Before you go ahead with purchasing a filter, run following two scans to see what RF interference is received by your antenna:
(1) Wide band scan - 24 Mhz to 1800 Mhz:
Spektrum - How-to Speedily Scan RF Noise in band 24MHz ~ 1800MHz
(2) Narrow band scan - 800 Mhz to 1200 Mhz
Do I Need A Filter?