Sometimes One of My Local Digital Stations is Unstable

Written by Clearly Resolved
Wednesday, 30 May 2007

[ Q ] I have excellent reception of all of the local digitals -- most of the time. But occasionally one of the stations will be break up, go to black for 10 or 15 seconds and then come back with a very strong signal. But then a minute or two later it all happens again. What's the problem, and what can be done to eliminate it?

[ A ] This sounds like it's probably a case of "multipath" interference.

Multipath is just what it sounds like -- the signal from one or more stations is arriving at your antenna from multiple paths, most likely as a result of it bouncing or reflecting off some topographical or man-made object as it travels from the station transmitter to your home. In the analog world, multipath resulted in out-of-sync, multiple images on screen that were often referred to as ghosts; in the digital world, the second signal causes the receiver to null out the signal until it can lock on a single signal. This explains why you intermittently receive picture/sound and then nothing.

Multipath can be fiendishly difficult to eliminate, but antenna placement can help. If you can, move your antenna a few feet left or right, up or down, or play with angle of orientation. The goal in repositioning is to try to find a spot where the antenna will not see the secondary signal that is causing the multipath.

Over time digital receivers have become more adept at rejecting multipath signals, with each generation doing a bit better than the previous. If you're using an older receiver, an upgrade may be a solution, but there's no guarantee that would cure your problem.

Also, a directional antenna (like a yagi attic/roof antenna) will perform better in installations where multipath problems occur. Multi-direction antennas are better at pulling in more stations without adjustments, but they sometimes increase the incidence of multipath issues.

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