(Continued from Part 1. This concludes the article.)
a Radio Survey
Performing a radio survey of the area first is a necessary step before purchasing or fabricating the appropriate antennas. You might find that no directional antennas will be necessary, the cost reduced, and the remaining budget used to purchase additional sensors. The range of any transmitter is in the end limited by or enhanced by the surrounding terrain. Given that very low power transmitters are being used, the 1 watt transmitted by these sensors, versus the 5 watts of a handheld transceiver, the challenge is greater. Having favorable terrain is necessary to extend the range of low-power transmitters. And even if the terrain is not entirely favorable, 1 watt of power is still adequate enough to refract over several hills to a receiving antenna. However, there is a significant loss of signal strength in the signal making that transit.
You must understand how an antenna propagates. Even a high gain omnidirectional antenna, such as a Slim Jim, can be aimed at the crest of a hill in order to send the signal farther. This antenna does concentrate its propagation lower and toward the horizon as compared to a unity gain 1/4 wave antenna that evenly distributes RF in all directions, and therefore has no gain. Aim the Slim Jim by placing the crest of the hill 90 degrees, or perpendicular to the length, or axis of the antenna, so that more of the signal is sent toward the top of the hill and will be refracted over and downward on the opposite side. A higher gain directional antenna is not always needed, but should the hill be high enough, additional power in terms of Effective Radiated Power (ERP) might be used to refract the signal over the hill. This is where a high gain directional antenna can be used to solve a problem. But first, we must find out where it would be best to use these sensors, given the low power of the transmitter, and where and what kind of external antenna, if any, might be needed. Therefore we need to perform a radio survey.Continue reading“MURS Dakota Alert IR Sensors and Antennas – Part 2, by Tunnel Rabbit”