I don't know exactly how the TV signal is made, but let's say it's a sine/cosine wave, doing the funny horizontal lines. Your friend should be creating such wave, using oscillators, adders, and multipliers, then the signal is modulated on the carrier frequency of the TV channel, or it can be also modulated over many frequencies, so as no matter what channel u change to, it'll stay jammed. Note that you don't need much power as such (50W
or more), cause the signal jammer is far closer and powerful, and TV signal are really (sometimes) weak, compared to locally transmitted signals.