EIRP Limit on a 2.4 GHz Point-to-Point Link
A rural internet service provider holds an NTC permit to operate a fixed wireless access network in the 2400–2483.5 MHz band. The permit condition states that any outdoor point-to-point link shall not exceed 100 mW equivalent isotropically radiated power (EIRP) unless a separate authorization is obtained. The provider's engineer proposes a link using a 250 mW radio, 1.5 dB of loss from 10 m of coaxial cable and connectors, and an 8 dBi flat-panel antenna. The site is 8.2 km from the access point. The installer has already mounted the antenna and wants to 'turn it on and file the paperwork after the first billing cycle.' The engineer's draft commissioning report states 'EIRP = 250 mW + 8 dBi = 1.58 W, within the 4 W allowed for the ISM band'-a line copied from a foreign vendor's application note. The operations manager argues that the 100 mW condition is only a guideline and that no one will notice a few watts at 2.4 GHz.
1. Calculate the actual EIRP in dBm and in mW. Is the link compliant? Show each step.
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Convert the radio power to dBm: 250 mW is 10 log10(250) = 23.98 dBm, approximately 24.0 dBm. Add the antenna gain and subtract the cable/connector loss: EIRP = 24.0 + 8.0 − 1.5 = 30.5 dBm. Convert back to mW: 10^(30.5/10) = 1122 mW, approximately 1.12 W. The permit limit is 100 mW, which is 20 dBm. The link is therefore over the limit by 10.5 dB, or about a factor of 11 in power. It is not compliant even though it is below the 4 W figure mentioned in the draft. The draft's 1.58 W is the uncorrected result of 24 + 8 = 32 dBm, which ignores the 1.5 dB cable loss; it is also written as though mW and dBi could be added directly. The actual EIRP is 1.12 W, still far above 100 mW.
2. Evaluate the draft report statement and the operations manager's argument.
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The draft statement is wrong in method and misleading in effect. You cannot add milliwatts and dBi; the calculation must be done in dBm and dB. The 1.58 W figure omits the cable and connector loss, and the '4 W allowed for the ISM band' is not the controlling limit for this station. The permit condition is the authorization under which the station may operate; exceeding it is unauthorized operation, regardless of what a foreign vendor note says. The operations manager's claim that the 100 mW condition is 'only a guideline' is incorrect and risky. Operating above the authorized EIRP can lead to NTC enforcement, shutdown, or penalties, and it may cause interference to other users. If the engineer signs a commissioning report that conceals the exceedance, he also exposes himself to professional liability for a false or misleading certification. 'No one will notice' is not an engineering or regulatory basis for compliance.
3. What should the engineer do before the link is energized, and what design change would bring it within the permit condition while preserving the 8.2 km path as far as possible?
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The link should not be energized as designed. The engineer should stop the installation from being turned on, correct the commissioning report, and notify the provider in writing that the design exceeds the permit condition. To comply with the existing permit, the radio transmit power must satisfy P ≤ 20 − 8 + 1.5 = 13.5 dBm, which is about 22 mW. Options include reducing the radio power to 13.5 dBm or less, using a lower-gain antenna, or changing the cable loss and then recalculating. Reducing power to 22 mW may not close an 8.2 km path with adequate fade margin, so the engineer should redo the link budget using the receiver sensitivity and required margin. If the compliant configuration cannot close the path, the correct route is to request a separate authorization or modify the permit before operation, not to exceed the limit. The final report should show the corrected EIRP calculation and the authorization basis.