@Angela Cole this is a real concern worth thinking through carefully. Worst-case cross-polarization between a vertical and horizontal antenna is theoretically around 20-25dB of additional path loss, but in practice with a tethered balloon you're rarely sitting at true 90° for long, and real-world environments add enough multipath scatter that you typically see something closer to 3-8dB of variable loss as the orientation drifts rather than a hard 20dB wall.
The bigger question is where that puts you relative to the MultiNav Pro+'s link budget. The module is built around the STM32WB07 with external LNA and PA stages that push receiver sensitivity well below what you'd get from a bare BLE chip — so there's meaningful headroom baked in. At the ranges you're talking about with a tethered balloon (presumably not pushing the 5km ground-to-ground ceiling), you're probably operating with enough margin that moderate polarization drift won't kill the link, but you might see RSSI swings of several dB as the balloon rotates.
Honestly the cleanest fix for your setup is to swap the balloon-side antenna for a circularly polarized one. A CP antenna only costs you about 3dB compared to a perfectly matched linear pair, but it completely eliminates the polarization-mismatch variable regardless of how the balloon tumbles — which on a tethered rig with wind-driven rotation is exactly what you want. Keep the ground station vertical, put a helical or patch CP antenna on the balloon side, and the link becomes much more predictable.
