

It would require as much, or more, power to drown out a TV broadcast signal at the source. I believe many of the old towers were 200kW-1000kW so it would have taken one hell of a pirate signal if interfering close to the main source. However, RF follows the same principle as light using the inverse square law so the further you get from the primary transmitter, the signal quickly becomes exponentially weaker for any receiver.
If you had a TV transmitter on a small hill that is a fair distance away from the target audience, like many were, splitting the distance with a directional antenna wouldn’t require nearly as much power from the pirate signal to overtake the original transmission.
If I wanted, I could interfere with ham radio signals with as little as a watt of power (in my immediate local area) even though people might be communicating through a ham radio repeater that transmits at a couple of thousand watts that is many miles away. (It’s actually a permitted emergency technique to “break into” active conversations. Actually, other ham radio operators are familiar with what interference sounds like, even for signals that can’t fully overtake a transmission. It’s customary to stop the conversation if detected and wait for the “break”.)
(sorry to add even more; I just made another comment about this and I am familiar with most of these concepts.)
Actually, that would be much easier. TV stations back then mostly received shows via satellite dish. Pointing a low power directional antenna directly at the dish’s LNB would work great. Satellite transmissions weren’t strong and were rarely encrypted back then so that would theoretically be super easy if you knew your RF and deep RF knowledge was much more common place +30 years ago.
I am not sure if they used point-to-point microwave antennas back then for TV, but it would be the same concept. (Microwave antennas are typically the round, cylindrical looking, covered antennas we see all over the place today.)