Noisefloor of my KIWISDR

Comments

• G8JNJ

  September 2017 edited September 2017

  Hi Dietmar,

  
  

  That looks very good.

  
  

  The big test is to see if the antenna you are using is good enough for the KiWi :-)

  
  

  The ITU publish a noise / frequency curve (Rec. ITU-R P.372-8) https://www.itu.int/dms_pubrec/itu-r/rec/p/R-REC-P.372-8-200304-S!!PDF-E.pdf (figure 2) which shows the variation in natural noise floor with frequency for various locations. 

  
  

  As long as you have a receiver and antenna that is slightly more sensitive than the level shown on the graph (best case B <100KHz / >100KHz C <4MHz  / D >4MHz curves), then you don't need to worry about your receiver or antenna. 

  
  

  In most cases because of the shape of the curve, this means that you only really need to concentrate on the higher frequencies, as the lower frequencies tend to have a much higher level of natural noise, even in very electrically quiet locations.
  

  
  

  To try this for yourself, pick a suitable clear frequency, perhaps somewhere around 15 - 20MHz, where the natural noise floor is likely to be moderately low and the KiWi has good sensitivity.

  
  

  Put a 50 Ohm termination on the KiWi RF input.

  
  

  Zoom to level 12 (Z13 has some problems at low signal levels) and increase the WF min sensitivity slider until you can see the KiWi noise floor on the spectrum display at around -150dB.

  
  

  Wait a few minutes for the display average to settle and then connect your antenna in place of the 50 Ohm load.
  

  
  

  Make a note of the new level of noise floor on the spectrum display.

  
  

  Ideally if the KiWi has enough gain and the antenna is good enough, your noise floor should rise slightly when the antenna is connected (as long as it's not due to unwanted interference). 

  
  

  If you find that the noise floor increases by more than 3dB, then the KiWi and antenna you are using are already sensitive enough, as the lowest level signals you will be able to hear are limited by the natural (galactic) noise level

  
  

  If the noise floor doesn't rise by 3dB (or at all) then your antenna needs to be improved.

  
  

  Generally speaking, when using a good (passive) antenna the Kiwi is usually sensitive enough up to about 25MHz. On higher frequencies the natural noise floor is lower and ideally you need an antenna that has some additional gain in order to achieve a 3dB increase in the noise floor. So if you repeat the same measurement method at for example 30MHz, you may find that you can't see any change in noise level.

  
  

  If you are using an active antenna, it's a bit more complicated to conduct these tests, as the amplifier will add it's own noise contribution. 
  

  
  

  In this case you need to replace the antenna part (whip, probe or loop) with an equivalent terminating impedance. In the case of a whip or probe something like a 12pF capacitor from the amplifier input to the common (ground) rail, or in the case of a loop you need to replace the loop element with a small inductor of equal value to that of the loop (typically 2.5uH). As before look for a 3dB increase in noise when the antenna element is reconnected.

  
  

  All of this assumes that your existing noise floor is low, but if it's dominated by local interference (as is the case in most urban environments), then you need to concentrate on reducing the level of interference, rather than worrying about the Kiwi or antenna gain. Although moving an existing antenna to a lower noise location, or using a directional antenna may be at least be a partial solution.

  
  

  Regards,

  
  

  Martin - G8JNJ

  
• dl2sba

  September 2017

  Hi Martin

  
  

  thanks for the detailed reply - I first have to understand it ;-) then I'll come back to you ...

  
  

  //Dietmar