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Strong local signal overloading - any suggestion?

Hi all!

I'm a newbie in SDR scene.

My QTH is Sz?kesfeh?rv?r, Hungary.

We have a 2000 kW "monster" that is our national broadcast, Radio Kossuth 540 kHz from Solt, 55 km far from my antenna.
I have an active MiniWhip mounted on our building, 30m high. (Lead with 45m of RG58 Coax, connected to the lightning rod, for the good grounding). There no chance to install a longwire or other antenna. We have 3 GSM repeaters on the roof, so there is no space for "passive" antenna.
On my Icom IC-R70 there is no problem (only some overload on 675, 1215 khz...)

The signal is more than S9+60!

R Kossuth fully overloads my KIWI, when the transmitter is on air (0330-2130 UTC) there is no other signal, overloads the full 0-32 MHz band.
My SDR is here:

I spent my one month-salary for the Kiwi, and now I very sad. :(

(When the TX is OFF, the KIWI works wonderful)


  • You have 2 options....

    1) add a AMBC High Pass filter which would attenuate everything below 1.7 MHz

    2) build a Notch filter for 540 KHz

  • The narrow notch filter is the option for me - LW and MW is very good here, with a dozens of stations at night.
  • I am building something similar to Martin but for the lower BC frequencies. Currently, I have a HPF
  • Yes, Martin's stuff looks good. But I need a DEEP and narrow notch for 540 khz. -10 dB att. is not enough for the S9+60+++ signal, I think.
  • edited February 2017
    One of these and a 100 uH or so inductor would do it

  • Hi,

    If you take a look at the spectrum display, you generally need the strongest signals to be below the -30dB line. They can be slightly higher, but you may have problems if propagation changes and increases the signal level.

    If you want to have a deeper notch depth you just need to change the L/C ratio, or use two or more notches tuned to the same frequency.

    Just choose the notch depth that is required to drop the signal levels below the magic -30dB line.

    For example:-

    a 10uH inductor and a 8600pF will give you a notch depth of about 25dB with a bandwidth of 750KHz at the -3dB points

    a 100uH inductor with approximately 1812pF will give you a notch depth of about 12dB with a bandwidth of 108KHz at the -3dB points

    In the case of the lower frequencies it is usually easier to use a variable inductor and fixed value of capacitance, as this makes tuning of the circuit a lot easier.

    Old analogue AM radios are a good source of suitable variable inductors. If you can find one of the metal canned adjustable AM 455KHz IF transformers (remove the small capacitor that can usually be found in the base of the coil) or the AM local oscillator coil, these make great notch filters. 

    Note that in order to get the best notch depth you need to use good quality polystyrene capacitors.

    Image result for am radio circuit board

    Martin - G8JNJ

  • jksjks
    edited February 2017
    There is some overload advice here:

    Martin's photo reminds me how I built my first series tuned notch filter a few years ago used at another location that had a very strong AM station nearby. At my local electronics store I found a flat ferrite rod antenna like the one at the upper left of the photo. I didn't have my RLC meter at the time, so I just measured the two windings and used the one with the greatest resistance (about 10 ohms). Then I tried different capacitor values in parallel with a 100 pF variable like the one at the lower left of the photo. I put the L/C combination in series across the antenna input.

    Eventually I found a fixed cap and var cap setting that caused a notch on 540 kHz (my problem frequency also). Working backwards I estimated the inductance of the coil to be about 550 uH. The tuning was quite sharp which meant the Q of the coil was good and the notch narrow. If you use the F/L/C formula for 540 kHz and 550 uH you find a single fixed cap of about 160 pF will work.

  • Hi All,

    I should have emphasised the point about choosing the appropriate L/C ratio.

    I suggest that you only use minimum depth of notch and narrowest notch width that you can get away with, in order to minimise the effect on nearby signals.

    Modelling the tuned circuit with either RFSim99  or Elsie (both of which are easy to use and free) will help you to be able to quickly choose suitable values.


    Martin - G8JNJ
  • It looks like a simple way to notch thing is to make a trap with a LC that resonates at the interferer's Freq.   I see lots of schematics where people simply hang that on the 50 ohm line. Doing it that way limits the Q and thus the narrowness and depth of the notch.  I just finished building a 3 notch assembly but I added 4:1 transformers on the in/out such that the LC notches are on a 200 ohm line, I got 15+ dB notches in the MW BC band that were very narrow. Unfortunately, I found I need a fourth notch!

  • Hi James,

    Ah yes - I'd forgotten about that trick. 

    Using 4:1 or 9:1 transformers is also useful when building diode RF limiter circuits.

    I haven't had to worry about really narrow notches on the MW BC band, so I didn't need to do that for my filters.. 

    But it's a really excellent suggestion :-)


    Martin - G8JNJ
  • Here's my analysis, somewhat after the fact, as I missed the need for a 4th notch!

    I had a HPF in there before and it totally knock 950 KHz out, so I missed the need for that.

    Starting from scratch, I replaced the HPF with a variable attenuator set to 15 dB. That's what the filter needs to be to keep the OV light off!

    I noted I had AM stations at S9+40 at 950 and 1180 and ones of S9+60 at 1280 and 1370.

    I have the parts coming to build a 6-bay notch filter which will be 4 bays  to start.
  • You can fab fixed attenuators for testing using the info here:

    If you have a antenna system with a preamp, you may have too much gain in general and can use one to reduce the reduce the gain
  • Here 1st pass of my new 4 channel (expandable to to 6) notch filter for AM stations.
    I used the 365 pF variable caps because that's what any AMBC RX device should have!!
    Inductors are 91 uH
    Transformers are 4:1
    There's a small 200 pF cap tucked in under he inductors to reduce loss at 1.8 MHz
    Notch depth on the 4 AM stations is 222-26 dB

  • the caps are 220 pF under the 4:1
  • before you try my system and are disappointed, NO this filter is not online yet.  2nd pass will go online this Friday.
  • Where you can buy variable caps in that big pF range? In the local electrical store, there is only 10-100 pF...

    Now I partially solved the problem (Thanks to Martin!).

    For 540 kHz 22uH+3.9 nF plus 560 pF capacitors (Two from this combo)
    For 135 kHz 100 uH+14 nF (3 from this combo) (I must add some nF because the center of the notch is around 160 kHz but I had no time for it yet)

  • You can note, there is also some intermod in my SDR. It is from my antenna amp, I must accept some compromise (No place for passive antenna and the high power tx's near me). The amp is from BF998 and BFG591.
  • Hi Laszlo,

    I'ts difficult to get hold of variable capacitors with a sufficiently large range of adjustment, which is why I suggested using a variable inductor instead.

    Try a search for "Polyvaricon capacitor" like the AM/FM tuning capacitor shown in one of my earlier posts on this thread.

    If you do buy one try to make sure that it has a maximum capacitance value of at least 200pF (or ideally more).

    Old AM/FM transistors radios are a good source but you can also buy them on-line


    Martin - G8JNJ

  • I have tried a lot of alternative parts in my filter. My goals were:

    1- deep enough notch depth on what ended up being 5 AM frequencies
    2- minimal impact on 160M + up
    3 - use of parts anyone can buy.

    I tried using some Mini-Circuit transformers in place of the hand-wound 4:1 and they caused too much loss at 160M
    I tried hand-winding some 100 uH toroids but they provide more notch than needed and are tedious to make

    In the end, all parts are can be bought an simply soldered together with the exception of the 4:1 transformers which are not too bad to wind.

    I'll post up some response curves next week and get it on my RX in place of the HPF which is still there
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