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Estimation of antenna temperature

To estimate antenna temperature one should know the power at antenna terminals. For this, a simple experiment is carried out using the spectrum analyzer and Low noise RF amplifier. The low noise RF amplifier in front of spectrum analyzer reduces the noise temperature of the receiving system, and then with this the estimated system temperature is mainly conributed by antenna temperature.

Fig: - Antenna temperature measurement setup The RF amplifier used here is actually two cascaded stages of J310 amplifier used in jove receiver. The amplifier has gain of 18dB and noise figure of 3.71dB. Spectrum analyzer usually has high noise figure of the order of 20dB.

Let's assume the spectrum analyzer noise figure of 25dB. Then the overall receiving system (as shown in figure) will have the noise figure of 8.66dB. This will lead to noise temperature of 1840.1 Kelvin@ 290Kelvin. Thus the main cotributor to the system temperature in this setup will be the antenna itself!

It is observed that the Noise floor seen on spectrum analyzer is typically -94dBm.

So considering the 18dB gain of low noise amplifier in front of the spectrum analyzer,

power at antenna terminals = -94-18 = -112dBm = 6.3 x 10^-15 Watt.

Equating power to K*T*B we get,
Tsys= P/(K*B)= 0.152 x 10^6 Kelvin= 0.152 Million Kelvin.

Since the contribution to system temperature by the receiver is much less (~1800 Kelvin). so the system temperature can be approximated as antenna temperature at 20.1MHz.
Tant=0.152 Million Kelvin.

The antenna temperature of 0.152 Million Kelvin is high, typically galactic background contribution to antenna temperature can be 50000 Kelvin. The high temperature of 0.152 Million Kelvin suggest some local noise causing increase in temperature. With this temperature one can receive only strong bursts of Jupiter. As for 10^6 Jansky Jupiter bursts power at antenna terminals will be -115dBm, still 3dB less than the antenna noise!

The proper way to measure antenna tempearture is the use of the noise sourses. So the above method will give a rough estimate.