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Putting all the transmitters at the same location is generally best for receivers, since they can more easily sort out many equal signals without the weak ones being distorted by the presence of stronger ones. The situation in Chicago is near ideal, with the exception that people downtown may need some attenuation on the input to make the overall level reasonable. Actually, some of the stations are on the Hancock building, and some on Sears (nost of digital TV on sears, but not all), and one of our favorite test sites in the past has been the Grant Park-facing apartment of a former employee who has line-of-sight to Sears and only reflected signals from Hancock.
Co-location where possible is one of the things that has made it possible to fire up digital TV transmitters duplicating the analog service without killing analog reception in the meantime. Many stations during this transition period are transmitting digital from the same tower as their analog signal, even on the adjacent channel.
Non-co-located sources can be a pain - you can experience an increase in interference almost without limit by being close to the interferor. This sort of thing has happened in the past for example with FM stations that are not co-located with the TV stations. This is most commonly a channel 6 problem, but when my Uncle got one of the first transistor TVs and lived near WMBI-FM, he even had trouble on channel 5, and I had to install an FM trap. This proximity problem is why there is currently a fight between interests who would like to use the TV "white spaces" for unlicensed wireless internet devices, and the broadcasters and licensed wireless mic people, who are saying such devices may ruin DTV reception and wireless mic use by creating an interference zone around every home and apartment that installs such new wireless gear.
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