Improved feed system design for a wideband dipole array

A high power, wideband HF phased array is being constructed as part of the High Frequency Active Auroral Research Program (HAARP) in Galkona, Alaska. Extensive testing of a demonstration prototype (DP) version of the array discovered undesirable feed resonance effects at discrete frequencies and excessive tower currents under simulated scanned conditions. Analyses and subsequent measurements pointed to even mode excitations of the dipole as the cause of these difficulties. To solve this problem the feed system was modified with the goal of controlling both the even and odd mode excitations. A novel wideband high efficiency hybrid-balun and balanced AMU (antenna matching unit) was designed and tested confirming the superiority of the new feed arrangement. A low-band version is being incorporated into the DP testbed and extends the low-band performance by 20%, with lower loss and reduced tower currents. The feed design is further evaluated by calculating the average efficiency of the array system, which is within 0.25 to 1.5 dB of its theoretical maximum, depending on frequency.