wanted: wireless magic tricks

Thu Aug 16 17:28:46 UTC 2001

On Thu, 16 Aug 2001, Leo Bicknell wrote:

> On Thu, Aug 16, 2001 at 12:57:19PM -0400, Greg Maxwell wrote:
> > FCC Part 15.247 limits EIRP. You can't legally put out enough power in the
> > unlicenced bands.
>
> Sure you can, you get more power with more directional antennas.
>
> Here's a company that will sell you everything you need:
>
> http://wavewireless.com
>
> They can get 25 miles.  Using a 36db Yagi you can build the same thing
> yourself, as many have done.  You do have to back off the transmitter
> slightly to stay within FCC limits, but 20+ miles 802.11b is easily
> done within FCC limits.
>
> I cannot speak to 802.11a, as I know nothing about how it works,
> or the frequences and power involved.

Directional antenns increase effective power output. FCC has limits not
only on transmitter power but also EIRP.

A 36dBi with 100mw input will yeald an EIRP of over 220 watts, well above
the limit of sanity and the law for EIRP of unlicenced equipment in the
ISM band.

You'd have to back off to less then 2mw to remain in the legal limits.

I just ran some quick numbers, and a unlicenced 20mile link like that
would be possible, but I wouldn't sell it.:

1.75mw -> 36dBi antenna -> freespace -> 36dBi antenna -> reciever + mis
losses, has an EIRP of about 3.9 watts (legal).

You'd have about 15db of link margin, and that's much too little for a
reliable link IMO.