Zone Preparation and Maintenance Essential for Trouble-Free Sensor Operation

As the developer of the first bi-static microwave sensor in 1971, and today’s leading provider of fixed and portable microwave links and transceivers, the team at Southwest Microwave knows first hand that two of the key ingredients to trouble-free microwave sensor operation are proper preparation of the sterile zone and consistent maintenance of the protected area once the system becomes operational. 

“The number one thing we see in the field when we’re called in to determine why a sensor isn’t functioning properly is that the site was not adequately prepared for microwave detection or that someone is not maintaining the detection zone,” says John Wietfeldt, Assistant Manager of Technical Services and Training at Southwest Microwave.

Remove Obstructions and Moving Objects
Wietfeldt, who teaches factory and field certification courses on microwave sensors for the company, explains that proper site preparation begins with the removal of obstructions and moving objects from the detection zone.

“Large fixed objects, like metal posts, will reflect microwave energy and distort the detection pattern,” says Wietfeldt.  “The area should be cleared of objects that move, shake or vibrate such as trucks, trees, bushes, fences or large areas of water. Large moving objects within the detection zone will be indistinguishable from an intruder and will cause nuisance alarms.”

Wietfeldt notes that the clear area must be at least as large as the detection zone.

“Don’t just think of the obstructions on the ground,” he instructs technicians during each class. “There may be a large tree in the area, and people may not be thinking about that hanging limb. Yes, it may be 10 feet off the ground, but if it’s inside the detection field, once it starts moving around, it will cause an alarm.”

Level the Terrain
Since the operation of a bi-static microwave link requires transmission of energy from transmitter (Tx) to receiver (Rx), it is critical to maintain a clear line of sight between the units. Therefore, the ground must be flat across the protected area. Any bumps, hills or ditches in the zone will shadow the beam and may provide crawl space for an intruder.

Hills or ditches may shadow an intruder

For both links and transceivers, Wietfeldt advises that hills must be leveled and ditches filled so that the protected area is flat to within 6 inches (15 cm). For detection of prone crawling targets, the detection zone should be flat to within 3 inches (8cm).

Ensure Appropriate Ground Material
Wietfeldt reminds installers that appropriate ground cover is essential to reliable microwave detection. The protected area can be any stable, reasonably smooth ground material such as concrete, asphalt, tilled earth, sand or gravel. If there is grass or vegetation in the protected area, it must be kept cut to a maximum of 3 inches (8 cm) in height. Snow should be removed to these same height specifications.

“If vegetation is being removed from a site before the sensors go in, we recommend that a plastic membrane be laid down prior to installation of the final ground cover. This will prevent future sprouting of grass or weeds,” says Wietfeldt. “However, if a plastic membrane does go down, it must be porous to allow drainage.”

Ensure Proper Drainage
Drainage is a critical issue where microwave detection is concerned. Wietfeldt urges users to be diligent in ensuring that water drains properly throughout the protected area, since large puddles may cause nuisance alarms.

Large puddles may cause nuisance alarms

“Standing water will reflect microwave energy and distort the detection zone. If you find that water is settling in a certain area, laying down gravel is a solution that will prevent pooling,” he advises. “Moving water will also generate an alarm. You want to prevent both of these scenarios.”

Consider Equipment Location
Aside from the physical characteristics of a site that must be considered prior to sensor installation, it is important to recognize potential man-made challenges.

“When specifying sensor location, consider areas that will provide the best level of security yet be free from nuisance alarms,” Wietfeldt suggests.  “Locating sensors inside a controlled access area will prevent nuisance alarms due to random foot traffic, vehicles or large animals.”

“Too many nuisance alarms are just as bad as not getting any alarms at all. For an end user, this scenario can become a ‘boy who cried wolf’ syndrome,” he cautions. With too many nuisance alarms, a legitimate alarm may be ignored.”

Wietfeldt also reminds installers to select locations which provide protection from accidental damage and tampering. Often, simple bumper guards will prevent damage from vehicles.