Practical_Antenna_Handbook_0071639586

492 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
Degrees kelvin (K) is the international way of defining all temperatures relative to
absolute zero. (No degree symbol is used with K.) To express temperature in degrees
kelvin (K) we add 273.16 to a temperature expressed in Celsius. The formula is
T(K) = T( ° C) + 273.16
(21.2)
Of course, temperatures expressed in Fahrenheit (°F) are related to Celsius by
so we also have
T(Fahrenheit)
= 9
T(Celsius) 32
5 + (21.3)
T(K)
= 5
T( F) 255.38
9 ° + (21.4)
Thus, water turns to ice at 32°F, 0°C, or 273.16 K. Water boils at 212°F, 100°C, or
373.15 K. Absolute zero corresponds to –459.67°F, –273.16°C, or 0 K.
Another important point on the various temperature scales is room temperature, typically
taken to be 27°C (or about 80°F) by some scientific and engineering specialties.
This may be higher than what you or I would want our room to be, but it has the advantage
of corresponding to 300 K—a nice round number for doing calculations!
Finally, by international agreement, T for terrestrial components and system elements
is assumed to be 290 K (about 17°C, or 62°F) unless otherwise stated. To improve
weak-signal detection at microwave frequencies and above, however, receiver input
stages are often cooled by liquid nitrogen. The markedly lower T of those stages results
in their contributing a substantially reduced noise power to the overall system noise
figure.
Example 21.1 A terrestrial receiver with a 1-MHz bandwidth and a 50-Ω input impedance
is connected to a 50-Ω resistor. The noise power delivered to the receiver input
stage is (1.38 × 10 –Â23 J/K) × (290 K) × (1,000,000 Hz) = 4 × 10 –15 W. This noise is called
thermal noise, thermal agitation noise, or Johnson noise.
Noise Factor, Noise Figure, and Noise Temperature
The noise performance of a receiving system can be defined in three different, but related,
ways: noise factor F n , noise figure (NF), and equivalent noise temperature T e ; these
properties are definable as a simple ratio, decibel ratio, or kelvin temperature, respectively.
Noise Factor (F n )
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