OK, I have seen 2 posts/emails just today on gents thinking of throwing an 8 watt amp into their system. So this has got to be posted.

BBHN uses microwaves, which act totally different from the low-n-slow bands. It REQUIRES point-to-point links with CLEAR Line of Sight and UNOBSTRUCTED Fresnel zones. If you are unsure of these 2 definitions, please Google/Bing/Your favorite search engine before moving on.

The trick to making a good mesh network is aiming GOOD antennas FIRST. If you have a clear path,  clear Fresnel, and a good antenna, you can go MILES with ONLY STOCK POWER (79mW/19dbm)

IT HAS BEEN DONE.

If you get a link, but it is real flaky, then maybe bump it up with a half-watt amp. If its still not solid, then go to 1W. No more than 2 watts should EVER be necessary.

If it takes more than 2 watts, then maybe you should rethink your paths. Can you add a repeater node in between? Can you raise the antennas higher to get over the obstructions?

But planning an 8W amp before even TRYING to make the link is foolish and can be downright dangerous. I wont intentionally walk in front of a 24dbi dish even with just stock 79mW into it. My contact lenses (and EYES) are expensive, and the concentrated microwaves will COOK water (coffee cup in the Radarange should be proof enough every morning!). Safety is the first (or second)  lesson for a Technician license.

Im not trying to belittle or chastise anyone, just bringing to the forefront the dangers of old HF/VHF habits when working in the microwave range.

Satellites shoot signals thousands/millions/billions of miles, and what is the transmitters max output? 2, 3, 5 watts? Yes, that is in free space vacuum, but it is the same principals and physics, even with air as an attenuator.

Sit down and figure out your path profiles and calculate the RF gains/losses. RadioMobile is a great tool to start with. Heywhatsthat (link under web links-path profiling) is another great profiling tool.

An 8 watt amp into a 24Dbi dish nets an EIRP of 2+KW. If you need that much, invest in a chainsaw first (to remove obstructions). At least you can see where the stuff is flying off it's blade.

And remember the one big Part 97 rule: USE THE LEAST AMOUNT OF POWER TO MAKE YOUR CONTACT.  IMHO, An 8 Watt amp should be considered illegal under this rule. Get a better antenna first ! Get more HAAT second. Add a relay node third. We are Hams... we are supposed to be creative! And with SAFETY always in mind.

We have done 5, 6, and 10 mile SOLID links across Austin, the Texas capitol, which include the University of Texas campus, with business and residential wifi, microwave ovens, and other 2.4 Ghz QRM present, with just gain antennas on each end and NO AMPLIFIERS.

I will close with my urging everyone planning mesh networks to realize this is not HF/VHF/UHF, and it works differently. You cannot change how it works, you have to plan the way it works. And most of all do it safely.

73 and Happy Meshing
Jim
K5KTF

The modulation scheme and protocols used by 802.11g makes this problem even worse than it first seems.

You can interfere with a lot of civilian wifi users. 

Unlike some other wifi standards like 802.11b, a single overly powerful 802.11g device can completely block out other users of a particular channel. 

If you're the strongest 802.11g RF signal a receiver hears on a particular channel, it can't hear anything at all from other wifi devices on that channel, except in gaps between your transmissions.  If you're sending out a nearly constant stream of data, the other users may not ever get a long enough gap to establish or maintain a data link. 

The reason for this is that 802.11g uses OFDM, not DSSS.  DSSS can share a channel, OFDM doesn't share the channel except between transmissions. 

Also, although there are 11 channels on 802.11g, they overlap, so there are only 3 non-overlapping channels.    One overly strong 802.11g signal will interfere with several other channels. 

There are several other problems caused by an overly strong amplified transmitter, but that's enough for a single post.

Right Mickey, and not just interfering with other wifi gear, but how about other nearby mesh nodes?

If one node has a blowtorch spraying out RF, other nodes in the near vicinity, that could normally make up hops/relays, may get de-sensed as all they would hear is the flamethrower.

Thats not how a mesh should be planned out.

The BEST way to start building a mesh is to plan it out via software, then once that shows it could be viable, go out and put up test nodes with GOOD ANTENNAS, and see what you get. Trying an omni to shoot 2 miles more than likely wont happen. 2.4ghz needs directional gain for any good distance. They are good for local (a block or 2) links, like relaying down from a tower/rooftop.

I saw an attempted link 2.4 miles, totally clear path and LOS, with an omni both ends: no connection. Maybe 1 to 2% of the time they would link. Throw a small dish on one end, came up to about 15-20%. Dishes on both ends: 100% ! Had we thrown amps on the omnis, it might have helped, but no where near as good as the dishes.

Another link we put up 24dBi dishes on both ends of a 9.8mile link. NO AMPS. Would get a connection about 10% of the time. Alot of Fresnel zone interference (wavy hills with apartments/buildings on it--cant chainsaw here without jail time :-) ). Added a 2w amp on the far end, and we are at about 80% connection, good enough to use now. May improve it further by raising the antenna on the near end (no-amp) off the flat (hospital) roof more. I prefer no amp on this end as there are 3 other antennas there (2 nodes), each making their own connections and do not want to de-sense them.

KTF

It gets worse the more you think about it.  

Don't forget that wifi links are two way.  If you put an amp on your own station, the far station can hear you, but you can't hear him, and can't communicate.  You can't establish a link despite the fact that he can hear you.  Boosting the signal on one end only doesn't usually give you a longer link distance. 

Since your station transmits much better than it receives, you won't hear that the frequency is busy for distant stations and wait your turn, but you will still interfere with them. 

You probably can't even effectively use a preamp on your node because the station you're trying to talk to will be lost among the sea of other low power 802.11g stations on the same channel.

Brainless high power amps only help if both ends of the link have amps.  In that case, you're making the interference problem even worse for other civilian and mesh users.

High gain antennas have several advantages over amps.

They are symmetrical between receive and transmit.  Your transmit signal is more powerful, but so is your receive signal.

You receive a spatially selective subset of stations.   There's less background noise from other wifi or other 2.4 GHz signals.

Your transmit signal doesn't interfere with as large an area.

If you have a high gain antenna on both ends, you get a multiplying effect, not just an additive effect.  A 10x gain antenna on both ends gives you 100x signal improvement.  You may also get a significant reduction in the general background noise of other 2.4 GHz noise sources because your beam width is smaller.