QRM: Radio China International - From Albania - OV +++

Comments

• G8JNJ

  May 2019

  Hi Giulio,
  
  Adding a series tuned notch filter across the KiWi RF input will help reduce the problems on 7345KHz.
  
  
  
  These values give the bets compromise between notch depth and attenuation on the adjacent 40m amateur band.
  
  For this simulation I have used an inductor with a Q of 50. But if you can use a better inductor, it will provide a deeper and narrower notch.
  
  31 turns wound on a T50-2 iron powder core would be a good starting point, but you would have to add or remove turns in order to tune the notch to the exact frequency.
  
  It will attenuate signals on 40m to a certain extent, so the signal levels will drop slightly, however as the natural noise floor is usually the limiting factor on the lower frequency bands, I don't think it will harm the Signal to Noise ratio.
  
  As a test you could run the WSPR decoder with and without the notch in circuit, and compare the average S/N before and after, but always compare reports of the same transmitting stations, don't compare different stations with each other.
  
  The other interference is something local to you, but sort out the 7MHz overload before investigating the other problems.
  
  Regards,
  
  Martin - G8JNJ

  
• IZ3EAW

  May 2019

  Thank You Martin,
  Now I have S9 OV from BC over 7.200KHz. Is it possibile to disable the PreAmp?
  Frankly I can't listen to anything on 40m, useless.
  
  Try to take a look on www.iz3eaw.it
  
  Regards,
  
  Giulio IZ3EAW
• G8JNJ

  May 2019

  No, you need to either notch out the strong signal which has a minimal reduction in overall performance across the whole frequency range, or put an attenuator in line which will reduce the performance on the higher frequency bands where the natural noise floor is lower and you need the additional gain.
  
  If you read the many posts on this forum regarding 0V indication, the notch is the best solution for this particular problem.
  
  I have notches for all the SW broadcast bands ahead of all my KiWi's.
  
  Regards,
  
  Martin - G8JNJ

  
• IZ3EAW

  May 2019

  Dear Martin,
  I am going to build the notch filters, could you tell me if you made a filter for each specific disturbing frequency or a slice of bc frequency?
  Giulio EAW
• n6gn

  May 2019

  Right at the moment, about 1700 UTC, your Kiwi is not indicating OV but the broadcast at 7380 is showing -24 dBm which with voice peaks may be pretty close to top of ADC. I suspect that you don't need very match attenuation or notch to make this not be a problem. No other signals are threatening.
  The Kiwi is working fine too, I can hear the weaker broadcast stations at -10 kHz and +15 kHz just fine. Noise floor nearby seems normal with weak lightning audible.
  I think you only need a little bit extra to be just fine.
• G8JNJ

  May 2019

  Hi Giulio,
  
  I use a separate notch filter for each BC band, as I have a large broadband antenna, but you only seem to have a problem on one frequency, so it is easy to fix with just one filter.
  
  The notch with values as shown, covers most of the 7MHz BC band, it is quite difficult to produce a much narrower notch that only removes one BC station. Look at the graph to see how much attenuation is presented at each frequency around the main notch.
  
  If the notch depth is too great, you can add a resistor in series with the capacitor and inductor. 4R7 will give you about 10dB notch depth and 10R will give you about 6dB notch depth.
  
  Regards,
  
  Martin - G8JNJ