MultiNav Pro+ BLE - Fresnel zone clearance at 50km, does it actually matter?

Harold Brown•July 25, 2026

Been planning a point-to-point link across a fairly flat rural area, roughly 45-48km between two fixed nodes. I've done plenty of sub-1km BLE stuff before but this is my first time stretching to near the rated 50km range on the MultiNav Pro+ BLE module. Started running the numbers on Fresnel zone clearance and now I'm second-guessing my antenna mounting heights. At 2.4GHz over that kind of distance the first Fresnel zone radius gets pretty wide - I'm getting something like 17-18 meters at the midpoint. My mast situation isn't great on one end, maybe 4-5 meters above ground level, and the terrain is flat but there's some scrubby vegetation in between.

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Adam•July 25, 2026

@Harold Brown yeah your Fresnel zone math is right and honestly that 4-5 meter mast is going to hurt you. At 2.4GHz over ~47km you're looking at a first zone radius around 17m at the midpoint, and the rule of thumb is you want at least 60% of that clear — so roughly 10+ meters of clearance above any obstruction along the path. Scrubby vegetation isn't just a physical blockage either, it absorbs and scatters at 2.4GHz worse than a solid obstacle would because of the water content in the leaves, so even sparse brush at the wrong point in the path can quietly kill several dB you can't afford at that range.

The 50km spec on the MultiNav Pro+ is the drone-to-drone figure where both ends are elevated and ground reflections are basically eliminated — ground-to-ground with one end at 4-5m over flat terrain with vegetation is a meaningfully different situation. You'll likely still get a link, the module's receiver sensitivity gives you a lot of margin to work with, but you're going to be operating closer to the edge than the headline number implies.

If raising the short mast isn't an option, I'd look hard at whether you can pick a path that puts any mid-path vegetation at the edges of the link rather than near the midpoint — that's where the Fresnel zone is widest and where partial obstruction hurts the most. Even getting that mast to 8-10m would make a significant difference in what you actually get through.

Devon Miller•July 25, 2026

Yeah Fresnel zone clearance absolutely matters at that distance, probably more than anything else you'll tweak - at 2.4GHz over 45km you basically need your antennas high enough to clear that ~17-18m radius plus whatever ground clutter exists at the midpoint, otherwise you're eating serious diffraction loss no matter how good your link budget looks on paper. I'd model the actual terrain profile (even a rough one from something like HeyWhatsThat or Radio Mobile) before you commit to tower heights, because a small hill or tree line you didn't account for right at the midpoint can wreck you way worse than a few dB of path loss.

Peter Adkins•July 25, 2026

Devon nailed it - Fresnel clearance at that distance is non-negotiable, and 2.4GHz is pretty unforgiving compared to sub-GHz stuff you might've used for long links before. One thing I'd add is don't just look at midpoint, the zone bulges worst there but a terrain obstacle at like 1/3 or 2/3 of the path can still hit you hard depending on geometry. Definitely run Radio Mobile with an actual elevation profile before you pour any concrete.

Amanda Crawford•July 25, 2026

Both Devon and Peter are spot on - also worth noting that at 2.4GHz you're going to want to aim for at least 60% of that first Fresnel radius clear of any obstacles, not full clearance, but the 60% rule is where diffraction loss starts becoming actually manageable. With flat rural terrain you might get away with lower mast heights than you'd expect, but that "flat" assumption has burned me before when I didn't account for crops or seasonal vegetation at the midpoint adding a few meters of effective height.