Installing a Diamond NR770H on my truck and running into some tuning challenges. Some dual-band antennas are designed so the 70-centimeter tuning depends on the harmonic of the 2-meter section, which means your UHF resonance is indirectly tied to how well the VHF portion was tuned. Therefore, always verify both bands after making UHF adjustments.

Started with 2m band showing 1.3:1 SWR at 146 MHz, but 70cm was way off at 2.8:1. After adjusting the loading coil slightly, small adjustments less than a millimeter can shift the UHF resonance noticeably because the 70-centimeter band's shorter wavelength means small physical changes create large frequency shifts.

Now both bands are under 1.5:1, but I'm curious about others' experiences with these trap-style mobile antennas. The interaction between bands seems stronger than I expected from the specs.

The ground plane is one of the most overlooked parts of mobile antenna tuning. Your vehicle's roof, trunk, or fender acts as the ground reference, and poor grounding can increase SWR and reduce efficiency. Ensure that the antenna mount makes solid electrical contact with bare metal.

Which mount are you using? I had similar issues with a mag-mount until I realized the paint was preventing good ground contact. Verify that coax connectors are tight and corrosion-free. Always perform final tuning with doors and trunk closed to simulate real operating conditions.

That's normal behavior for trapped antennas. In many dual-band designs, the two resonant circuits interact through shared conductors or traps. When you tune the 2-meter section, it changes the electrical length of the antenna, which can slightly alter the 70-centimeter resonance. Always tune VHF first, then UHF.