Understanding beamforming is actually quite simple. It used to be about analog delay lines. Now it’s just a matter of complex math in your computer.
Beamforming is all about timing. Consider an array of at least two antennas, spaced perhaps λ/2 apart. Distant signals arrive at each antenna at different times. The timing differences depend on direction of arrival. If the distant signal is broadside (perpendicular) to the array, it arrives at each element simultaneously. On the other hand, if the signal is coming in-line (end fire) to the array, the timing difference is highest.
By measuring the timing differences, you can use simple trigonometry to determine the direction of arrival. So much for directional receiving.
On the transmitting side, if you vary the timing of when your transmitted signal arrives at each element of the array, you can force the signal strength to be higher in certain directions. This is the essence of beamforming. Local AM broadcasters started using phased arrays to create directional patterns in the 1930’s. They still do. Beamforming has been at the heart of sonar and radar since the 1950’s.
Historically, beamforming was analog. You simply connect each array element using a different length of cable, causing timing delays. If you do it right, the timing delays cause the array to favor certain directions over others. When beamforming started to go digital, binary codes were used to switch in various delay lines to steer the antenna pattern. An early digital delay controller from Lincoln Laboratories is shown in the above picture.
Today, beamforming can be fully digital. If you stop and think about it, the antenna elements are indifferent to whether signals are phased in hardware (transmission lines, matching circuits, hybrid couplers) or software.
Understanding beamforming is easy. Configure timing delays to each element in your array and you can adjust direction of the beam, including the peaks and nulls of the pattern. You can do this with two elements, or hundreds. You can do this with radio antennas or microphones. Oh, and in most cases, beamforming works the same for receiving and transmitting.
So, there’s a simple explanation without any math. This video description may help, too.
Understanding Beamforming – Why Spatial Filtering?
There are two important methods that can be implemented by SDR that incorporate Spatial Interference Filtering Techniques. One is software beamforming and the other is digital anti-phase cancellation.
If you are using a dual channel SDR, with each channel connected to a different antenna, you have the ability to do spatial filtering. By adjusting phase and amplitude between the two receivers, you can get some directivity. In particular, you can place a null towards the local RFI source. Medium wave DXers can also use this technique for directional phasing to separate multiple signals on the same channel.
Taking things a bit further, with proper antenna selection, you can phase reverse the RFI interference on one receiver, and subtract it from the signal plus noise signal on the other receiver. This is how the ANC-4 and MFJ1026 devices work. Tomorrow, you will do this in your radio, rather than an external box.
A final benefit is spatial diversity reception. If only all our SDR radios contained dual-channel coherent receivers!
Coming up next, we will talk about how almost everyone is using spatial filtering except us!