7. Increase antenna height
In most applications you will be using UHF and microwave frequencies, so line of sight (LOS) rules apply. If your receive antenna cannot "see" the transmit antenna, good luck in making a reliable link. The way to overcome this problem is to increase antenna height. Range automatically increases with height according to the formula:
D = √(2ht) + √(2hr)
D is the range or distance in miles, ht is the transmitter antenna height in feet, and hr is the receiver antenna height in feet.
Putting the antennas on a tower or tall building will get you in the clear for best reception, but remember; if you use a tower, your transmission line length may increase decreasing the power at the antenna thereby offsetting some of the benefit of height. There are cases where height is more important than line loss just because the signal will get through if it is LOS. You can always make up the loss with a LNA or PA, or you can put the transceiver on the tower with the antenna. Let the transmission line be twisted pair carrying the data.
8. Consider the modulation schemes
When designing a digital radio system, you will be looking at things like carrier to noise ratio (C/N) and bit error rate (BER). The modulation scheme used by your system will determine the relationship between the two. This is no place for a college course on the subject, but what it boils down to is that some modulation methods give a better BER with a lower C/N. FSK is better than ASK/OOK. BPSK is better than FSK, and so on. If you are really dealing with a critical link, be sure to check into this. You may even want to consider direct sequence spread spectrum (DSSS), which spreads the bandwidth of the signal to provide process gain that will add to the range. Cutting data rate can also usually lead to lower BER and longer range.
9. Minimize obstructions
When you evaluate the applications, try to eliminate as many obstructions as possible. Walls kill range more than anything. They won't stop a signal, but they will attenuate the daylights out of it. If walls have to be involved, try to minimize the number implicated. If you can, put the transceiver outside the wall. If outdoor obstructions are present, locate antennas appropriately to avoid them. Put the antennas on towers or find some way around the obstruction. Relocate the whole application. Take a hard look at this, as obstructions will make or break the application faster than anything—and this goes for indoor operation as well.
10. Use a repeater or mesh network
A repeater is just a transceiver operating on a different frequency that serves as a radio relay station. Many VHF and UHF radio systems use repeaters where they are mounted on a high building or a tower on a hill. The transmitter sends the signal to the repeater, which receives the signal and retransmits it on a different frequency. It works great. The big problem is that the system becomes more complex and more expensive. And, repeaters are often not available for or allowed in some wireless services because of the extra frequencies needed.
A good alternative is to use a mesh network. A mesh uses lots of transceiver nodes that talk to one another over a short distance. If the nodes are spread out, a wide range can be covered. To send data, one transceiver sends the signal to one of its closest neighbors. This neighbor node serves as a repeater/router and transmits the data to the next node and so on. With lots of nodes, a truly extended range can be achieved. ZigBee is a natural choice because mesh is inherent. Other wireless technologies can be adapted to mesh but it requires extensive proprietary design. Wi-Fi, the popular WLAN 802.11 technology, is now regularly used in mesh applications you can buy. An IEEE mesh standard (802.11s) is in the works. A real benefit of mesh is that it not only extends the range but also increases reliability since if one node goes down, the signal can usually find an alternate path through the mesh.
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