Link Budget Failure on a 5.8 GHz Point-to-Point Link
A 5.8 GHz point-to-point link spans 3.2 km between two rooftops. Each end uses a 23 dBi dish and a radio with 20 dBm transmit power and a receiver sensitivity of -85 dBm at the required data rate. The installer measured -78 dBm at the far end during a clear day. After two weeks, throughput drops and the received level reads -88 dBm. There is a new line of sight obstruction: a building under construction is partially in the Fresnel zone. Cable loss is 1.5 dB per end; connector loss is 0.5 dB per end.
1. Compute the expected free-space path loss and the predicted received level, then compare with the measured values.
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FSPL(dB) = 32.45 + 20 log10(f_MHz) + 20 log10(d_km) = 32.45 + 20 log10(5800) + 20 log10(3.2). 20 log10(5800) = 75.27; 20 log10(3.2) = 10.10; FSPL = 117.8 dB. Received power = 20 dBm + 23 dBi + 23 dBi - 117.8 - 1.5 - 1.5 - 0.5 - 0.5 = -55.8 dBm. That is far better than the measured -78 dBm, indicating about 22 dB of excess loss even before the obstruction, likely misalignment, obstruction, or a damaged feeder. After the building appears, -88 dBm is 32 dB below prediction and 3 dB below the -85 dBm sensitivity, explaining the throughput drop.
2. What role does the Fresnel zone play, and how would you quantify the obstruction?
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The first Fresnel zone radius at midpoint is r = 17.32 x sqrt((d1 x d2)/(f_GHz x D)) with distances in km and D in km. At midpoint d1 = d2 = 1.6 km, f = 5.8 GHz, D = 3.2 km: r = 17.32 x sqrt((1.6 x 1.6)/(5.8 x 3.2)) = 17.32 x sqrt(2.56/18.56) = 17.32 x 0.371 = 6.4 m. To keep at least 60 percent clearance, the path must clear about 3.9 m at midpoint. If the new building intrudes into that radius, diffraction loss of many dB is expected. Survey the profile and compute clearance at the obstruction's actual position, not just midpoint.
3. Recommend a remedy and justify the trade-offs.
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Options: raise one or both antennas to restore 60 percent Fresnel clearance, relocate to route around the building, or increase gain/transmit power within regulatory EIRP limits. Raising antennas is preferred because it addresses diffraction directly; a 3 m raise may recover enough clearance if the obstruction is near one end. Increasing dish size from 23 dBi to, say, 29 dBi adds 12 dB of link margin (6 dB per end) and would bring -88 dBm to about -76 dBm, restoring margin, but costs and wind loading increase. Reducing the data rate lowers the sensitivity threshold and can restore service as an interim step. Verify final level against the -85 dBm threshold with at least 10 dB fade margin.