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Thanks to WBBM using RF channel 12, you need a high-VHF/UHF antenna. As to what model, let's step through calculating the requirements:
TVFool's predictions suggest that you have about 20dB margin to the good in signal strengths for the Chicago stations. To protect against local interference and the more unusual reception conditions not accounted for in the prediction, we like to keep a 10 dB or so margin in the system. That's still ok, you're now 10 dB to the good.
However, the model shows that the path from the transmitters to you is 2-edge, meaning the signals have to bend over two terrain features to get to you. In practice, this adds more variability to signals than predicted, so factoring in another 6 dB or so is good practice. No worries, yet, as you are still 4 dB to the good.
The killer is mounting in the attic. It's hard to say exactly how much loss you will encounter, but it's safe to say it will be at least 10 dB and could be 20dB. Now you are between 4 and 14 dB below decodable signal levels. You will need that much antenna gain (not pre-amp gain) to have watchable signals. Thus, larger antennas (with more gain) will give you better reception reliability.
The minimum antennas I would recommend are the Winegard HD7696P or the AntennaCraft HBU-44. I would also recommend avoiding using a pre-amplifier due to the strength of the local LPTVs.
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