3.2 km Bridge Degraded After a Silo Is Built
Calder Vale Farms links its milking parlor (Building A) to a grain office (Building B) 3.2 km away with a 5.8 GHz point-to-point bridge. Each end uses a 20 dBm radio, a 24 dBi grid antenna, and 2 m of LMR-400 with about 1 dB total loss per end including connectors. Antennas sit at 18 m on A and 12 m on B. The bridge ran at 300 Mbps for two years, but after an 18 m grain silo was built 1.1 km from A, throughput fell to 20-40 Mbps and sometimes drops entirely in heavy rain. The installer re-aimed both antennas and measured -67 dBm RSSI, which the vendor calls acceptable because it is above the -70 dBm threshold. A spectrum sweep shows no other 5.8 GHz users. All gear is 802.11ac with 40 MHz channels.
1. Calculate the free-space received signal and compare it with the -67 dBm reading. What does the difference tell you?
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FSPL = 32.44 + 20log10(3.2) + 20log10(5800) = 32.44 + 10.10 + 75.27 = 117.8 dB. Each end has EIRP 20 - 1 + 24 = 43 dBm, so free-space received power is 43 + 43 - 117.8 = -31.8 dBm. The reported -67 dBm is 35 dB worse than free space. That is far too large to be misalignment or cable loss; it points to an obstruction in the path. A 24 dBi grid at 3.2 km should be near -32 dBm, and even a modest fade would leave it above -45 dBm. The -67 dBm reading is acceptable only against a receiver sensitivity threshold, not against the link's own baseline, and it explains the rate collapse: 40 MHz 802.11ac needs high SNR for 256-QAM, and -67 dBm on an obstructed channel forces low MCS.
2. Check the silo against line of sight and the first Fresnel zone. Show the geometry.
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At the silo (1.1 km from A), LOS height = 18 + (12 - 18)(1.1/3.2) = 18 - 2.06 = 15.94 m. First Fresnel radius r = 17.32 x sqrt(d1 x d2 / (f x d)) = 17.32 x sqrt(1.1 x 2.1 / (5.8 x 3.2)) = 17.32 x sqrt(2.31/18.56) = 6.11 m, so 60% clearance is 3.67 m. To keep the silo out of the 60% zone, LOS at that point must be at least 18 + 3.67 = 21.67 m. Actual LOS is 15.94 m, so the silo top is 2.06 m above LOS and 5.7 m above the 60% boundary; it blocks the path and cuts deeply into the first zone. That is consistent with 15-35 dB of diffraction and scattering loss and with the observed throughput collapse.
3. Recommend and justify remedies, including a rough antenna height if raising tower A is chosen.
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Check the rain outages separately first: at 5.8 GHz, rain attenuation over 3.2 km is well under 1 dB even in heavy rain, so full outages during rain point to water ingress in an N connector or a failing PoE injector, not rain fade. For the silo: (a) raise A. With B fixed at 12 m, height at the silo is 0.656H + 4.125; setting that to 21.67 m gives H = 26.7 m, so a 27 m tower or mast at A. Raising B to 20 m instead reduces the A requirement to about 22.5 m. (b) Route around the silo with a two-hop relay, for example a midpoint pole with clear LOS to both ends. (c) Relocate B's antenna higher on the grain office. Do not switch to 2.4 GHz: the Fresnel zone grows and the obstruction worsens. Re-aim and re-test with a link planner using actual terrain and silo coordinates.