[h2odragon]

The TV stations used a big transmitter up on the Sears tower for their real signal. That got fed by a little microwave dish antenna pointed out to a relay "in the city". So if you could get an angle on the dish with a transmitter of sufficient power, you could feed your own signal into whatever channel.

"Hauling equipment up to the roof without getting caught" and "having haul-able equipment" are the technical challenges. IIRC they used a ~900Mhz system for that and even today I would have to spend some money to put a video signal out in that range, and another chunk to do it at any power.

In 1987 someone had to creatively borrow some very high priced kit to do that.

[leephillips]

Thanks for the details! (But would you need to haul equipment up to the roof? Couldn’t you insert your transmitter anywhere in the line of sight?)

[jessaustin]

Not an expert, but some lines of sight stretch from the top of one building to the top of another. Chicago doesn't have any mountains nearby, so those lines would just extend out into space and never touch the ground.

[mark-r]

The frequencies involved would be blocked by buildings, so the "line of sight" would necessarily be above them. Thus, rooftop.

[h2odragon]

Yeah, I just assume "roof" cuz that's where antennas will be. That kind of thing is as close to "tight beam" as can be managed with antennas, usually; and that can be quite small.

[GekkePrutser]

Borrow, or build! 900Mhz was pretty high for the era but it was already in the realm of home building. The power amps for such frequencies would be hard to get but it wouldn't have to be perfect so using a part out of spec would have been feasible.