A wideband balun from coaxial line to TEM line

Present requirements for ultra wideband systems involve the design of antennas having bandwidths of 1 to 4 octaves. Suitable directive antennas are TEM horns but the connection between the input line to a TEM horn and the associated electronics must be designed with care. The ideal input arrangement is an end-fed connection from input coaxial line to a parallel plate line which flares into the TEM horn. This arrangement requires a balun (balance-to-unbalance transformer) and a balun design transforming directly from coaxial line to TEM line is desirable. A design approach used was to transform directly from a coaxial line to a TEM line based on a variant of the technique of Duncan and Minerva (1960) for the first half of the balun and then using a method of moments technique to determine the impedance of the rest of the structure. The length of the complete balun is λ/4 at the lowest frequency. The disagreement between the design specification and the measurements is less than 2 ohms over the balun length in spite of the necessity to include strengthening ridges in the design. (5 pages)