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One of the limiting factors in this exercise will be the overall noise figure (NF) of your detection system. NF is the additional noise over theoretical minimum that the receiving system adds to the detection process.
Note NF is independent of detection bandwidth.
A spectrum analyser, especially older ones has a typical noise figure of 25 dB, hence a Low Noise Pre-amplifier (LNA) will be absolutely essential. The overall noise figure of a detection system is determined by the NF of the first amplifier in the chain (not the gain of that amplifier). You will be looking for an overall noise figure of less than 3dB. Problem with LNAs are that while being very sensitive, they can get easily 'swamped' by signals that are too large. The swamping signals are most often not the ones you are searching for. The resulting intermodulation caused by the overdriving of your LNA can produce spurious and misleading results. Hence it is important to limit the bandpass right at the front-end to be as narrow as possible around the 1421 MHzyou are interested in.
My expertise runs out about there, but I'm guessing that the limiting of bandpass is carried out in the feed horn or waveguide in front of the LNA.
Good Luck.
Brian C
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