|Image of the Self-complimentary 4-arm Archimedes spiral antenna.|
At first glance, the 4-arm spiral is only a 2-arm self complimentary with 4 arms - but if you look closely it is a lot more than that. The addition of 2 extra arms improves the axial ratio, but it also introduces some extra capabilities. The 4-arm spiral can operate very much like the 2-arm spiral (often referred to as sum mode, or mode 1 operation), but a difference mode (mode-2) is also possible.
By using the correct feed network and signal processing, each arm of the 4-arm spiral can be fed 90 degrees out of phase, allowing mode-1 operation which produces a radiation pattern with a maximum in boresight. When feeding each arm 180 degrees out of phase mode-2 operation produces a boresight null. The 3D radiation patterns produced by these two operating modes as shown in the image below....
|Typical mode-1 and mode-2 RHC gain patterns across the operation band.|
A beam forming network is often used to combine these two modes of operation vectorially, to form Σ + Δ, Σ - Δ, Σ + jΔ, Σ - jΔ corresponding to four squinted beams. This method is popular in tracking radar applications where the antenna can be steered in the right direction when the target is moving away from the center null into one of the 4 quadrants.