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• Update 1.0.3 20.01.09.

# Overview

FSC (Field Strength Calculator) is a simple application for calculating radio frequency propagation using the Friis transmission equation. Given two antennas in free space, the ratio of power received by the receiving antenna, Pr, to power input to the transmitting antenna, Pt, is given by where Gt and Gr are the transmitter and receiver antenna gains, λ is wavelength and R is the distance.

Underlying assumptions are:

• antennas are in unobstructed free space, with no multi-path;
• Pr is the available power at the receive antenna terminals;
• Pt is the power delivered to the transmit antenna;
• antennas are correctly aligned and polarised alike;
• bandwidth is sufficiently narrow.

FSC assumes a receiver input impedance of 50+j0Ω in relation to receiver Antenna Factors if specified.

The Friis transmission equation assumes propagation loss to be inversely proportional to the square of distance. For some applications, propagation may be expressed with a different distance exponent, for example some FCC field strength interpolations / extrapolations specify factors of 30log(d1/d2) which is equivalent to a distance exponent of 3. FSC supports distance exponents other than two, and the modified Friis equation is: Pr/Pt=GtGr(λ/(4πR^E))^2 where E is the distance exponent.

(Note that some other methods of dealing with propagation in the near field region use an exponent other than two applied to the whole term. FSC applies it only to the R term.) Fig 1 shows a screen shot of FSC.

The method used is consistent with ITU-R P.525-2 CALCULATION OF FREE-SPACE ATTENUATION.