If you decide to answer, PLEASE DO NOT JUST ANSWER ONE QUESTION AND LEAVE IT LIK

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If you decide to answer, PLEASE DO NOT JUST ANSWER ONE QUESTION AND LEAVE IT LIKE THAT! I won't give you the rating for points. I really do appreciate your help if you do decide, but please don't try to just answer one and get the whole points. Can somebody please help me answer these questions? I've been trying to do them, but I can' get the right answers. Thank you very much if you decide to help me. A) Assume that a transmitter on Earth must communicate with a spacecraft on the Moon. Parameters are: Earth-based atenna gain=40dB Lunar receiving atenna gain=20dB Distance=386,000 Km Frequency= 3GHz Minimum lunar signal level = 100fW What is the power required for the Earth transmitter? B) Since the lunar atenna in A) is pointed at earth, it will "see" a temperature of about 290 K. The effective noise temperature of the lunar receiver referred to its input is 400 K and the IF bandwidth is 100 kHZ. Neglecting the losses., determine the system carrier-to-noise ratio (in dB) at the lunar receiver output. C) A pulse radar system has a pulse repetition rate of 50 Hz and a pulse duration of 100 uS. What is the maximum unambiguous range (in km)? D) For the radar system in C), determine the minimum measurable range E) The effective area of a certain antenna at 300 MHz is 1m^2. What is the absolute gain? F) What is the detected output S/N Ratio (in dB) for an AM receiver with 100% modulation?

Explanation / Answer

Noise figure is the logarithmic ratio of the output of a noiseless receiver to the output of the receiver under test. Both are connected to a matched input resistor at a standard temperature of 290K. Thus the ideal receiver represents the noise from the resistor only, and the real receiver represents the resistor + receiver noise. This comes down to:

F = noise factor = (Sin/Nin)/(Sout/Nout)

The receiver noise factor (F) is equal to the ratio of the SNR at the input to the SNR at the output. The noise across the resistor at a known temperature (290K) can be calculated from kTB to come up with absolute power levels. This amounts to -174dBm/Hz for 290K.

The noise figure (NF) is the logarithmic ratio, so 10log(F). This is also obtained by:
NF = SNR_input_dB - SNR_output_dB
as subtracting dB is the same as division.

The receiver noise can be related to an equivalent noise temperature at the input = Tn = the equivalent temperature the resistor would be to give the same noise as the receiver. This is derived from noise factor as:
F = 1 + Tn/290.
Transposing:
Tn = 290(F-1). Note F = noise factor, not noise figure.
Using the dB ratio noise figure:
F is antilog(NF/10) so:
Tn = 290 * (10^(NF/10)-1)

When this includes the antenna it is called system temperature. The antenna pointed through the air along the ground will see the earth temperature of ~290K so this will mask out the receiver noise unless it is a noisy system, so system temperature usually applies to antennas pointed at space. In a system I worked with, the system temperature was 33K, so about 0.5dB noise figure, mostly due to losses in the antenna feedhorn, diplexer and filters before the low noise amplifier portion. Note that a 3db noise figure is a noise factor of 2 and close to an equivalent noise temperature of 290K. Microwave receivers with no amplifier before the diode mixers are something like 10-12dB noise figure.

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