It’s becoming more and more a standard part of the average radio amateur station, the X-phase QRM Eliminator. In the fight against manmade noise on HF, a QRM Eliminator can be an excellent tool. Ay least, if the noise source is close by and that it is also a single noise source. Once in the shack with the QRM Eliminator, the goal is to achieve a low-noise reception.
Main antenna and auxiliary antenna
The QRM Eliminator is basically a device that picks up two identical signals. By shifting one signal 180 degrees in phase to the other signal, you can reduce a specific incoming signal. The device therefore uses an auxiliary antenna, the “aux antenna”. The aux antenna feeds the received signal to the QRM Eliminator. It mixes the signal from the main antenna with that from the auxiliary antenna. By putting the noise signal from the aux antenna in phase opposite to the main antenna, the interference signal is “eliminated”.
Bij shifting phase 180 degrees, the sum of A and B becomes zero.
Gain knobs and phase knob
To eliminate a specific noise signal, the signal received through the auxiliary antenna must be as strong as the received signal through the main antenna. That’s why the QRM Eliminator has a gain knob for both antennas to control the strength. In order to then be able to shift the phase of the specific interference signal from the aux antenna, 180°, there is a third button; the phase knob.
A type of noise that changes
Now my local noise comes from a close solar panel installation with two microinverters on the roof. The electrical output of that installation varies depending on the time of year and the weather conditions. The characteristics of the noise therefore also change. At a high output from the solar panels, it’s a loud static kind of noise. When the panels produce a low output, the noise sounds like resembles woodpecker in the forest. In the winter I therefore have to adjust the QRM Eliminator differently than in the summer. I use a certain order to be able to adjust quickly and easily.
Turning the knobs
I’ve had a QRM Eliminator from US4LG Igor for some time now. In the mean time I’ve become quite handy in adjusting the device. I always do that in a certain order:
- I start by first setting the buttons to the same position.
- Then I turn the gain knob of the aux antenna until the interference signal is reduced the furthest.
- Then I turn the phase knob to further reduce the interference signal.
- Then I turn the gain knob of the main antenna to possibly further reduce the interference signal.
I use the S-meter but mostly by ear. But you can also use the spectrum display of your transceiver or SDR receiver. Then you can literally see that the interference signal “turns away” compared to a DX signal.
Different adjustment per frequency
The adjustment is frequency specific. My experience is that if you have the best adjustment on the 20 meter band at 14050 kHz, for example, you have to turn the knobs slightly at 14300 kHz. This is especially true if you change from one band (wavelength) to another.
Does it eliminate wanted DX signals?
I often get the question whether DX signals are also eliminated. The answer is no. That’s because those signals have traveled along multiple paths. Through the ionosphere and through reflections from all kinds of objects in the path of the signal. The DX signal arrives as a composite of different signals in different phases with different polarisations. Therefore it’s almost impossible to eliminate it. Eliminating interference that is DX, such as the well-known Russian Over The Horizon Radars (OTHR), therefore does not work.
What is the best aux antenna?
When you are looking for an optimal broadband auxiliary antenna. Read about my experience using the Miniwhip active antenna to pick up local noise effectively.
When you need ‘high end’ QRM elimination, you might want to read about getting rid of QRM using diversity receive mode.