Answer 2—A ghost on a TV screen is caused by a reflected RF signal that arrives slightly later at the TV antenna than the direct signal from the transmitter. To locate the source of the reflection—which could be anything from a tall building or water tank to a particularly steep hill—you need to narrow down where it might be based on the information at hand.

You can estimate the amount of time delay between the direct path and the reflected path from the horizontal displacement of the ghost on the screen. In this case, you have about 10% of the visible portion of each horizontal scan line. In the case of an NTSC signal, this represents 10% of 51 µs, or 5.1 µs of delay. Taking the speed of light (3 × 108 m/s) into account, this delay represents an additional path distance of 1.53 km.

The set of points that represents a fixed additional distance between two points forms an ellipse with the two given points (the transmitter and receiver) at the foci of the ellipse. You can take a map that shows both locations, cut a length of string that represents the length of the reflected path (using the scale of the map), anchor its ends at the transmitter and receiver locations, and use it to sketch the ellipse on which the reflector must lie.

With the map in hand, it should be straightforward to search along that path for likely reflectors of TV signals.

Contributor: David Tweed

Published February 2004