I'm getting severe video breakup whenever my quad goes behind any obstacle, even at close range (30m). Flying in open field works fine up to 400m+. My setup: Rush Tank VTX (200mW), Eachine EV800D goggles, and what I suspect might be mismatched antennas.

The weird part is distance isn't the issue - it's purely line-of-sight related. Could this be a polarization mismatch? How do I verify if my VTX and receiver antennas are both RHCP or LHCP? The antenna housings don't have clear markings.

Already tried different channels, moved antenna away from battery wires, and checked for loose connections.

Mismatched polarization will severely reduce your range - if you mix LHCP and RHCP antennas, they will still work but your range is greatly reduced because LHCP antenna rejects signal from RHCP antenna and vice versa. For close-range obstacles, this becomes critical.

You can tell polarization by opening the antenna cap and looking at the copper wire direction from bottom to top - if it curves right it's RHCP, if it curves left it's LHCP. Behind obstacles you're losing the already weakened signal completely.

Had the exact same issue - turned out my VTX came with LHCP antenna while my goggles had RHCP. If you mix them you get about -13dB attenuation, and 3dB is about half the range. Replace one set to match the other and you'll be flying through trees again.