Freespace loss calculator7/24/2023 (Note that microwave calculations are based on EIRP, or Effective Isotropic Radiated Power, which is more commonly used in microwave engineering.) TAP™ field strength calculations involving Effective Radiated Power (ERP) generally assume the ERP is computed based on transmitter antenna gain above a dipole (dBd). The TAP™ programs for field calculations (Broadcast, Carey, Bullington, Okumura, Longley-Rice) use antenna gain values referenced to a dipole (dBd), as is common in broadcast, land-mobile, and other industries. The equation referenced above is commonly used for computing free-space loss between isotropic antennas (see, for example, Engineering Considerations for Microwave Communications Systems, GTE Lenkurt, 1970, page 35 or Microwave System Path Design Considerations, TeleSciences, 1991, page 7, or Tech Note 101,equation 2.16). This is a case of both methods yielding the right answer, but expressed differently. You will note that if you subtract the free-space loss (in dB) from the transmitter ERP (in dBm) to get the receiver input power (in dBm) that when you use TAP™ to compute free space field, then use the Required Field utility to convert the field value (in dBu) to receiver input power (in dBm), the TAP™ value is about 4.3dB higher. Where D is the distance in miles and F is the frequency in MHz is commonly referenced. Added on: August 2, 2013, in: RF Engineering Articles To calculate free-space loss, the equation 36.6 + 20LogD + 20LogF,
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