Dual loop antenna system
A combination of two rotating broadband magnetic Alford loops is a powerful means for getting a better short-wave reception.
We can do two thinks simultaneously:
** after nulling the loops not always point in the same direction. Using the two loops as an array for directivity, the loops have to point in the same direction. Polarization diversity reception is only possible when the loops are not point in the same direction.
Noise cancelling (local QRM)
Noise cancelling is a very effective way of eliminating a single local noise source. You need two separate antennas, preferably two antennas with different polarization e.g. two loop antennas at right angles as shown in the figure above (you don’t want the directivity of an array). Both antennas receive the noise source with a fixed amplitude ratio and with a fixed phase difference. The noise source is eliminated by combining (adding) the signals of both antennas with the matching phase difference and amplitude ratio. Assuming both antennas are receiving the noise source. When one of the antennas (loops) doesn’t receive the noise source, noise cancelling is not needed of course and only that antenna can be used.
In principle by adding more antennas more sources can be cancelled.
Directivity with an array
This is a very effective way of reducing noise (interfering signals). Not only local noise.
We can make an array with two (or more) antennas with identical radiation patterns placed on a certain distance to each other. The phase difference and amplitude ratios of the antenna signals when combined (added) define the overall radiation pattern of the array. The effect is directivity with the advantage that it is electronically controlled. The distance between the loops is preferably more than 0.125´l and less than 0.5´l.
The radiation patterns have to be identical, because we use summing of the fields of all antennas. Not only the magnitude and phase count, but also the polarization. The loops in an array have to point in the same direction for it !!!
For noise cancelling this is not necessary and even not advised, because the noise source is nearby and has a fixed polarization. All loops receive the noise source with a fixed amplitude ratio and with a fixed phase difference.
The above picture (azimuth, 30º elevation) shows what is possible with two loops (far field). Using the phase difference when combining the signals of the loops, one can select the direction(s) of noise reduction. Of course one has to take the elevation into account also to get the whole picture.
The above picture shows a 3D-radiation pattern (far field) of an array of two vertical small loops using summing of signals.
When using two antennas there is a maximum of just one interfering source or one arc of directions to be suppressed. 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 (<0.125´λ), the gain decreases. The sharp null makes it difficult to suppress interfering skywave signals, because on shortwave the direction of arrival is constantly changing over time.
Placing two antennas with different polarization on a certain distance to each other makes diversity reception possible. Fading by multipath reception or by polarization mismatch works out different for the two antennas. Using two identical and synchronized receivers gives you always the strongest signal. The stereoscopic picture when using headphones helps the brain in filtering out the noise. Alternatively you can use only one receiver and switch between the two antennas by hand, selecting the best signal. DSP makes an enhanced stereo image possible.
In an array, when combining two or more signals, noise and signals add up differently.
The uncorrelated noise of the antenna amplifiers add up as power. So adding or subtracting the noise of two amplifiers always gives 3dB more noise at the output. The signals add up linear. When in an array the signals from the antennas are equal and in phase, after summing the output level is 6dB higher. This improves the signal to noise ratio with 3dB (when no other noise sources present) and improves the average signal strength at the same time because spacial diversity is used ! It is easy to accomplish with a phaser or noise canceller by first nulling the wanted source and then invert one of the input signals.
The man-made and atmospheric noise can or can not add/subtract like uncorrelated noise. This depends on the radiation patterns of the antennas. When equal (in an array), man-made and atmospheric noise add/subtract not like uncorrelated noise. Directivity gives then the noise reduction.
If the distance between the loops in an array is less then 0.125´l, the overall gain will decrease when nulling an interfering source. At the lower frequency bands this is not a problem, because the received noise level is much higher than the noise generated by the antenna amplifiers and the phaser or noise canceller.
Conclusion: Depending on how the signals of the antennas in the array are combined and on the radiation pattern of the antennas, the signal to noise ratio will improve.
Combining the signals from the loops (MFJ-1025/Phaser/DSP)
The MFJ-1025 is not good enough for the job. Its large signal behaviour is insufficient. It gives you not the full 360 degrees phase shift (only abt 280 degrees). It does not tell the phase shift, which is important when using it with an array. And tuning the phase is very not linear (and after a while its contacts wear out). Very unsatisfactory. It makes tuning by hand very time consuming.
A better Phaser is needed for this job.
An other way to go is by using 1 receiver per loop/antenna and combining the signals using Digital Signal Processing (DSP). The receivers have to be synchronous. Preferably the gain ratios (AGC) have to stay constant. The only practical way is by putting the AGC in the DSP or putting the DSP in the IF. However using the FT1000D and DSP as postprocessing give very useful results!
Very promising is the Flex-radio (http://www.flex-radio.com/). There is no AGC in de analog part and you can make almost identical receivers.
DSP makes it possible to find automatically the best way of combining the signals. And faster than you can do it by hand. And in principle in more ways than the MFJ-1025 can.
It can not only reduce the QRM, but also improve diversity reception.
Last update: October 28, 2006