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

Attachments:
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Lonecrow

Comments

  • 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
    WA2ZKD
  • Hi James,

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

    Regards,

    Martin - G8JNJ
    WA2ZKD
  • 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
    WA2ZKD
  • 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

  • Hi all,

    In my location I have serious BCB overload problems and from  AM stations closeby. I see signal levels in excess of -8 dbm on the output of a "Trask" amplifier. This antenna amp has a good ground and has an antenna length trimmed back to 1 cm only!
    The strongest AM station probably about 20 KW is less than 1 km away and also has a lot of spurs and harmonics. Perhaps I am also on their rf ground network or share the same AC mains section.The next nearest about 3 km away.

    To see wat is going on and where the interference products are coming from, I remembered about 10 years ago Dallas Lankford published some articles about LNV (low noise vertical) antennas and I decided to use a version without any amplifier. After trying various hi mu toroids, mainly stripped from old PSU power supplies, it turned out to work pretty well from 200 Khz to 3 Mhz.The antenna length can be increased to the point where peak signal levels remains just below -30 dBm on the Kiwi.

    For the Trask antenna and other active vertical antennas that worked just fine in other locations a notch filter on the ouput did not really help all that much since the imd already starts in the amplifier. I may have to resort to using an ugly high impedance notch filter at the input just to be able to use these at VLF and HF frequencies.

    Active loops work ok when oriented horizontally for the higher frequencies. Placed vertically I can put a 20 dB "null" on only one of the AM stations.

    Anyway these so-called LNV antennas might be worth a try for folks in similar situations with active antenna overload.
    Have a look at ref http://peditio.net/utility/fpdf/rod.pdf if you are interested.

    best regards, Ben.
  • my fix for AM OV
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