I've been running a Diamond NR770HB on a trunk lip mount for local repeater work, but I'm considering moving to a roof-mounted NMO setup for better performance. How much of a difference does antenna height really make for VHF/UHF mobile operation? I'm mainly doing 2m/70cm FM, some simplex, and accessing repeaters within 50 miles. The trunk mount is convenient but I wonder if I'm giving up too much signal. Has anyone done a side-by-side comparison?

The center of a vehicle's roof is a very good place to mount a VHF/UHF antenna, though creating an optimal ground requires drilling a hole. You could use a mag mount instead, but the only ground path is through capacitive coupling, which is lossy. Trunk mounting is second best.

Antenna location makes all the difference - it's the metal mass directly under the antenna that counts, not what's alongside. Increasing the antenna height may also increase distance, so mount your antenna as high as possible. I moved from trunk to roof NMO and gained about 10-15 miles on weak repeaters.

I've done exactly this comparison with the same antenna! Roof mount gave me noticeably better performance, especially for simplex work and fringe repeaters. The roof is probably the easiest location to place VHF/UHF antennas, but make sure seam mountings and clip mountings are well grounded (bonded) to the frame.