Help interpreting frequency scan results


I ran a frequency scan using the instructions on this forum, and wonder if anyone could help interpreting the output? I’ve attached the output file here.

I have a 3dBi 1090mhz antenna (this one), fed in to a FA Filter and then an FA Pro Stick Plus (blue).

A few questions:

  • There seems to be a faint yellow hue across the entire spectrum, whereas other results I’ve seen have a pure black background. Is this indicative of background noise across the spectrum?
  • There is a strong signal at around 923mhz, which I’ve seen on other forums could be indicative of the mesh networking that smart meters use. I do have a smart meter in the home, so this is entirely plausible. Is that signal strong enough to saturate the receiver and cause problems with ADS-B reception?

Any help would be most appreciated!


Cell Phone/Pager signal between 920 Mhz and 960 Mhz. Indicates Filter is likely to benefit, but only by trying a filter you will know.



Thanks for the reply - to clarify, I’ve already fitted the FA filter, and this scan was taken after the filter was fitted.


You need a better filter such as ceramic, as FA filter does not effectively filter out frequencies so close to 1090 Mhz (such as 920 or 960 Mhz). It does however remove effectively the GSM850 (850 Mhz) Cell frequencies.

Please see this post which has graphs. The graphs “image 3 of 5” and “image 5 of 5” show this very clearly.
Also, scrolling up/down and reading other posts in this thread may be helpful.


I have several of these filters with the uputronics pre-amp/fitlers.

They work really well but are not cheap.
I also custom ordered some single pole cavity filters for 1090 and 978Mhz(Not used in the UK) for less. They ended up being about $US30 each.



Looks like I need to invest in a better filter then, to cut out that 923mhz noise.

What could be causing background noise across the range? Looking at other graphs on the linked threads, there is a constant yellow tint to mine which I believe is indication a level of background noise across the entire spectrum. I’ve attached an example, the top is taken from another thread and the bottom is the same range but from my scan.

Any ideas? Could that be causing an issue?



Possibilities I can think of are:

  • RF noise in the space around your antenna.
  • Poorly shielded antenna cable.
  • Noise generated in hardware (bad connectors, electronic chips).
  • Malfuntion of scanning/graphing software.

Please repeat the scan to see if this is a persistant phenomenon, or just a one-time occurance.


Thanks for the link, abcd567. I now have the blue stick with the FA filter. As per image 5 of 5, more work is needed. I received the RTL-SDR Blog LNA/filter combo. I’m now waiting for the Bias T interface. I’ll add it to the existing configuration and see what happens.

Any source of affordable ceramic filters?

I posted a scan a few days ago. It shows a big hump +/- 25 MHz around 1090 GHz. Any ideas what that is or what could cause it, if not RF?

Also, completed a series of tests to determine best gain settings, and settled on 25.x dB.



I think obj once wrote that you cannot compare the colors between those scans, it is not an absolute scale, it just shows the differences between strong and week signals in one plot. I guess this means that you do not receive strong signals like a cellphone tower, therefore the scan has no black areas and yellow peaks, just shades of yellow blackischness…


Ah, maybe that would explain it. Can anyone confirm?

Third scan just now shows exactly the same thing:



You can see the relative strengths (dB) of Cell/Mobile and ADSB signals as well as strength of noise if you do the scan on Windows using software “rtlplan.exe”. This method plots the heatmap like RPi, as well as a graph showing relative strengths of sinals in dB. This method is given in OPTION-2 of the thread "Do I Need A Filter?"

Here is scan at my location, done with DVB-T plugged into Windows Computer