Need to attenuate LW 252 Khz

Hi All,

I need to attenuate LW 252 Khz here in Ireland as it's 40+ db over and causing images and performance issues throughout the entire spectrum at times, issue on both my SDRs.

I don't want to attenuate 0-1.7 Mhz or anything like that because I want people to be able to listen to Lw/MW broadcasts so just want to reduce 252 Khz by about 20 db.

Any ideas ?




  • a simple L-C notch should do that...
  • Simple you say ? :-)
  • You don't need anything elaborate but here's some guidance
  • Hi Mark,

    Here's a simple notch that should do the trick and give you a 20dB deep notch on 252KHz.

    It's just a series tuned circuit wired across the antenna feed into the KiWi.

    You may need to fine tune either the inductor or capacitor value to get it spot on frequency, but these values should get you close.

    If you buy a standard value pre-wound inductor, just play with parallel combinations of capcacitors to get near the correct value by watching the KiWi S-meter when tuned to 252KHz.


    Martin - G8JNJ
  • If your noise floor is reasonably high at LF, you should be able to see the tuning of the notch as a local minimum, a "noise-stimulus scalar network analyzer". Look at the percentage error and adjust the capacitance by the square of that number.
    Or else go "all out" and get one of These :)
    IMO, really quite a good value for this sort of thing.
  • edited August 2019
    The QRP-Labs filter boards fit nicely on the kiwi (of course changes in the routing of the boards are necessary).

    Unfortunately, I have too many peaks across the entire spectrum to notch.
  • Thanks for all the replies guys, very helpful ! :smiley:
  • Watch out if you use the wire-antenna connection on the kiwi - it is not nearly so good as the SMA and you will very possibly have increased spurious coupled in, at least that's what I've concluded from experience and measurement.
  • edited August 2019
    Short signal and ground path is crucial.
    It is meant to be a cheap and easy solution if somebody only needs to notch 1-2 frequencies. The entire thing is cheaper than two SMA connectors alone (at least in HB9).
  • The ground path through the wire connector is worse than the SMA in terms of common mode paths that can increase the noise floor. This is true whether or not leads are short since the coupling mechanism isn't due to slightly increased series inductance and magnetic or capacitive coupling.
    On the antenna switches I designed and fabricated, I at first tried to use that connector to get into the Kiwi to but finallly gave up on it because of these sorts of problems.
    If one uses preamplification or an electrically large antenna & suitable filtering to increase levels ahead of the input connector it will tend to suppress these other coupling mechanisms relative to the desired signal so things will work better but, in general, I think you'll find that common mode current on the antenna line will couple better (worse) if this connection is used instead of the SMA. I did.
  • edited August 2019
    I have taken measures to suppress common mode currents on the antenna line.
    Common mode impedance is >1 kOhm from 100 kHz to 30 MHz.

    Since I have two kiwis now, things got easier. Low pass filter to one, high pass filter to the second kiwi.
    Only the dynamic between 4-12 MHz is difficult, particularly the 41m broadcast band. Sometimes I have -5 dBm broadcast stations 10 kHz from the 40m ham band which has lower than -130 dBm/Hz noise floor.
  • It does depend on what noise floor you are striving to achieve. This morning, 0500 local time, located at a very quiet site with a well calibrated system from the known antenna efficiency and 'ground noise' through an LNA and all measured hardware to the kiwi SMA connector is reporting -144 dBm/1-Hz on 7 MHz.
    You may find that at that level unwanted ingress may be larger than this with the wire input connector.
  • @HB9TMC

    Hi Stefan,

    Just based on quickly looking at your screen grab (I can't find your KiWi on-line, so I wasn't able to play with it to be sure), as your noise floor is at least 20dB above that of the KiWi SDR, I'm assuming you are using some form of active antenna. If so you could possibly just put a 10 or maybe even a 20dB attenuator in front of the KiWi without significantly degrading the S/N ratio, as the antenna and associated electronics will probably be defining this anyway.

    If you decode WSPR stations it would be a simple matter to try different values of attenuation whilst observing the S/N of the spots you receive in order to find the best 'sweet spot' value.


    Martin - G8JNJ
  • edited August 2019
    I was referring to the noise level on 40m to illustrate that I don't have much room for filters. I'd need to attenuate 7205 kHz by >30dB, in order to avoid ADC clipping.
    But by attenuating 30dB, the RF noise floor on 40m would fall below the receiver noise floor.

    I get -10 to -5 dBm "bare foot" on my L-antenna. Only during gray line or sporadic-e though.
    Culprits are Woofferton (300 kW) and Issoudun (500 kW), both at good skip distances.

    The noise floor from 8.5 MHz to 28 MHz is dominated by QRM from PLC, VDSL and the neighbors SMPS.
  • Hi Stefan,

    OK understood.

    I mentioned it because of your previous comment "Unfortunately, I have too many peaks across the entire spectrum to notch."

    The point I was trying to make is that you have some margin for attenuation across the whole spectrum, which makes any remaining notching that may be required a lot easier to achieve.

    I use a combination of amplitude slope equalisation and eight individual BC band notches to 'level off' the overall 0-30MHz spectrum in order to avoid similar problems and maximise the S/N as best I can on the frequencies outside the BC bands.


    Martin - G8JNJ
  • HB9TMC writes:
    "I get -10 to -5 dBm "bare foot" on my L-antenna. Only during gray line or sporadic-e though.
    Culprits are Woofferton (300 kW) and Issoudun (500 kW), both at good skip distances.
    The noise floor from 8.5 MHz to 28 MHz is dominated by QRM from PLC, VDSL and the neighbors SMPS."

    Those certainly are big signals. I have a similar problem at n6gn/k2 where there are seven! 50-100 kW ERP class NIST transmitters 60 kHz - 25 MHz,, entirely LOS at 19 km. The site itself is not near other noise sources but to get down to the propagated noise floor under that sort of onslaught is quite a challenge for a kiwi.
    Presently this is not occurring above about 10 MHz. The Kiwi noise floor is too high to permit this from even a well matched dipole and a large (half-wave class) antenna delivers so much signal from these big sources that it truly is 'being stuck between a rock and a hard place".
    My current efforts toward achieving a propagated noise limit are presently focused on a hybrid antenna approach, mentioned in another thread. But to create a system that doesn't overload, big signals must be reduced or filtered which makes system noise and unwanted spurious including IMD, that much more of an issue.
    This is a dynamic range tuning problem which is why I mentioned the common mode ingress mechanism differences at the Kiwi inputs. Somewhere inside the kiwi there is a point that is truly a 'ground' but we presently are not returning CM currents to that point with either the SMA or the wire inputs. The wire input is somewhat worse. This seems to be how the remaining lines (mainly above mid-HF) are entering the n6gn/k2
  • Issoudun has twelve 500 kW transmitters, Woofferton ten (5 x 250 kW, 1 x 500 kW, 4 x 300 kW), nicely spaced across the shortwave spectrum. Not all of them are active at the same time, anyway.

    I thought also about making a slope equalization filter. But I'm not sure if it is worth the effort. I'm constantly working on the antenna and changing things, so the frequency/power distribution on the antenna does change too.

    Since I put common mode chokes on the kiwis connections, I have only noise left from which I know the sources (I think).
    Fun fact: The GPS's coax was radiating common mode noise originating from the kiwi.
  • "Fun fact: The GPS's coax was radiating common mode noise originating from the kiwi."
    and thereby also providing a lower impedance path for CM currents from other sources, both within (LAN) and outside the Kiwi on other lines, LAN, PS, antenna.
    "It's a jungle out there." :)
  • I've gone over to GPS repeaters partly due to the risk of pickup or radiation from the GPS coax.
    My stock puck antenna was working but not well, turns out coax had a nick that allowed water ingress.
    I cut that coax down to about1m and put a new 90degree SMA on it.
    Repeater connected to the main GPS with re-radiator in a coffee can, I put the cut down stock in there and feed two Kiwi's.
    That did help with the common mode noise here and means I can just stick GPS antennas from other kit in there too.
  • Have I already mentioned transmitter Roumoules (1000 kW)? :D

    I'm actually using an active GPS splitter (this one) but it doesn't stop common mode noise from spreading. It does provide a bit lower impedance point though, the attached cables work like radials.

    I've put common mode chokes on both antenna connectors, and I haven't had any CM problems since.
    8W5000 core for shortwave, N30 for MF/LF.
  • That is too posh/high-brow for me ;-)
    The type I was referring to was the car gps repeater, no galvanic link between Kiwi and external antenna, quite hard for common mode to get accross that gap (I hope).
    Google search term "gps repeater car" gets them, cheap as chips (direct from China) and work fine for this use. Not sure how many downstream GPS antennas they can service as the range is very short.
    Only reason I put it inside a RF enclosure is the rules in the UK about repeaters.
  • Re. Martin G8JNJ's notch filter: I built one for 198 kHz using 26 uH in series with 24370 pF (10nF + 10nF +3900pF + 470pF all polystyrene) and found that the notch was 34 dB deep, which was more than necessary. I 'softened' it to 20 dB with a series 2.2 ohm resistor. This makes the response from 193-203 kHz flat to within 1.5 dB, meaning that users can still listen to Radio 4 if they want to without distortion.

    I now have the Wellbrook loop's azimuth such that it reduces 198 kHz by 6 dB anyway, so I no longer need the notch.

    Richard G4DYA
  • >
    > found that the notch was 34 dB deep, which was more than necessary. I 'softened' it to 20 dB with a series 2.2 ohm resistor.

    Hi Richard,

    That's good, you must have found an inductor with a good value of Q.

    I've only ever been able to achieve a maximum of about 20dB using surface mount or small molded axial types.

    You can widen or narrow the notch bandwidth by choosing different L/C ratios. The values I used were designed to provide some attenuation over the whole of the LW BC band, with a deep notch at the band center and minimal effect outside of the band (148-284KHz @ -3dB).

    You can decrease the depth of the notch by adding low value series resistors as Richard suggests. When using surface mount inductors, I typically add 6.8 Ohms to this circuit to obtain a notch depth of 10dB, with 0 Ohms I can get about 17 to 20dB.

    Here's a plot showing the measured performance of my BC band filters.


    Martin - G8JNJ
  • I wound the inductor on a T50-2 toroid. Quite a lot of turns, too many to count and I didn't keep a note!

    Richard G4DYA
  • >
    >I wound the inductor on a T50-2 toroid. Quite a lot of turns, too many to count

    For others who wish to copy Richard's circuit, it was probably about 73 turns according to mini ring-core calculator (free)


    Martin - G8JNJ
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