The best client antenna in 900 depends on the typical weather / environment, not just specifically best antenna.
To combat heavy foliage in very Rural areas (dry summer months), M2inc's - 17 dbi Yagis have been invaluable to gain maximum RSSI, to penetrate the trees. However, they become useless in Winter weather, when they get ice buildup on them.
In a ice/snow heavy environment, panel antennas are MUCH better, for example the built-in 10dbi antenna of Trango 900 radios, to get max allowed RSSI in a weather resistent panel enclosure. The F/B is poor (only 12 db), but often the best choice for ease, cost, and Dual pol flexibilty.
In high noise areas, such as Urban or colocated near paging gear, a high quality antenna like MTI's 10 dbi panel, offers maximum F/B ratio, to block out interference. Not much can out perform them, but at a trade off of cost and flexibility of pol change on the fly.
When Yagi's can be mounted low for easy access, (within Gorilla Ladder height (18 feet), and for residential where I can afford to take the risk of not having pol change on the fly (usually consistent noise floor on a polarity), I don't hesitate to install a Yagi as my first choice. Often Verticle is less desirable interference any way, based on paging companies. However, for critical links, installing the M2inc yagis are risky. They mounting method is horrible. It allows a lot of play for the Yagi to move in heavy winds. If mounted high on a steep roof, I avoid the Yagi unless they are absolutely necessary, because they need mcuh more frequent attention. For example to wipe the snow off of them, or re-align.
There are someother Yagis that have more secure double point mounts, around 12-15 dbi, if you can afford to give up the 2 db.
As for verticle Omni type client antennas, for example for mobile apps, I have no advice.
RapidDSL & Wireless, Inc
IntAirNet- Fixed Wireless Broadband
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