EMI May Render Your Car’s Beeper Inoperative
COMMENTARY
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 9/27/2005 6:34:00 AM
| Read more commentary from Martin Rowe. |
Because they are RF transmitters, beepers need a receiver in the vehicle. Like all receivers, the remote-keyless receivers are susceptible to interference. I noticed that my two beepers consistently failed while shopping in a particular area. The problem? Interference from nearby TV and FM transmitters. All of Boston’s TV stations and several FM stations use a set of four 1000-ft towers in the Newton/Needham area. The lone Newton tower transmits analog Channel 7 (WHDH-TV) and its DTV counterpart on Channel 42 (WHDH-DT). The tower sits in the middle of a popular shopping area, while the others are in a more residential/office area.
Although I have shopped in that area under the WHDH tower many times, I never paid attention to the problem until my daughter attended preschool a few blocks away. Every morning and evening, I struggled to get my keyless-entry system to work. Then, I noticed that the beeper worked better if I stood on the driver’s side and within a few inches of the car.
What does it take to interfere with a keyless-entry system? To find out, I asked the transmitter engineers at WHDH for field-strength measurements. The plot shows measurements at 11 locations near the tower. The first nine locations are along Needham St. where the stores and other businesses reside. The preschool where my daughter attends is directly across the street from the location with the highest measured field strength, which is 573 mV/m.
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| EM field strength near a TV tower can interfere with remote beepers. Data for this graph courtesy of WHDH |
On a recent shopping trip, I experimented by measuring the farthest distance from the car that the beeper would work. At distances of roughly 1000 to 2000 ft, my beeper would either not work at all or would work only within a few inches of the car door. (One of those locations was the preschool parking lot.) At locations 3000 ft and beyond, the beeper would work at distances of 15 to 18 ft. The graph shows a significant drop in field strength at about 1300 ft, but my beeper wouldn’t work at that distance where the data indicated that it should. But, this was a rather unscientific experiment and I was not at the exact locations of the field-strength measurements. Furthermore, the field-strength measurements were conducted at the channel 7 and channel 42 frequencies only. Other channel frequencies could have interfered with my beeper.
I didn’t find an exact correlation between the field-strength data and my experimental data. But, I can still conclude that using a remote-keyless-entry system in areas of high EM fields will cause them to fail or to work only at reduced distances. Keep that in mind the next time you think the battery has failed.
Remote-keyless beepers may fail when used near broadcast towers, but they recover as soon as you move away. I have, though, found a good way to make the remote beeper fail permanently. No, I didn’t run over it with the car. I walked into the ocean with my keychain still in my pocket. Salt water will kill your beeper. Fortunately, my wife was with me so we just used her beeper until I could get a replacement.
