47 C.F.R. § 101.143
(a) The distance between end points of a fixed link in the private operational fixed point-to-point and the common carrier fixed point-to-point microwave services must equal or exceed the value set forth in the table below or the EIRP must be reduced in accordance with the equation set forth below:
| Frequency band (MHz) | Minimum path length (km) |
|---|---|
| Below 1,850 | N/A |
| 1,850 to 7,125 | 17 |
| 10,550 to 13,250 | 5 |
| Above 17,700 | N/A |
(b) For paths shorter than those specified in the table in paragraph (a) of this section, the EIRP shall not exceed the value derived from the following equation:
EIRP = MAXEIRP-40*log(A/B) dBW
Where: EIRP = The new maximum EIRP (equivalent isotropically radiated power) in dBW. MAXEIRP = Maximum EIRP as set forth in the Table in Section 101.113(a). A = Minimum path length from the Table above for the frequency band in kilometers. B = The actual path length in kilometers.
Note to paragraph (b): For transmitters using Automatic Transmitter Power Control, EIRP corresponds to the maximum transmitter power available, not the coordinated transmit power or the nominal transmit power.
Note to paragraph (c): Links authorized prior to April 1, 1987, need not comply with this requirement.
[61 FR 26677, May 28, 1996, as amended at 65 FR 38330, June 20, 2000]