FYI that 2.4ghz antenna will probably hinder you more than help. It is built specifically to receive 2.4ghz signals. If you are really interested in getting it to work a real cell antenna will probably work wonders for you and on the plus side if you leave the company you can attach it to your car afterwards for roadtrips through low signal areas
Also, I don't believe it's only the length of the antenna which sets it up to receive certain frequencies, there is more to it than that (IIRC spacing of the coils/etc)... You can certainly find antennas of different lengths built to receive the same frequencies.