The method described in the patent is breathtakingly beautiful in its simplicity. Have inter-modulation because of poor quality radio frequency (RF) connectors made with steel or nickel? Well just pass a strong magnetic field through them to reduce the inter-modulation.

Very nice and a very handy technique to help engineers and technicians troubleshoot busy radio sites with multiple radio transmissions.

Being so elegantly simple leaves one wondering if this method is obvious or not.

Does anyone have any radio references talking about using a static magnetic field to tame the influence of cheap RF connectors made of a ferrous material?

  • Thanks for putting my post in the Prior Art Request category George.
    – JSH
    Nov 22, 2013 at 3:17
  • This patent link may be a bread crumb. This patent doesn't claim the idea of quelling inter-mod using a strong static field as novel, rather it talks about this topic in paragraph 4 in the Background Art. "The occurrence of IMD can be suppressed by applying a sufficiently strong DC magnetic field to the ferrimagnetic member[. . .]." Of course steel and nickel connectors are ferrOmagnetic which is different, but the fact the patent treats this similar phenomena as common knowledge makes one wonder.
    – JSH
    Nov 22, 2013 at 3:29
  • I think this patent is prior art US4156857. The claims seem to describe the whole test setup rather than an intermod mitigating technique. Nonetheless, the Description and Background sections describe how using magnets around a intermod prone material quells the generation of the intermod. One might argue the wording is speaking of the mitigating technique as if it is well known.
    – JSH
    Nov 22, 2013 at 3:51

2 Answers 2


How about this from 2009? Ferromagnetic Passive Intermodulation:

Ferromagnetic materials are a known cause of passive intermodulation (PIM). For the first time we show that by biasing them with a static magnetic field, the PIM can be reduced significantly. Specifically we show that a standard gold plated SMA can have it's PIM reduced by 35 dB, down to the level of the DIN 7/16 silver plated connector specifically designed for low PIM performance

Humorously, it came from Purdue's IDEAS RF Lab. The lab apparently has a YouTube account (https://www.youtube.com/user/ideasrf and https://engineering.purdue.edu/IDEAS).

  • This is, indeed, the same group requesting the patent. The priority date of the Patent Application, 2009-06-04, is 20 days or so before the YouTube video posting date, 2009-06-24, so they wisely managed the timing of the release of information... if I understand the real meanings of Priority Date.
    – JSH
    Jan 7, 2014 at 16:45

I'll go out on a limb and suggest the wording in this 1977 patent...


"[. . .] the interaction of the local r.f. magnetic fields with the magnetic moments associated with the atoms of the various metals making up a passive microwave component. One of the stronger of this class of interactions is that associated with ferromagnetic resonance, [. . .]

The invention focuses on this class of phenomena which is dependent upon the magnitude of a static magnetic field (generated internally or externally), whereby the generation of harmonics and other spurious signals by this effect is minimized through the use of appropriately placed magnets.

[. . .]

Utilizing additional conventional techniques to reduce harmonic generation at various junctions within the filter improved the second harmonic response to only about -70 dBm. However, applying small magnets 23 near the standard "non-magnetic" stainless steel coax connectors in accordance with the method of invention, easily improved the response to -90 dBm. Further adjustments in the placement of the magnets provided a response of less than -110 dBm, which was the sensitivity limit of the measurement system used."

...properly describes the very phenomena the Purdue team suggests is novel in their claim #1.

Good useful research? Yes. Novel? No.

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