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RADIO RECEPTION IS ESSENTIALLY A MATTER OF SIGNAL-TO-NOISE RATIO (SNR) SIGNALS must be some amplitude above the noise floor of the system in order to be received properly All electronic systems (receivers and antennas included) have inherent noise, even if there is no power flowing in them One of the goals of the antenna designer is to minimize the noise so that weak signals are not obscured One of the basic forms of noise seen in systems is the thermal noise Even if the amplifiers in the receiver add no additional noise (they will!), there will be thermal noise at the input If you replace the antenna with a resistor matched to the system impedance that is totally shielded, there will still be noise present The noise is produced by the random motion of electrons inside the resistor At all temperatures above absolute zero (about 27316 C) the electrons in the resistor material are in random motion At any given instant there will be a huge number of electrons in motion in all directions The reason why there is no discernible current flow in one direction is that the motions cancel each other out even over short time periods The noise power present in a resistor is: PN where PN is the noise power, in watts T is the temperature, in kelvins (K) K is Boltzmann s constant (138 10 B is the bandwidth, in hertz R is the resistance, in ohms KTBR [201]
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Note: By international agreement T is set to 290 K Consider a receiver with a 1-MHz bandwidth and an input resistance of 50 The noise power is (138 10 23 K) (290 K) (1,000,000 Hz) (50 ) 2 10 13 W This noise is called thermal noise, thermal agitation noise, or Johnson noise A resonant antenna can be modeled as an impedance consisting solely of a resistor with a value equal to the feedpoint impedance If an antenna has a 50- feedpoint
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418 Antenna noise temperature impedance, then it will generate exactly the same amount of thermal noise as a resistor of the same value
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The noise performance of a receiving system can be defined in three different, but related, ways: noise factor Fn, noise figure (NF), and equivalent noise temperature Te; these properties are definable as a simple ratio, decibel ratio, or kelvin temperature, respectively
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For components such as resistors, the noise factor is the ratio of the noise produced by a real resistor to the simple thermal noise of an ideal resistor The noise factor of a radio receiver (or any system) is the ratio of output noise power Pno to input noise power Pni: Pno Fn [202] Pni T 290 K In order to make comparisons easier, the noise factor is usually measured at the standard temperature (To) of 290 K (standardized room temperature), although in some countries 299 K or 300 K is commonly used (the differences are negligible) It is also possible to define noise factor Fn in terms of output and input signal-tonoise ratios: Sni [203] Fn Sno where Sni is the input signal-to-noise ratio Sno is the output signal-to-noise ratio
Noise figure (NF)
The noise figure is a frequently used measure of a receiver s goodness, or its departure from idealness Thus, it is a figure of merit The noise figure is the noise factor converted to decibel notation: NF where NF is the noise figure, in decibels Fn is the noise factor LOG refers to the system of base 10 logarithms 10 log Fn [204]
Noise temperature (Te)
The noise temperature is a means for specifying noise in terms of an equivalent temperature Evaluating the noise equations shows that the noise power is directly
Noise factor, noise figure, and noise temperature 419 proportional to temperature in kelvins, and also that noise power collapses to zero at the temperature of absolute zero (0 K) Note that the equivalent noise temperature Te is not the physical temperature of the amplifier, but rather a theoretical construct that is an equivalent temperature that produces that amount of noise power The noise temperature is related to the noise factor by Te and to noise figure by Te KTo log