Why do radio astronomers use temperature rather than dB or another measurement to measure noise?

Radio astronomers use temperature because the random electrical noise generated by a resistor is directly proportional to its physical temperature. This creates a natural link between thermal physics and radio noise.

The fundamental relation is:

P = kTB

where:

• P = noise power (watts)
• k = Boltzmann’s constant
• T = temperature (kelvin)
• B = bandwidth (Hz)

Because of this relationship, any noise source can be described by the temperature of an ideal resistor that would produce the same amount of noise. This is called the noise temperature.

Why not dB?

A dB value is only a ratio. For example:

• 3 dB means a factor of 2
• 10 dB means a factor of 10

but it does not tell you how much actual noise power exists unless you know the reference level. A temperature in kelvin has a direct physical meaning.

Advantages of temperature

1. AdditiveNoise temperatures add directly:T_total = T_sky + T_feed + T_cable + T_receiverwhereas dB values must be converted back to linear units before combining.
2. Independent of bandwidthThe noise power changes with bandwidth, but the noise temperature does not. A source with a temperature of 100 K remains 100 K whether observed with a 1 kHz or 1 MHz receiver.
3. Physical interpretationA receiver noise temperature of 50 K means it produces the same noise as a perfect 50 K resistor connected to its input.
4. Connects naturally to astrophysicsAstronomical sources are often described by their brightness temperature, the temperature a blackbody would need to have to emit the same radio intensity.

Example: Your 1420 MHz observations

Suppose your system has:

• Sky background: 20 K
• Feed losses: 10 K
• Receiver: 60 K

Then:

T_system = 20 + 10 + 60 = 90 K

If the hydrogen line increases the antenna temperature by 1 K, the signal is immediately seen as a 1/90 (~1.1%) increase.

Expressing all of these contributions in dB would make the analysis much less intuitive.

Historical reason

Early radio astronomers such as Karl Jansky and Grote Reber were measuring weak thermal radiation from the sky. Since the radiation obeys the same statistical laws as heat, temperature became the natural unit and remains the standard today.

In practice, radio astronomers often use:

• Kelvin (K) for receiver and sky noise.
• Jansky (Jy) for source flux density (1 Jy = 10^-26 W m^-2 Hz^-1).
• dB mainly for gains, losses, attenuator settings, and amplifier specifications.

So temperature is preferred because it is a physically meaningful, additive measure of noise that follows directly from the fundamental physics of thermal radiation.