A new tapered double balun for spiral-mode 2

The tapered balun (J.W. Duncan and V.P. Minerva, 1960) is an excellent feed for very broadband, balanced antennas, such as spirals and sinuous antennas. Its bandwidth is limited only at the low-frequency end, which can be made arbitrarily low by making the taper longer. Furthermore, it can be easily designed and fabricated in an inexpensive printed circuit medium. A four-arm spiral antenna that operates only in mode 2 (R.G. Corzine and J.A. Mosko, 1990) really has only two electrical terminals since opposite arms theoretically can be connected together. However, practically, it is not obvious how to achieve this connection in a rugged and inexpensive way. A clever tapered coplanar-line balun has been reported (E. Gschwendtner, 1999) but a cavity will interfere with it, and the authors do not discuss how to make it balanced at the feed point. This paper describes a new tapered double balun that solves these issues and provides an improved technique to excite mode 2 for four-arm spirals.