Sight Surveys are Required!
- There is no way to design a proper link (regardless of how simple it sounds) without looking at the geography involved. It's easy for an un-trained person to perform a simple sight survey if they know what to look for. If the link is complicated, some expertise is required. Your vendor can help you decide how much of a problem the sight survey will be. As a minimum, the sight survey should cover...
- Length of the Path
- Matching Equipment to the Site Requirements
- Line of Sight
- Obstructions and the best way to deal with them
- Antenna mounting requirements, towers vs. pole, roof mounts, in-building mounts
- Cable routes between antenna and radio/LAN unit
- Power availability for the radio/LAN unit
- LAN environment... throughput requirement, protocol
- Any unusual situation, such as a building owner who doesn't allow outside antenna supports
- What is the length of the path? Do not over-extend the link. If a manufacturer states that the unit is good for 4 miles, don't try to make it work 6 miles or even 4.2 miles. In most cases, DCB de-rates the maximum path specified by manufacturers to allow for a more reliable link. Remember that all units in the USA must comply with the same Federal Communications Commission rules and regulations. They are all subject to the same maximum power lmits. The laws of physics that govern path losses apply to all radios equally. If one manufacturer claims 50% more range than all the others, it's really suspect. Find out why they make that claim, and get references to prove it.
Match the antenna to the path requirement
- Simple links of a few hundred feet can be installed with small "rubber duck" type antennas mounted on the radios. Links of a few hundred feet to a few thousand feet are often made with "flat-plate" antennae attached to the side of buildings or inside windows. Multi-mile links require "yagi" antennae or reflector dish type antennae that may need a tower or metal pole mount for support.
Minimize Coax Cable Length
- At these frequencies, signal loss in the cable between the radio and the antenna is significiant. Minimize that loss by minimizing the cable length. Consider this when locating the radio and antenna. Use quality, low-loss cable. FCC regulations require unique connectors for many units, normally all units that are designed for installation by the general public. Make sure the coax connectors are appropriate.
Line of Sight is Everything
- When using 2.4Ghz band license-free radios (and with a few exceptions, the 900Mhz band) the two antennae must be within line-of-sight (LOS) of each other. Read that literally...LOS means that you can see the remote site antenna from the other location. That is, assuming you have powerful enough eyes or a telescope. At these frequencies, if you can't see the other antenna, the radio signal can't. There can be no obstructions between the two. That includes buildings, water towers, billboards, LEAVES, mountains, and trucks. If you perform the site survey during the winter, will spring leaves and tree growth block the path? Sometimes you can move an antenna from one side of a building to the other to get around an object. At 23Ghz, there is a "fresnel zone", or clear area that must be maintained around the line of sight. For example, you can't "skirt" a hill in the middle of the path without giving some extra clearance.
- The antennae must be mounted to a rigid structure. Do not use thin-wall pipe that will flex in the wind. The antenna is highly directional, and a wind or ice load that makes the antenna move a few degrees can ruin the link. We prefer 2" to 4" antenna mast pipe. Often a non-penetrating roof mount is used when LOS allows the antenna to be mounted 8' or less above a roof. Some links are short enough to allow a "flat plate" type antenna taped to a window pane, or to allow inside mounting of the antennae.
Deal with Obstructions and Long Path Length
- Obstructions to LOS can be handled several ways. Moving an antenna can sometimes cure the problem of a building in the way. Adding a tower may be required for additional height to get over a tree. A repeater can be installed to allow the signal to go around an obstruction or extend the usable range to meet your path requirements. If repeaters are required, it's often better to have a professional perform the site survey and analysis. If long paths or obstructions are encountered, a wired link may be more economical.
Use the Most Appropriate Band
- In urban areas, the 900 Mhz ISM band is becoming quite crowded and may be unusable for your application. Although units operating in the 900 Mhz band have greater range, the 2.4 Ghz ISM band is still quite open, and it's directional nature tends to keep it available. All ISM band units must tolerate interference... you don't "own" the frequency. If the requirements are more stringent, consider 23 Ghz licensed microwave units. They are more expensive, and require the time delay and expense of FCC licensing (or equivanent in other countries), but the frequency is guaranteed to be free of interference "forever".
- These systems should be as reliable as any wired link. To insure high reliability, use the lowest bandwidth that matches system needs. Lower speed links are more tolerant of external interference and path loss when used on long paths. The most reliable links use directional antennae instead of omni antennae. Use an omni antenna only when it is required for multi-drop systems or on very short paths. Unless you are using the frequencies in the 23 Ghz band (or higher), rain does not affect signal quality. When a system that uses these frequencies is designed, allowance is made for interference from the rain.
Again, the reality is that there are many reliable, economical, wireless data communications links. By following a few simple rules and working with a reputible vendor, a wireless link may be the best way for you to solve your communications problems. Although knowing these basics will help you discuss your application with our engineers, these rules are simply the beginning. When discussing the project with our engineers, you may be asked more questions to help determine best equipment for your project. Applying LNAs, bi-directional amplifiers, and FCC rules can quickly become complex. We're here to help!
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