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Your question relates to radiometry. Best to demonstrate with a microwave antenna and receiver.
The "smAllest" signal is in fact noise of an object (or medium) to which the antenna is pointed by its single main lobe.
To define the output noise temperature of an antenna, one needs to know three important quantities:
1. the effective temperature of a distant body (like e.g. the Sun) : T = To x E,
where T is the effective temperature and To the body temperature, both in Kelvins. E is body surface emissivity, 0<E<1,
2. the solid viewing angle of the body from the observation location. The Sun diameter seen from Earth is 0.5 degree, so the solid viewing angle is 0.5 x 0.5 = 0.25 in square degrees.
3. the solid angle of the antenna main lobe. If you use a 30-cm parabolic dish at 12 GHz, the beam width is ~ 5 degrees, so the solid angle will be ~25 degrees squared.
By a simple geometrical projection we get for the case of the Sun : T= 10 000 K at 12 GHz, the ratio of the above solid angles is 0.25 / 25 = 0.01 The output noise temperature of our antenna pointed to the Sun will then be
Ta = T x 0.01 = 100 K,
while the background temperature should be ~ 3 Kelvins of the galactic background. In fact, the side lobes of our small antenna will add some 50 Kelvins due to the Earth temperature, ~290 K, projected through those side lobes to antenna output. So by scanning the Sun, the antenna output temperature will vary between 50 K (minimum) to ~150 K (maximum).
If you have a satellite TV receiver, you can try this experiment, only take care no satellite interferes. Try this at noon when the Sun is "high" above the satellites in orbit.
Hi jiripolivka..thnx for taking time and answering...
i think my question was not clear..
i have designed a helical antenna array (normal mode helix antenna) will use it as a near-field EMC scanner ..so these helix antennas will measure the electromagnetic emissions from PCbs or other devices ..i need to know what is the sensitivity of my scanner ...it means the sensitivity of the helix antennas.. how to proceed ...
thnx
Hi jiripolivka..thnx for taking time and answering...
i think my question was not clear..
i have designed a helical antenna array (normal mode helix antenna) will use it as a near-field EMC scanner ..so these helix antennas will measure the electromagnetic emissions from PCbs or other devices ..i need to know what is the sensitivity of my scanner ...it means the sensitivity of the helix antennas.. how to proceed ...
thnx
Your question is clearer now. My explanation can be used to define the minimum physical noise temperature at the output of any antenna including yours.
Your question is not fully correct: the sensitivity is a feature of your RECEIVING SYSTEM consisting of the antenna AND receiver (or scanner if you wish to call it so). You should know the important parameters of your receiver: its noise figure, noise bandwidth before detector, and possibly its gain before detector, and detector response.
The smallest received signal is called the "noise floor". The noise power at receiver input is defined by its thermal noise power, Pn = kTBN, where k is the Bolzmann constant, T is the noise temperature (remember what I explained about the antenna?) and N is receiver noise factor. B is the noise bandwidth before detector in Hertz.
A simpler estimation can be found in dBm from Pn = -174 + 10 log B + F,
where -174 dBm comes from the log (kT) and the ambient temperature, ~290 K, B again the bandwidth in Hz and F is 10 logN, the noise figure.
All this is referred to receiver INPUT. You must add the gain from input to detector, and then know your detector constant, typically 1500-2000 mV/mW.
From the above you can estimate the receiver noise floor, below which no signal can be detected.
The "signal" range can start at the noise floor, and end up where the detector is saturated.
So to conclude, it is not correct to ask about "antenna sensitivity" but the "system sensitivity".
Like in photography, there is no sensitivity of a camera lens but the system consisting of a lens (antenna) and film or CCD sensor (receiver).
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