Comparison of the predicted performance of typical HF and VHF Yagi-Uda antennas with the theoretical limit of all their elements being driven

The Yagi-Uda or Yagi antenna typically consists of 2 to about 20 half-wave dipole elements at HF or VHF. The Yagi array consists of a single driven element with all other elements being parasitic. Usually there is only one reflector element on one side of the driven element with the rest of parasitic elements being directors deployed in-line on the other side of the driven element. The theoretical maximum gain of any n element close-spaced uniform array antenna is achieved when the element excitations are such that n-1 nulls are formed in positions that maximally suppress radiation in all directions other than the main beam. If the elements remain sufficiently close spaced the gain is found to essentially unaffected as the element spacing is varied keeping the overall length of the antenna the same and the null angles the same. This paper considers the case of linear arrays with uniform element spacings only and assumes that the results obtained are well representative also of the non-uniform linear arrays with the same null angles and the same overall length. For comparison with the Yagi, which has an end-fire pattern, the driven element arrays are always adjusted to give the main lobe in the end-fire direction. This paper confirms that the maximum directivity is found when the antenna elements are infinitely close together, and this corresponds to maximum super-directivity with a very small matched bandwidth. The pattern is however retained over all frequencies for which the antenna has close-spaced elements. The purpose of this paper is to compare existing measured and simulated Yagi patterns and forward gains (directivities) with those for the equivalent all driven linear array