When designing a short-range wireless system, you look for one that will give you the maximum range (distance between transmitter and receiver) and reliability that fits the application. Often, the technology choices do not always give you the range you need. So before you go nailing down a technology or standard, it is best to do some upfront calculations. First, look at the available technologies and determine their maximum reliable ranges. The table linked below is an estimate given by the various standards organizations and alliances. It will at least get you into the ballpark for an initial selection. (see table)
Next you can get a good estimate of the range by going back to the basic formula for received power:
Pr = Pt(Gt)(Gr)λ2/16π2d2
Pr is the received power, Pt is the transmitter output power, Gt is the gain of the transmitting antenna, Gr is the gain of the receiving antenna, λ is the wavelength in meters, and d is the range or distance. Playing around with all the variables and solving for d will let you determine just how much range you can get... theoretically. The formula assumes an open unobstructed, but not necessarily line of sight, path between transmit and receive antennas. If you have lots of obstructions like walls, other buildings, trees, or other stuff, your range is going to be much less than the theoretical. If you have worked with radio before, you know that testing is a must in the real world.
In the formula above, the antenna gains are power ratios, not dB. If the gains you have are referenced to a dipole as many specs give, these values must be multiplied by 1.64 to get them into a isotropic gain reference value for this formula. The distance d is given in meters.
Now here are the top 10 ways to extend range. Some are related to the formula above, but some are not:
1. Use a lower frequency
Look at the aforementioned formula and note the range is proportional to the square of the wavelength (λ). And, of course the λ = 300/f where f is the frequency in MHz. So decreasing the frequency of operation is a quick way to get more range. Lots of wireless standards operate in the 2.4 GHz band, but that does restrict range. If you really need to get maximum range, drop down to the 900 MHz band or even the 433 MHz band.
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