AM Overload Solved

Here's the final pix of my notch filter I used to solve the severe AM overload I had. More info on it in a follow up post
5376 x 3024 - 3M
5376 x 3024 - 3M


  • This was to fix the extreme overload I got from local stations on 950, 1180, 1280, 1370 and 1460.
    No magic to the design, used 365 pF var. caps and 91 uH inductors
    The 1:4 and 4:1 transformers at the input and outputs raise the Z that the notch filters work "against" to 200 ohms and get the 20 dB notches
    I used 220 pF caps on the 200 ohm windings to reduce the loss at 160M 
    I tried toroid inductors and alternate 4:1 and other xfrmrs. I settled on the T80-2 toroids for those. If you go too small on those the loss goes way up. 
  • Even though the notches reduce some adjacent channel signals, the resultant performance is impressive. For example, 1010 (NYC) is nestled between 950 and 1180, both local channels I notch, and I could still receive it. AMBCDX is possible, impossible with the former HPF. I logged 30+ stations in an hour.
  • Thanks for posting this information. Currently I have one notch filter set for 1380 KHz. It was the only local station causing problems. The filter I made is bit wide a attenuates a wider bandwidth than I would like. Still it is quite useful. The 4-1 transformer idea is a great bit of information.
    Ron  - KA7U
  • Hi James,

    Your notches are doing a great job, as your SDR seems to be a lot happier now :-)


    Martin - G8JNJ
  • KWEI is back on the air after being down for quite some time. 8.4 KW during the day @ 1260 KHz, and just 3 miles upstream from me. Now the OV light is on. Dang nabbit.
    Ron - KA7U
  • 5.8 microhenry and 3000 picofarads, problem solved.
    Ron - KA7U
  • I have been trying various combinations of notch freq. and find that I only need 3 notches total. 950, 1280, 1370.   Those three TX are less than 2 miles from me!!
  • edited April 2017
    I have found that a pretty nice single notch can be made using ready made off the shelf components. 30+ dB. If you are interested, contact me and I'll provide more info. KD2OM using one on 1310 KHz
  • The circuitry is very simple. Series LC that resonates at the frequency of interest. If all you have is a single notch you can use this transformer to step the Z up to 800 and another back down to 50

    The LC hang on the 800 ohm line.

    for my MW notches I used 91 uH molded coils like these

    and for nostaglia, a 365 uuF variable cap like this

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