6db gain attenna or lower, and you would be legal. You may compute line losses here if you are using a long coax feed, but I'm keeping this example simple, and assuming a short coax with a negligible loss. To do the actual computation, you would have to know the length of the coax, and its db loss per a given length. Cable manufacturers post this info.

A couple of further thoughts about amps. First, they are expensive. Second, by their very nature, amplifiers will amplify the NOISE as well as the SIGNAL. So if you have a noisy setup, is effectly just "louder." You will kill six or more channels from being usable, unless you add some sort of side band filter. Most professional installers shy away from them unless absolutely needed. You might want to do a site survey without an amp before spending the money. Third, this is RF radiation. At the low levels that you are dealing with from a card, you don't really have too much of a problem. But when you boost it via amps and antenna, you are approaching an area where you *might* have adverse health effects, if you set this stuff up incorrectly. Just be careful. My advice is to stay away from amps until you know what you are doing with them. By the very nature of the question you are showing that you need to do some more research first.

2) 2.4Gz phones can and do cause interfence. They are in the same band. So do wireless video cameras (X10's for instance) and many microwave ovens, and other 2.4GHz radiating devices. So your phones could be a problem, but think of other potential sources too.

As to ChicagoDon's "scary threat" of a $10k fine, well, that is the standard figure that the FCC usually credited with hitting people with. I believe provisions for fines are also listed in Part 15. The FCC usually deals with commerical broadcast stations, so big fines are the norm there. If you made the stupid mistake of intentionally violating the regs, and were caught, most likely the FCC would order a "siese and desist" order, and if you still didn't comply, then seize the equipment. But they COULD hit you with a huge fine, and there is no requirement that I know of that they issue a warning first. Just make an intellient decision knowing all the facts. If you do decide to intentionally violate the law, you can't say "But no one told me." And if you knock out someone else's setup in your neighborhood by being illegally overpowered , the FCC is who they complain to.

A couple of other things to consider: It has been reported in the trades that the FCC has doubled the enforcement budget this year. And those same trades say they are reportedly very interested in 802.11b.

One final word, read through these forums and use the search function. This has been asked and answered before, in differing amounts of detail. Other postings might give you some more food for thought.

Cheers,
Thorn

03-20-2002	#4 (permalink)
Posts: n/a	Hey Joe, don't be too hastly... You'll just get flamed if you start in with the attitude on your second post. Maybe DonChicago did not answer to your satisfaction, but it doesn't mean it was devoid of information. To answer your questions, and add some detail: 1)The base setup that you describe is illegal. Here's why: The FCC regs are in Part 15, which is rather long and involved. If you want the full details, the FCC does make it available on their website. I can quickly summerize the section which you are interested in however. The maximum gain for the 2.4GHz ISM band (which 802.11b this stuff is in) is 36db for a "Point To Multi-Point" design. This is what you are describing. A PTMP is a base-to-mobile or base-to-multiple-bases setup. A "Point To Point" design can radiate an u to 48db gain. A PTP is two dishes or similar antennae pointed directly at each other, with no other systems receiving signals between them. This is mainly for long distance links. Now, your next logical question is "What is my gain with the setup I described?" Here's the equation: 1 watt = 30db, so 30db + 15db (gain of antenna) = 45db. Clearly over the limit for a PTP setup. To be legal with a 15db antenna, you could have no more than a 21db amp, or about .7 watts. (The db is a logirithmic scale, so that's not quite right, but close enough for explanation purposes.) Stick with a 6db gain attenna or lower, and you would be legal. You may compute line losses here if you are using a long coax feed, but I'm keeping this example simple, and assuming a short coax with a negligible loss. To do the actual computation, you would have to know the length of the coax, and its db loss per a given length. Cable manufacturers post this info. A couple of further thoughts about amps. First, they are expensive. Second, by their very nature, amplifiers will amplify the NOISE as well as the SIGNAL. So if you have a noisy setup, is effectly just "louder." You will kill six or more channels from being usable, unless you add some sort of side band filter. Most professional installers shy away from them unless absolutely needed. You might want to do a site survey without an amp before spending the money. Third, this is RF radiation. At the low levels that you are dealing with from a card, you don't really have too much of a problem. But when you boost it via amps and antenna, you are approaching an area where you might have adverse health effects, if you set this stuff up incorrectly. Just be careful. My advice is to stay away from amps until you know what you are doing with them. By the very nature of the question you are showing that you need to do some more research first. 2) 2.4Gz phones can and do cause interfence. They are in the same band. So do wireless video cameras (X10's for instance) and many microwave ovens, and other 2.4GHz radiating devices. So your phones could be a problem, but think of other potential sources too. As to ChicagoDon's "scary threat" of a $10k fine, well, that is the standard figure that the FCC usually credited with hitting people with. I believe provisions for fines are also listed in Part 15. The FCC usually deals with commerical broadcast stations, so big fines are the norm there. If you made the stupid mistake of intentionally violating the regs, and were caught, most likely the FCC would order a "siese and desist" order, and if you still didn't comply, then seize the equipment. But they COULD hit you with a huge fine, and there is no requirement that I know of that they issue a warning first. Just make an intellient decision knowing all the facts. If you do decide to intentionally violate the law, you can't say "But no one told me." And if you knock out someone else's setup in your neighborhood by being illegally overpowered , the FCC is who they complain to. A couple of other things to consider: It has been reported in the trades that the FCC has doubled the enforcement budget this year. And those same trades say they are reportedly very interested in 802.11b. One final word, read through these forums and use the search function. This has been asked and answered before, in differing amounts of detail. Other postings might give you some more food for thought. Cheers, Thorn