Noises Off – WiMo QRM Eliminator

As I posted at the end of June I was almost on the point of giving up on HF due to the high levels of QRN/M suffered at home.

In desperation I had been looking at some of the noise cancellers available from MFJ and others. I had heard conflicting options on their effectiveness but was willing to try one if I could obtain one cheaply. The MFJ units are in my opinion expensive to purchase new so I bid on several that came up second hand on eBay but they invariably also went for silly money. I was even contemplating building an home-brew one from the numerous designs available.

Then I discovered the WiMo QRM Eliminator, made by the German company WiMo Antennas and Electronics. Several online reviews and numerous YouTube videos seemed to indicate its effectiveness and taking up the then summer discount offer I ordered one for the princely sum of €147 including postage, around £116.

Due to heavy demand and low stock levels following the International Ham Radio Exhibition at Friedrichshafen I was told the unit wouldn't be available till the middle of August, so I was pleasantly surprised when the unit turned up last Friday morning.
I have connected it up and briefly tried it out and am impressed, as this video illustrates.

Even using one for a short time I would agree that they are something of a black art to set up and use and can understand why people might consider them useless, but this may be due to lack of understanding of how they work.

I have created a diagram showing a typical scenario.

The operators main antenna is designed to pick up distant signals, not necessarily DX but signals not emanating from the immediate vicinity, however they will also pick up locally produced QRM as shown, perhaps generated by a neighbours TV or PLT device. 

The QRM eliminator has a second antenna which isn't as efficient as the main antenna and ideally just receives the local QRM at a similar level to the main antenna. The device then takes this second signal and inverts the phase so when it is mixed with the main antenna signal the QRM is cancelled out.

The principal is quite simple, if you take two in phase signals and combine them you will end up with a signal with a larger amplitude. However if the signal is 180 degrees out of phase, the positive and negatives of the waveform cancel each other, producing a null signal. 

The tricky part is making sure the noise antenna just picks up the noise, if it picks up the main signal then that will also be cancelled out. The WiMo unit has three controls a gain and two phase controls it is a case of altering all three to maximise the cancelling effect without losing the main signal.

At present I just have several meters of wire running as the noise antenna along the side of the shack and this seems to fairly effective.

The unit is powered from 12V and can be left in line, but requires the PTT/TX-GND signal from the CAT/Linear socket from the transceiver to activate a bypass when transmitting and I don't have the appropriate 8-pin plug at present. If the unit is powered off the bypass is automatically engaged.

All in all, first impressions are good and looks like a worthwhile purchase.
Andrew Garratt, MØNRD, is a regular contributor to and writes from East Midlands, England. Contact him at [email protected].

7 Responses to “Noises Off – WiMo QRM Eliminator”

  • jeff herr ww6l:

    what is the real world insertion loss? and its noise floor?

  • David WB4ONA:

    This thing connects at the antenna(s). I bet if you open it up there’s probably a little active antenna preamp with gain pot, a passive combiner circuit, a transmit bypass relay, and a couple of polyvaricon caps for the phase controls (I don’t know why you need two phase controls, maybe one polyvaricon doesn’t give you enough adjustment range?)

    If that is pretty much what is inside, then this little box is quite pricey for more than $210 USD each plus shipping from Germany. I wouldn’t mind seeing some home-brew examples.

  • David WB4ONA:


    There are no specifications in the very short operating manual I downloaded for this box (the link is in the original post).

    But that shouldn’t really matter. At HF, atmospheric noise dominates, especially at the 20m and lower frequency bands. And it would not be difficult to design the probe and main antenna input circuitry to contribute trivial noise.

    As for insertion loss, if a filter combiner is used (maybe that’s why they use two phase controls?), then the phasing combiner loss may be negligible when tuned. If a passive 3dB combiner is used then there is (obviously) a bit more than 3dB insertion loss, which isn’t a catastrophe for general purpose HF operation – especially because you are fighting so much interference noise in the first place, otherwise why would you be using this box?!

    73’s, David

  • Andrew M6GTG:

    If you look at my earlier posts you can see the level of man made interference I am contending with at the moment makes HF operation almost impossible at times. This device offers me a fighting chance of hearing stations. A small insertion loss of increase in the noise floor would be a small price to pay.

    From looking at the home brew circuits available I know there probably isn’t a lot of sophisticated electronics in these devices. You can find the schematic for the similar MFJ-1025/6 Noise cancellers in their manual for instance.

    This unit may be expensive in the US with shipping but visa-versa prices for MFJ units here in the UK either from dealers or importing directly seem to be nearly double the equivalent cost in the US.

    73 Andrew

  • Mike VE3WDM:

    Andrew it’s great that you have found something that works just FB for you. I had a similar issue at my old QTH and used the MFJ 1026 unit. It’s very true as you said it takes some time to get used to how to fine tune the unit. Over time you will be able to adjust the unit very fast and it will be more effective as you practice.

  • Marco HB9CAT:

    I confirm it does help in heavy QRM situations like mine.
    To visualize the effect I tested Wimo’s QRM-Eliminator in JT65:

    The Box was turned on at the beginning of the period, I turned it off after 20 seconds, and back on 15s later; QRM is easy to see but more importantly the right JT65 transmission would be lost without it.

    Marco HB9CAT

  • Craig G0KVL:

    There’s not a lot in these units, WB4ONA David commented on that subject, there isn’t exactly 2 phase adjustments, from what i can see is 1 phase and 1 pot for gain adjustment of the Aux antenna, why it’s labelled as 2 phase adjustments I’m not sure.

    If you Google and see the internals it’s virtually if not identical to a homebrew version that originated from Germany called the X-Phase QRM Eliminator.

    The circuit is extremely simple I have build my own unit for around £30 with virtually enough components for a second unit should I choose to make another.

    Insertion loss would be Dependant on a pair of relay contacts and SO239/PL259 plug/sockets i guess.

    The main antenna and Aux antenna each go through a J310 FET with a gain pot for each.

    Craig G0KVL

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