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What's possible with 900Mhz outdoor wireless links - and what isn't.

While we prefer using the license-free 5.8 Ghz ISM (industrial, scientific, and medical) band for outdoor wireless video data links whenever possible, the 5.8 Ghz band does require optical line-of-sight (LOS) path between antennas. That's because any obstacles in the path severely attenuate the 5.8 Ghz signal to the point where useful data communications is limited or even impossible. However, there are times when LOS simply isn't practical. One common example is the need to transmit video through a heavily wooded area where there are cost or height restrictions that prevent you from installing a tower high enough to clear the tree canopy. In these kinds of situations, 900 Mhz radios can sometimes be used to great effect. However, these are not a panacea as they are sometimes portrayed, and you need to be very aware of the limitations and trade-offs inherent in 900 Mhz, prior to attempting a deployment.

Some considerations to keep in mind include:

  • Limits to link distances 900 Mhz has the ability to propagate through trees and buildings - but not forever. As a practical limit we recommend a maximum distance of no more than 1/2 mile through dense trees, and even then you'll find the performance noticeably degraded when compared with an equivalent LOS link. On the other hand, a link that has just one stand of trees at the mid-point may work well for many miles.
  • Susceptibility to interference The very thing that makes 900 Mhz advantageous, is also its greatest weakness: A strong 900 Mhz signal from , for example, a nearby cell or paging tower, (or even closely-located 900 Mhz baby monitors) can go through obstacles and create massive interference with your local 900 Mhz installation.
  • Limited available bandwidth There is only about 30 Mhz of channel space available in the 900 Mhz band, as compared with almost 400 Mhz in the outdoor 5.8 Ghz band. This means that you can't readily deploy more than a handful of 900 Mhz wireless nodes in a given area without them stepping on each other. This effect can be reduced by very careful layout planning and antenna installation, but it's much more difficult to optimize the installation than on 5.8 Ghz, since, to minimize self-interference, you will need to physically separate antennas on dual-radio 900 Mhz nodes by at least 15-20 feet. and use high-gain antennas that must be aimed accurately to perform properly.
  • Antenna free-space considerations are more critical. Since 900 Mhz operates at much lower (and longer) frequency that 5.8 Ghz, you will need more space between the active areas on your antenna and the nearest obstacles to avoid detuning and attenuation effects. Strive for a minimum of 10 feet between the front of the antenna and the nearest large obstacle like a tree or building. And if possible avoid locating near metal roofs or shooting over a nearby metal roof since these can noticeably effect the 900Mhz antenna signal path.
  • Bio-hazard High-powered 900 Mhz links are a biohazard both due to the power and the frequency. Strive to install the active end of any 900 Mhz antenna so that its beam pattern is at least six feet above and 10 feet away from any area where humans can enter.
  • Maximum available throughput is typically lower than 5.8 Ghz.  We've found that on 900 Mhz you just can't pull as much data through a link as with 5.8 Ghz. Typically, the limit seems to be around 10-20 mb/s of net data throughput, and much less if you're pushing the envelope as to range, obstacles, or local interference. That may be fine if you're only deploying a few cameras on that leg of your system.
  • Antennas must be aimed very accurately With 5.8 Ghz on a close-in LOS link you can sometimes get away with casual visual antenna aiming, not so on 900 Mhz. And since you usually can't see the partner node's antenna (that's why you had to use 900 Mhz in the first place), getting the antennas pointed at each other can be quite a challenge. One trick we've found to be very effective, is to buy around a dozen red helium-filled balloons and raise them above the partner wireless node's location. Once they are high enough, you can often spot them from your wireless location and use them to complete the initial aiming of antennas. The final adjustment, of course, still needs to be done using the built-in RSSI metering function in the wireless node.

The bottom line: If you have a choice of using 5.8 Ghz on some legs of your system, do so. Use 900 Mhz only where absolutely necessary.

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2 Comments

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    William Fisher

    Thanks for the detailed explanation. Does newer software like AirMax help reduce or eliminate interference issues? Hmmm?

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    Tom Sharples

    Sorry for the delayed response William, I don't get over here very often.  I haven't used the  UBNT self-contained 900Mhz gear, altho we do use their OEM hi-power radio modules and 5.8Ghz gear extensively.

    In theory Airmax should help with interference, however we've seen installer reports on the ISP bulletin boards indicating that the 900Mhz UBNT self-contained radios don't seem to work very well (poor range)  compared with some of the other alternatives. That may be due to the low-gain built-in antenna. We like to use separate high-gain panel antennas with our 900Mhz deployments (or even dishes where absolutely necessary) so that may be why we get more range. Hope this helps.

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