Figure this from YDI’s Loss Calculator (below). I plugged in a 10 mile range using a -90db receiver, two 12db antennas and a 100 mW radio figuring 3 db cable loss on each end and a 124 db free space loss:

1. Free Space Loss of a 10-mile, 2.4 GHz link is approximately -124 dB.
2. Deduct another 6 dB for cable and connectors or a total loss around -130 dBm.

Then add antennas gains and receiver sensitivity gains:

1. Radio receiver sensitivity can add a gain of 90 dBm
2. With two, 12 dBm gain antennas, total gain is 24 dBm.
3. A 100 milliwatt radio (100 mW) is equivalent to 20 dBm.
4. Total gain would be (90+24+20) or +134 dBm gain
   Add the total gain (-134 dBm)
   Minus total loss ~ (-130 dBm)
   Net result equals ~ ( +4 dBm)

Wow. A whole 4 db of “headroom”. That won’t do. Immovable objects sometimes get in the way, too. COFDM-based 802.11g, might handle multi-path better but sensitivity could be worse. Let’s figure we need a minimum of +10 dB. That lowers the distance to about 5 miles. We need more power. Jim Geier says, “The FCC allows EIRP greater than 4 watts for antennas having gains higher than 6 dBi.” Adding an 18dB patch antenna to the USB enabled 802.11b client adds +6dB. Another +4 dB might be added with a 250 mW 802.11b radio to bring us back up to 10 miles. With luck. Is it legal? I think, so if I understand Jim correctly. Here’s a more detailed loss calculator.

At 5 GHz, free space loss isn’t THAT much worse. Our calculator says a 10 mile “shoot” drops -132dB at 5 GHz vrs 124dB loss at 2.4GHz. If a remote truck used a 5.8GHz 21dBi directional panel, it could be possible to compensate the 8dB higher path loss with a +8dB antenna gain. You might get lucky with 5-8 mile range. We estimate that a 200 milliwatt, 5.8 Ghz client radio ($150) such as the Atheros AR5001A with a 21 db gain antenna ($50) should travel about 1 mile with a 15 db fade margin to a neighborhood tower with a 6dB antenna. The client can use high gain directionals but the Multi-point tower has limited gain. Pumping anything more through the air might encourage a visit by the federales. Tim Pozar explains Part 15 rules for us dunderheads. Antenna suppliers include Hyper Link, Super Pass, Telex and YDI.

AirBridge might be key because it’s “plug and play”, has a low noise radio and puts out decent power. Perhaps a 802.11g version could be beneficial, too. If not, short-range 802.11a/b/g bridges might use Mesh and Smart Antennas to bring it home. Free.

Unlicensed 2.4 and 5.8 GHz bands don’t have the range of licensed carriers like MMDS or cellular. But unlicensed networks can go anywhere on a moments notice. Licensed providers can’t.

Posted by Sam Churchill on Thursday, May 23rd, 2002 at 3:10 am.