Recently tested my Larsen NMO2/70B in different mounting positions on my sedan and got some surprising VSWR readings. Roof center mount shows 1.3:1 on 2M, 1.4:1 on 70cm, while trunk lip mount reads 2.1:1 and 1.8:1 respectively on the same bands.

Ground plane availability and mounting placement significantly affect mobile antenna performance, but I didn't expect such a dramatic difference. The trunk mount is more convenient for my daily driver, but those VSWR numbers worry me about transmitter stress and pattern distortion.

Running 50W on a Yaesu FTM-400XDR with RG-58 jumper. Should I drill the roof or can I improve the trunk mount performance somehow?

Roof mounting consistently outperforms trunk placement - the difference in coverage is significant. I've done the same comparison on multiple vehicles and always see better performance with roof mounting. The ground plane effect is much more effective with the antenna centered on a large metal surface.

Those VSWR readings on the trunk mount suggest poor ground plane coupling. You might try adding ground straps from the mount to the vehicle frame, or consider a different trunk mount style. NMO mounts work great when properly installed - the 3/4" hole isn't that big and can be easily patched when you sell the vehicle.

As a newer ham, this is exactly the kind of technical detail I need to understand! Is the 2.1:1 VSWR harmful to my radio, or just inefficient? I'm hesitant to drill my car roof but want good performance. Would a mag mount on the roof be a good compromise for testing before committing to the permanent mount?