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I have a tracking station for UHF band satellites which comprises of a cross yagi of 12 dBi and a wideband LNA of 40 dB Power Gain and 1.3 dB Noise Figure. I observe a Noise floor of about 300 kHz centered at the frequency of 436 MHz on the spectrum analyzer. The floor power varies between -75 and -80 dBm, while the floor at other frequencies is -100 dBm.

I tried reducing the RBW and VBW of the analyzer, but the floor doesn't change. I tested both antenna and LNA separately and couldn't find the reason for this floor. Please help me.

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  • $\begingroup$ 20db is a large difference, but there's a chance that you are just seeing that the antenna doesn't have constant gain across the band. The higher noise floor could correspond to higher gain or better impedance match at certain frequencies. $\endgroup$ – user10489 Feb 19 at 22:46
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300 kHz channel at 436 MHz sounds a lot like your spectrum analyzer might be seeing that are at twice the frequency – 872 MHz would be a typical 2G cellular network frequency, and 300 kHz might be around the bandwidth of a 2G channel.

Try and tune to your SA to 872 MHz.

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Are you sure you're not picking up RFI from one of the electronic devices that's part of your station? Some power supplies have offset knobs you can adjust (the RFI will change frequency); SDRs frequently have similar adjustments to move their self-interference if it's in the way (as is common for budget SDRs). You'll have to hunt via process of elimination but I'' betting it's part of your station (including wall warts, inverters, etc.). Adjust or replace the noisy components; try "choking the hell out of" any cables (as an elmer once said to me) with an RFI choke or six.

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