Is the noisy spectrum from 20MHz to 330 MHz normal?

Question

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.

Average with sample size of 10

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:

1. 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?

2. 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?

3. 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?

4. 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.

Answer 1

Is this expected?

This is actually better than many people would expect, yes, assuming the floor at ca -130 dBm/300 Hz ~= -153.5 dBm/Hz is mostly thermal noise + spectrum analyzer noise temperature.

You basically have very narrowband interferers, and their power is very low.

From the perspective of a receiver, this polluted spectrum is going to be seen as noise

Well, the maximum hold figure suggests that, as said, you're mostly dealing with narrowband interferers, which also are relatively low duty cycle. Any reasonable communications system would be designed to deal with these.

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).

Both, of course. As said, this interference scenario is better than I would have expected, so thermal noise seems on average to be the dominant source of error energy.

But I could not find any reference to this consideration made by any link budgeting text online. Why is that so?

I can only guess, but you call "noise" what I would call "a narrowband interferer". Of course, interferers are very prominent in systems design literature, spectrum regulation and undergrad lectures.

Shouldn't the greater of the noise floors (environmental vs. internal/thermal) be used for link margin calculations?

most systems can be designed such that narrowband interferers don't matter much, as long as they don't contribute more energy than thermal noise (e.g., through multicarrier systems, frequency agility, spread spectrum…). Same for short-term phenomena (through interleaving, channel coding,…).

So, no, the maximum PSD doesn't matter as much as the average. That's what limits your channel capacity.

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?

see above.

Finally, is it possible that this excessive pollution

nothing excessive about these two figures. You're having a good day, spectrum-wise!

is not caused by man-made sources, but some mistake of interpretation or test setup on my part? What could have I done wrong?

The fact that these are narrowband phenomena suggest they are man-made. If you care about the spectrum as estimated by a scanning spectrum analyzer, there's little reason to mistrust your measurement. But it seems you're not really sure what kind of measurements you need – and we honestly can't help with that without knowing what kind of receiver you actually are thinking about. Interference has specific effects on specific receivers, and we can't generalize this.

Answer 2

The 0-300 MHz spectrum is full of strong signals, but it should be at least half empty, meaning that in many places an instrument with excellent selectivity would not see anything above thermal/atmospheric noise.

I suggest taking a closer look at an interesting band, perhaps 86-110 MHz, to look at the FM broadcast signals.

Consider the following instrument set-up comments too:

1. By setting the reference level to -40 dBm, and deliberately turning off the attenuator, the spectrum analyser will be quite susceptable to overload from strong signals. As your band (and antenna) includes the FM broadcast, you should expect some chaos from that. Attenuator 0dB is more for things like noise figure measurements, and bench experiments, or for antennas with a pre-selector filter.

---

2. It seems to be set up for an extremely fast sweep, for the bandwidth you show. Have you forced the sweep time down on your spectrum analyser, or reduced the number of points? For a 300 Hz RBW, to scan over 300 MHz, you should have about 1 million points, which will take about an hour to sweep. Some background reading here and much more here.
   If you force it to sweep faster, then at best it will just be sampling occasionally across the band, with unpredictable results as to whether it will detect something at all, and the amplitude it displays.
   So rather start with an appropriate sampling for your RBW and span - in this case, use a 1 MHz RBW, or reduce the span and look at separate bands.

---

3. It's not safe to look at the trace value and compare that to the expected thermal noise power density. First of all, you are using the Auto Peak detector, which will tend to over-read on noise. But even with an RMS detector, there are a lot of subtle points about reading noise on the trace, which will give you the wrong answer.

The correct way of reading noise and noise-like signals on a spectrum analyser is by using the marker noise feature, which compensates for everything for you automatically.

I recommend reading Agilent application note 1303 Spectrum Analyzer Measurements and Noise: Measuring Noise and Noise-like Digital Communications Signals with a Spectrum Analyzer

This application note has some great background to noise measurement in different bandwidths. It's aimed at spectrum analysers but many of the principles are useful to other measurement systems.

Found all over the web, but not on the official site as far as I can see. Here is one copy.