How to Maximize Range While Minimizing Power in Wireless Transmissions
This article will discuss design considerations that provide the maximum transmission distance for digital wireless radio communication in the FCC unlicensed bands, while minimizing the system power supply requirements, thus facilitating solar or wind powered remote applications.
System integrators with radio experience are aware that the FCC imposes rules that limit the transmitted RF power in the non-licensed ISM bands. These rules are different for each of the three bands, 900 MHz, 2.4 GHz and 5.8 GHz. There are also different rules for multi-point vs point-to-point topologies. For simplicity, we will only investigate point-to-point range optimization where the conducted output power and antenna gain must be properly designed to ensure legal operation. The conducted power (power coming out of the radio’s connector) limit is 30 dBm or 1 Watt for all three bands, but may need to be reduced when high gain antennas are used.
- At 900 MHz, the user is allowed to use maximum conducted power when used with an antenna and cables whose respective gain and loss sum to 6 dB or less
- At 2.4GHz, the calculations are similar but the conducted power must be reduced by only 1 dB for every 3 dB that the antenna gain exceeds 6 dBi
- At 5.8Ghz, no reduction in conducted power is required regardless of the antenna gain
The most important decision when designing long range radio systems is to use short RF cables (<1 ft) from the antenna to the radio. Short cables require less conducted output to overcome cable losses and more importantly the short cables don’t act as an attenuator for the receiver’s “ears”. The lower conducted power allows more range and less current draw from the radio’s power amplifier. The lower power also creates less heat to dissipate, enabling outdoor installations that are often requisite to using short RF cables.
900 Mhz has the lowest frequency and longest wavelength compared to 2.4 and 5.8 Ghz. Lower frequencies have lower attenuation through materials that impair the radio’s transmission path. These materials can typically include trees and small buildings. Higher frequencies require a clear visual path (line of sight) with no visible path impairments. Lower frequencies also have an ability to bend slightly around hard path impairments like the ground or large buildings. Applying wave bending characteristics and fade margins should be done by experienced wireless technicians. Given these considerations, we believe that 900 Mhz is typically the best choice given real world path impairments and will focus hereafter on this selection.
AvaLAN Wireless is a developer and manufacturer of long range industrial wireless radio technology. AvaLAN’s products are designed to enable wireless connections in perimeter or remote locations. Specializing in the unlicensed 900 MHz, 2.4 GHz, 4.9 GHz and 5.8 GHz radio spectra, AvaLAN offers a number of Ethernet bridge products and point-to-multipoint wireless networking products. AvaLAN Wireless Systems, Inc. can be contacted by visiting www.avalanwireless.com.