I suggest an antenna preamp LSA151 that covers the full 54 to 1000MHz band has 15dB gain, and uses 75 ohm coaxial input and output ports.
For LSA151 you will have to be careful that there are no strong signals above 310MHz that could overload the preamp.
If you use an antenna tuned to 310MHz that will help a lot.
Make sure that the LSA151 (or a preamp) is placed right at the antenna. There should only be short cable between the antenna and the preamp. The long cable should be placed between the preamp and the receiver.
Note that the LSA8 may not be ideal for use as a preamp because it only has 8dB gain. Preamps are generally 15 to 25dB gain, with low noise figure. If you are worried about overloading the CM15A then you can put a filter between the preamp and the CM15A. Perhaps the CM15A already has an input filter? But don't put a filter between the antenna and the preamp, or you cancel the benefit of the preamp's low NF.
Here are a couple of web sites for you.
Useful information:
http://www.irving.org/ftplib/scanning/whipant.txt
Antenna length calculator:
http://www.crompton.com/wa3dsp/hamradio/antcalc.html
You should use approximately 9 inches for 1/4 wavelength at 310MHz. But read the first link to find out more about it. Maybe you would want to make it 1/2 wavelength at a different frequency to help "reject" an interfering strong signal in the area.
Like this next photo, the wire should not be thicker than #19 AWG, otherwise the contact fingers inside the LSA will never again accept the proper pin diameter. It is ok to push the larger #14 wire in there, but don't try to use a smaller wire later because it will not connect very well anymore. So just to grind a wire to can insert in a hole easily.
Cost on February 2007 is approximately $24 + S&H
http://www.lindsaybroadbandinc.com/