Answer 3—One possible way to mitigate a visible ghost on a TV screen is to create a ghost of your own that cancels the one from the outside.

A transmission line of the appropriate length with a short at the end of it produces an inverted reflection with a delay that is proportional to twice the length of the line (after taking into account its velocity factor). The strength of this artificial reflection can be varied by replacing the short with a variable resistance that is less than the characteristic impedance of the transmission line. By connecting this stub to the antenna terminals of the TV set and tweaking the length of the line and the value of the resistance, it should be possible to create a “negative ghost” that cancels most, if not all, of the effects of the original positive ghost.

Note that this suggestion, while simple to implement, relies on canceling the RF energy of the ghost signal, and gets more sensitive to tuning as the RF carrier frequency goes up. It is probably completely unworkable at UHF frequencies. It may be possible to create a similar effect in the video signal after demodulation by using a delay line and an inverting amplifier.

Contributor: David Tweed

Published February 2004