Point-to-Point 5 GHz Link EIRP and DFS Compliance
A rural ISP plans 20 point-to-point links over 18 km using 5 GHz licence-exempt radios. The radio's maximum conducted output is 23 dBm; the selected antenna has 14 dBi gain and 1.5 dB cable/connector loss. The regulator's rules state: 5.47–5.725 GHz has a 30 dBm EIRP limit and requires DFS; 5.725–5.85 GHz has a 36 dBm EIRP limit and no DFS; conducted power must not exceed 23 dBm in either band. The link budget requires 34 dBm EIRP at both ends to maintain 99.95% availability. The vendor shipped radios preset to 23 dBm and antennas aimed at the lower band. A competitor's installer says DFS is optional if the link is fixed and licensed. The ISP wants to avoid a 3 dB fade margin shortfall and regulatory trouble.
1. Calculate the EIRP, compare it with both bands, and determine the compliant options.
Read the worked answer
EIRP = conducted power + antenna gain - cable/connector loss = 23 + 14 - 1.5 = 35.5 dBm. In 5.47–5.725 GHz the limit is 30 dBm, so the configuration exceeds it by 5.5 dB and DFS is required. In 5.725–5.85 GHz the limit is 36 dBm, so 35.5 dBm is compliant with 0.5 dB to spare; conducted power is exactly at the 23 dBm maximum. To use the lower band, the radio would have to be reduced to 17.5 dBm to produce 30 dBm EIRP, which is 4 dB below the 34 dBm link-budget requirement. The competitor is wrong: DFS is not optional for the lower band merely because the link is fixed. The upper band is the only configuration that meets both the EIRP limit and the stated link budget.
2. Recommend a configuration and explain the effect on link margin and reliability.
Read the worked answer
Configure the upper band at 23 dBm conducted with the 14 dBi antenna, giving 35.5 dBm EIRP and a 1.5 dB margin over the 34 dBm requirement. Lock the power setting so field replacement radios cannot default higher. If more receive gain is needed, use a larger antenna but reduce conducted power to keep EIRP at or below 36 dBm; a 17 dBi antenna with 20.5 dBm conducted and 1.5 dB loss gives exactly 36 dBm and improves receive sensitivity. Do not solve a fade-margin shortfall by raising conducted power above 23 dBm or EIRP above 36 dBm. Recalculate both ends and verify that the availability target is met with the actual antenna and cable losses.
3. What documentation and field checks should accompany deployment?
Read the worked answer
Keep type-approval evidence, antenna gain specifications, cable loss measurements, and a per-link EIRP calculation. Record the band, conducted power, antenna model, and whether DFS is required. Lock configuration and verify with a power meter or radio diagnostic that output is as documented. For the upper band, confirm that no DFS is required and that emissions stay within the 36 dBm EIRP limit. Monitor interference and availability after turn-up. If a link is later moved to the lower band, redo the calculation, enable DFS, and reduce power. Retain records for regulator inspection and for change control.