Couple of comments:
Regarding ISM spectrum sharing: the 2.4 GHZ band (2400-2500 MHz) and the 5.8 GHz (5725-5875 MHz) are certainly shared with ISM devices- microwave ovens, induction heaters, etc. However, the 5.2 and 5.4 GHz unlicensed bands (UNII) are not shared with ISM devices. However, these bands are subject to FCC regulations that mandate radar sensing and avoidance. This means that if your radios detect the signature of a military radar system on their active channel, they will automatically shut down and begin a waiting period before switching to another channel. Mandatory 60 second outage.
There are generally three classes of point-to-point high speed unlicensed data radio gear out there today:
1. Wi-fi based gear with some additional hardware and a user interface suitable for point-to-point use. Ubiquiti, Tranzeo, HGA, etc. Pretty self-explanatory. Sub-1000 range.
2. Gear using a wi-fi chipeset (Atheros, Broadcom, etc.) with a proprietary firmware load. Trango, Alvarion, Ligowave, etc. $2000-5000 range.
3. Gear using a custom designed RF interface. Motorola, Dragonwave, etc.
Given your requirements, I'd encourage you to look at classes 2 and 3. Getting any decent amount of reliability from vanilla 802.11 equipment is (as you've found) difficult. Gear in categories 2 and 3 from above will generally have a built in spectrum analyzer of some sort that will be able to see interference not caused by 802.11 devices, performance monitoring systems (BER reporting, event logs, etc), SNMP capability, etc.
Definitely choose a system with an integrated antenna. You want a directional antenna such as a patch array (panel) integrated with the radio. Messing around with RF cabling, connectors, etc. is not necessary with what you're trying to do. Minimize the potential points of failure.
Lightning protection is a concern. Most of this gear is PoE powered, so you'll have a single cat-5 going to the roof. Make sure it's protected with an Ethernet surge suppressor that is properly grounded. Follow the radio manufacturer's recommendations here. Your antenna mount must also be grounded according to NEC requirements.
The Motorola PTP400 series radio that was recommended is one of the best unlicensed point to point radios out there. However, it's been EOL'd and replaced by the PTP500. Seems like these are both out of your budget, though. As an alternative, you might consider looking at the Trango TLink45. This radio uses a proprietary firmware and an Atheros WiFi chipset. It has a rudimentary spectrum analyzer, SNMP, ARQ (important), and adaptive rate modulation. It also has a dual-polarity software switchable antenna. This greatly increases your ability to avoid interference. It will run in the 5.3, 5.4, or 5.8 GHz unlicensed bands. They retail at about $4000 for a pair, but Trango routinely runs specials. They were on special for $1700 per pair in April.
The WISPA list is a great resource for help with projects like this.
Patrick Shoemaker
Vector Data Systems LLC
shoemakerp@vectordatasystems.com
office: (301) 358-1690 x36
http://www.vectordatasystems.com