Q: How can I get the range that I need with so little power? The Government rules for license-free operation appear very limiting.

A: For most projects, the US FCC rules allow an adequate transmit power for low frequency non-spread spectrum applications. Operations in the higher frequency bands can use frequency hopping or direct sequence spread-spectrum. The FCC allows a higher power level under these rules.

The Canadian rules are very similar to the US rules.

European governments do not allow spread-spectrum operation, they do allow a bit more power than the US rules. Let's review wireless range and power requirements.



Q: What range can I expect?

A: With optimum frequency choice, an efficient antenna, best receiver sensitivity, and maximum transmit power, 1500 feet is a possible free-space range for non-spread-spectrum, license-free operation in the US or Canada. The key, here, is "free-space." Most applications cannot expect a range of this magnitude due to attenuation from walls, buildings, trees, etc. A greater range is possible with the maximum power level allowed by some European rules.

In the US or Canada, a range of several miles is possible using spread-spectrum. This range increase is not a result of spread-spectrum processing gain. It is a result of a significantly higher allowable transmit power level, and higher allowable antenna gain.



Q: How can I Maximize my system range?

A: More transmit power provides a greater range for any wireless project. But, there is more to achieving a workable range than just increasing power. Antenna gain and receiver bandwidth affect range every bit as much as transmit power. Some other components which affect system range are:

Antenna gain and pattern
Receiver bandwidth
Modulation and Spread-spectrum
Bit error rate
Receiver sensitivity

The government in my market allows more transmit power for license-free operation. Should I use more power?

If the transmitter is battery powered, increasing the power output will seriously affect battery life by more than one might expect. Transmit amplifiers are not efficient.

A transmit amplifier stage typically adds a gain of 10 dB. In order to increase the transmitter output from a few milliwatts to 250 milliwatts requires two stages. The efficiency of each stage is about 40 or 45 percent. To this must be added the efficiency of the power regulation or supply components. A combined efficiency for this higher power level is often less than 35 percent. Even with a very low duty cycle, the power drain for a higher output power level may significantly impact battery life.


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