A little more info: an antenna is only resonant at one frequency (well, and harmonics, but that's another discussion). If everything is ideal, you'll have a 1:1 SWR at that frequency, and the SWR will get higher the further you stray from that frequency (above or below). The rate at which it climbs varies by antenna design (not germane here). Higher SWR means more power wasted--power that's
not being sent out where somebody can receive it.
The fact that your antenna works at the upper end of the band, but falls off at the lower end, suggests that the antenna is tuned too high. That can be caused by a few things. The first, and most basic, is that it's simply too short (smaller things have higher resonant frequencies--think of the pipes on an organ, or of bells). The fix for that would be to add length. Another is that it's close enough to other metal that it's being de-tuned. That's also highly likely, given its proximity to a large metal structure. This is where an antenna analyzer would be really helpful. Think of the analyzer as automating the task of trying the frequency, noting the SWR, stepping up a few kHz, then taking another measurement, etc., and graphing it all:
You can do it by hand, but it's very time-consuming. With an analyzer, you can run an analysis, reposition the antenna, then run it again. Much quicker, but considerably more expensive than a simple SWR meter (which is why you're more likely to find someone with an SWR meter). I actually have a dual-needle
power meter (forward and reflected power), and the scale has SWR markings that you read where the needles cross:
Anyhow, my advice to find a ham club stands. We tend to be very helpful people (especially if you offer a ride in exchange for the help!), love to talk about our hobby, and who knows--you might just decide to become one yourself. Goes great with flying!
Good luck!