Practical_Antenna_Handbook_0071639586

CHAPTER 21
Antenna Noise Temperature
Radio reception is essentially a matter of signal-to-noise ratio (SNR). Signals must
be at or above some amplitude relative to the noise floor of the system in order
to be detected properly for their intended use. All electronic systems (receivers
and antennas included) generate noise internally, even if there is no power flowing in
them. While we normally think in terms of the antenna designer maximizing signal
strength to overcome noise, often a second goal is to minimize the noise from some or
all of the various sources.
In general, at HF and below, the external signals and noise sources detected by the
antennas described in this book (including even the very low efficiency antennas of
Chap. 14) are strong enough to swamp any noise generated within a well-designed receiver.
Thus, we typically have to take special measures to override internally generated
receiver noise only at VHF and above, and the balance of this chapter is primarily for the
benefit of those who are concerned with weak signal reception above 30 MHz or so.
One of the basic forms of noise seen in systems is thermal noise. Even if the amplifiers
in the receiver add no additional noise (they will!), there is thermal noise at the
input. In fact, if we replace the antenna attached to the receiver input with a totally
shielded resistor matched to the system impedance, some noise will still be present.
This noise is produced by the random motion of electrons inside the resistor. At all temperatures
above absolute zero (about –273.16°C), the electrons in the resistor material
are in random motion. In the absence of an external bias voltage creating a uniform field
acting on the resistor body, the short-term random motions of the electrons cancel each
other out to the extent that no discernable current can be observed.
Thermal noise in a resistor can be modeled as a voltage source, V¯N, in series with a
Ânoise-Âfree resistor R, where V¯N is the root mean square (rms) value of the fluctuating,
thermally generated noise voltage. If ÂR—Âin Âohms—Âis constant over the frequency range
of interest, V¯N is proportional to kTBR, whose components are defined below. When
the resistor is connected across a matched load, the noise power transferred to that load
is given by Eq. (21.1). Note that the noise power delivered to a matched load is independent
of the value of the resistor.
P
N
= kTB
(21.1)
where P N = noise power, in watts
k = Boltzmann’s constant (1.38 × 10 –23 joules/K)
T = temperature, in degrees kelvin (K)
B = bandwidth, in hertz
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