I'm brand new to amateur radio. While waiting for my license to appear in the database, I purchased a Baofeng UV-5R and a Comet GP-1 dual band (2m/70cm) antenna to put on my roof. I have used an N-type female to PL-259 adapter to attached 25 feet of LMR-400 coax to the base of the antenna. I am using a whip to attach the coax to my UV-5R. There is a local repeater on 442.000 MHz that always produces a strong signal. With the rubber duck antenna that came with my HT, I always see all the bars unless I'm in the basement. When I attach the roof antenna, the signal from the repeater increases and decreases from nothing to full strength erratically and I hear some awful sounding interference from time to time (though not regularly in any sort of pattern). I actually tried a different antenna (Tram 1490), but the same thing happened. I tried connecting the antenna to the handheld without the whip, but that didn't help. The coax is new, so I'm pretty sure it's not that. I haven't checked the N-type to PL-259 adapter, but that's new too. I am perplexed as to why the signal would be so erratic with a roof antenna. Why would the signal be consistently strong and clean with the rubber duck that came with the HT and so erratically up and down with the roof antenna? Is it somehow picking up more interference that the short rubber duck isn't affected by? Any help would be greatly appreciated.