The RF environment which is the amount of electrical and atmospheric noise present, can influence the receiver's ability to pick out the signals radiated by the alarm transmitters. Atmospheric interference fluctuates with the weather conditions and is to some extent outside the installer's control. Electrical interference is more prevalent in industrial and commercial premises, due to the use of heavy industrial plant, such as rotating machines and air conditioning units. In the domestic situation a major source of interference is the home computer. Receivers in particular should not be installed in their near vicinity.


One other phenomenon known as multipath reflection can affect a receiver's performance even when it is sited well within its operational range. Dead spots are produced where the resultant signals are reduced to zero. This canceling effect is due to the radio waves arriving out of phase via different reflected paths. It is important in this respect to ensure the receiver is not sited within one of these areas. Usually a receiver movement of one metre within the site area will effect a solution.

All radio systems suffer from these effects to some extent; manufacturers who claim immunity for  their equipment have obviously found some means of defying basic physical laws! However, the majority of these effects, if encountered, can be reduced to negligible proportion by good system design. In the main this means adequate aerial heights, employing a high performance receiver dual conversion if possible, and ensuring the equipment is not working at the extremes of its operational range Whilst the majority of applications for radio alarms involve domestic premises, they do offer considerable scope where areas are widely spaced or require external cabling. Ranges of up to 3KM are possible with the current configuration of high gain aerial and dual conversion receiver.

Conclusion

The history of intruder alarms is one of constant evolution, from the early relay based devices to the modern sophisticated micro-electronic systems available today. Radio alarms represent a significant advance in this process, their scope and flexibility offering exciting prospects in the design of modern systems.