I have recently been testing some VHF/UHF antennas for reception in my Lab, and have noted something I could not explain. Following are the spectrum plots captured on our R&S FSV spectrum analyzer, with some antennas (monopole and BiConical):
Green Plot for BiConnical, black for Monopole, blue plot taken with 50 ohms termination as reference noise floor Max Hold Plot; This trend of spectral pollution is reliably repeated at different takes, within 10 seconds.
From the max hold plot, it seems that almost the entire band is severely polluted. There are sharp tone like transmissions at many places, and then there is a general rise in noise floor for the first 200 MHz. My questions are:
Is this expected? From the perspective of a receiver, this polluted spectrum is going to be seen as noise. Which will adversely effect the receiver SNR. How do radio operators handle this situation in this band?
If it is expected, the link margin in a link-budget calculation should be determined by this environmental noise, not by the receiver's Thermal Noise floor (kTB + NF). But I could not find any reference to this consideration made by any link budgeting text online. Why is that so? Shouldn't the greater of the noise floors (environmental vs. internal/thermal) be used for link margin calculations?
The average plot is a lot cleaner, and actually meets the Analyzer noise floor. From a narrowband receiver's perspective, which of the two plots is more meaningful? Why?
Finally, is it possible that this excessive pollution is not caused by man-made sources, but some mistake of interpretation or test setup on my part? What could have I done wrong?
Any help, along with any resources to learn more about these things would be deeply appreciated. Thanks.