What antenna do you have now?
Good brands to look at would be Antennacraft, Antennas Direct, Channelmaster, Winegard. There are certainly more, but those are the ones I know of. These are reputable antenna companies that publish thorough technical specs on each of their antennas. Here's an example with what I would consider a lot of technical info -
http://www.antennasdirect.com/cmss_f...s/DB4E-TDS.pdf Any antenna builder should be able to back up their performance claims with detailed technical documents.
The exception to this rule may be the RCA ANT751 antenna, which has good demonstrated performance but RCA does not supply much info at all.
http://www.amazon.com/RCA-ANT751-Dur.../dp/B0024R4B5C However, it is attractive for its high reported performance, build quality, and modest price and size. It's likely most similar to the Antennacraft HBU11 or HBU22.
http://www.antennacraft.net/Antennas/AntennasHBU.html
123' is a long run. However, you can likely get all the Green stations at ca 240 magnetic (except KRCW 5) with an ANT751 pointed that way. In that direction, the horizon must be unobstructed by trees, buildings, power lines, etc. With 123' of cable, you may need to add an amplifier on the mast to boost the signal and overcome the cable losses.
Alternately, you can start with more antenna gain (ie a bigger antenna) to get more signal and overcome the long cable run. You need an antenna that is optimized for both the VHF-high stations 7-13 and the UHF stations 14-51. The reception characteristics of these two bands are very different, so mixing the signals from two antennas, each tuned to the specific band, is one way to address this. Or you can go with a bigger antenna that captures both bands, like the HBU33 or HBU44.