Abstract :
In order to couple up to twelve 250 watt V.H.F. Transmitters to a single antenna, it was necessary to design an antenna which could carry the combined power of 3 kw and have a low VSWR over the 150-175 Mc band. The main qualities required of this antenna were good pattern and impedance bandwidth with a gain of at least 6 db over a dipole, excellent RF connections for low intermodulation and high efficiency, solid dc grounding for lightning protection, and rugged construction for long life under all probable weather conditions. The bandwidth and gain requirements were met by using four broadband colinear coaxial dipoles with a centre-to-centre spacing of one wavelength. These dipoles were fed in parallel with equal lengths of teflon insulated coaxial cable. This particular feed system, which employs only two matching sections, was chosen for its simplicity, from several types all of which met the basic requirements. The use of conical chokes between the dipoles kept the side-lobe level to about ten db below the main lobe. Without these chokes the side-lobe level would have been unduly high, which is often the case with full wave spacing, so that a closer spacing with more radiating elements would have been necessary. This would have resulted in a more complex feed system and a much costlier product.
