Comparing Noise Cancelling Techniques
In principle with DSP, the same noise cancelling can be performed as, for example, the MFJ-1025 or ANC-4 performs on high frequency signals. DSP can cancel interference much better and faster using adaptive filters. When cancelling a local interfering source, two different antennas can be used.
Although a lot is possible with adaptive filters, there are limitations in the level of noise reduction achievable with two antennas used as a receiving array. In such an array, the signals from the antennas are summed after applying a complex weight to the signals. When using two antennas there is a maximum of just one interfering source or one arc of direction to be suppressed. Figure 12 shows an example of a possible 3-dimensional radiation pattern of an array of two loops, as shown in Figure 3A.
Figure 12 — A three-dimensional radiation pattern of an array of two vertical small loops, using summing of signals.
The arc of nulls can be very sharp, but the beam is always broad and the achievable directivity is limited. When the distance between the loops gets smaller compared to the wavelength (<1/8 λ), the gain decreases significantly. The sharp null makes it difficult to suppress interfering sky wave signals, because on shortwave the direction of arrival is constantly changing over time. The advantage is that such an array can operate if two or more signals share the same frequency.
The frequency domain processing using time difference of arrival or the phase difference for selecting frequency components as described in this article, behaves much differently. Beams and nulls are in arcs of direction as shown in Figure 4, but very sharp beams and very broad nulls are possible. Also, multiple beams and multiple nulls are possible. The achievable directivity is only limited by the signal-to-noise ratio. The gain is not affected when the distance between the loops gets smaller compared to the wavelength. A further advantage is that it is possible to profit by elliptical polarization.
All in all, each technique has its own advantages and disadvantages. There is not one single best technique. The best you can do is having all techniques at your disposal. Digital Signal Processing makes this feasible.
Last update: September 24, 2006