1.2 km Warehouse Bridge: EIRP, Fresnel Zone, and a Bad Antenna Swap
A regional logistics company needs a 5 GHz point-to-point bridge between two warehouses 1.2 km apart; fiber is not available. The integrator mounts a 23 dBi dish on each building at 6 m above grade, feeds it with 8 m of LMR-400 (3.5 dB loss at 5.8 GHz), and sets the radio to 20 dBm. The site's regulatory profile allows a maximum of 36 dBm EIRP. A tree line stands 40 m from Warehouse A, with the canopy top at 9 m. After alignment, the link reports -78 dBm RSSI, 15 dB SNR, and roughly 8 Mbps throughput with heavy retries. The integrator proposes swapping in 30 dBi dishes to "push more signal through the trees." The customer also asks whether the current installation is legal and whether rain will make the link worse.
1. Calculate the current EIRP at each radio and the first Fresnel zone radius at the midpoint of the 1.2 km path at 5.8 GHz. Is the proposed 30 dBi swap a valid fix?
Read the worked answer
EIRP = radio power - cable loss + antenna gain = 20 - 3.5 + 23 = 39.5 dBm. The site limit is 36 dBm, so the link is 3.5 dB over the limit today; the radio must drop to 16.5 dBm (or use lower-loss cable / lower-gain antenna) to comply.
Fresnel radius: F1 = 17.32 x sqrt(d1 x d2 / (f x (d1 + d2))), with distances in km and f in GHz. At midpath, d1 = d2 = 0.6 km: F1 = 17.32 x sqrt(0.36 / (5.8 x 1.2)) = 17.32 x sqrt(0.0517) = 3.94 m. About 60% clearance (2.4 m) is the practical minimum.
The 30 dBi swap is not a valid fix. It raises EIRP to 20 - 3.5 + 30 = 46.5 dBm, further out of compliance, and it narrows the beam, making alignment harder. More gain does not clear an obstruction: at the tree line (40 m from A), F1 is only about 1.4 m, and the canopy sits 3 m above the line of sight, blocking essentially the whole zone. The path must be raised or rerouted first; only then should power be set for legal EIRP.
2. Explain why the measured RSSI is so much worse than a free-space prediction, and list corrective steps in priority order.
Read the worked answer
Free-space path loss at 1.2 km and 5800 MHz is 32.44 + 20log(1.2) + 20log(5800) = 32.44 + 1.58 + 75.27 = 109.3 dB. Expected receive level = EIRP - FSPL + receive gain - receive cable loss = 39.5 - 109.3 + 23 - 3.5 = -50.3 dBm. The link reports -78 dBm, roughly 28 dB worse than free space. That gap is not distance; it is tree canopy intrusion into the Fresnel zone, causing absorption, scattering, and diffraction. The 15 dB SNR confirms a weak signal rather than heavy noise.
Priority order:
- Clear the path: raise both antennas on masts/towers so the canopy is well below 60% of F1 (or reroute the link).
- Re-align dishes carefully and verify both polarities match.
- Reduce radio power to 16.5 dBm to meet the 36 dBm EIRP limit.
- Re-measure RSSI, SNR, and MCS; expect SNR of 25 dB or better and a stable modulation scheme.
- Only then evaluate rain fade margin, since 5.8 GHz attenuation in heavy rain is modest compared with the 28 dB obstruction loss.