I wouldn't amplify if I were you, adding an amp in the mix will amplify the noise as well.

I would try using a short length of lmr400 if you can. Stay away from RG58 if possible

Check the attenuation charts: http://www.universal-radio.com/catalog/cab...perf.html#atten (lower is better)

Then try the test again when it isn't raining and see if you get a better signal/throughput.