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Friis formula or Friis's formula (sometimes Friis' formula), named after Danish-American electrical engineer Harald T. Friis, is either of two formulas used in telecommunications engineering to calculate the signal-to-noise ratio of a multistage amplifier. One relates to noise factor while the other relates to noise temperature.
Friis' original idea behind his transmission formula was to dispense with the usage of directivity or gain when describing antenna performance. In their place is the descriptor of antenna capture area as one of two important parts of the transmission formula that characterizes the behavior of a free-space radio circuit.
The free-space path loss (FSPL) formula derives from the Friis transmission formula. [3] This states that in a radio system consisting of a transmitting antenna transmitting radio waves to a receiving antenna, the ratio of radio wave power received P r {\displaystyle P_{r}} to the power transmitted P t {\displaystyle P_{t}} is:
The noise power from a simple load is equal to kTB, where k is the Boltzmann constant, T is the absolute temperature of the load (for example a resistor), and B is the measurement bandwidth. This makes the noise figure a useful figure of merit for terrestrial systems, where the antenna effective temperature is usually near the standard 290 K ...
In its simplest form, the path loss can be calculated using the formula L = 10 n log 10 ( d ) + C {\displaystyle L=10n\log _{10}(d)+C} where L {\displaystyle L} is the path loss in decibels, n {\displaystyle n} is the path loss exponent, d {\displaystyle d} is the distance between the transmitter and the receiver, usually measured in meters ...
The noise factor (a linear term) is more often expressed as the noise figure (in decibels) using the conversion: = The noise figure can also be seen as the decrease in signal-to-noise ratio (SNR) caused by passing a signal through a system if the original signal had a noise temperature of 290 K. This is a common way of expressing the noise ...
Harald Trap Friis (22 February 1893 – 15 June 1976), who published as H. T. Friis, was a Danish-American radio engineer whose work at Bell Laboratories included pioneering contributions to radio propagation, radio astronomy, and radar. [1]
There are two formulas or equations named after Danish-American radio engineer Harald T. Friis. Friis formulas for noise; Friis transmission equation
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