Electrically small D. F. antenna

Of prime importance in evaluating the usefulness of a direction finding antenna are the pattern characteristics obtainable over the frequency range of interest. In general, the most desirable direction finding antenna is one which offers the narrowest beamwidth in the plane in which direction is to be determined, since this narrow beamwidth lends itself well to accuracy and ease of operation. However, where physical size is severely limited the well known inverse relationship between an antenna's size in terms of wavelengths and its beamwidth precludes the use of a narrow beam antenna. In applications where size is thus limited alternative approaches must be considered. A direction finding antenna has been designed whose size is restircted to a cylindrical volume approximately.15 \lambdain diameter and.35 \lambdain length at the lowest operating frequency. The antenna consists of a metallic cylinder containing two diametrically opposed slots. These slots are polarized at an angle of 45 degrees from vertical, hence the antenna is equally responsive to both vertically and horizontally polarized signals. Further space conservation is realized by packaging the drive and control mechanisms associated with the antenna within the slotted cylinder antenna structure. In operation the antenna functions in a manner similar to an Adcock array, that is the slot elements are 180 degrees out of phase. Since this relationship is achieved by simply "inverting" one of the elements it is independent of frequency, and the antenna's pattern characteristics remain constant. The antenna's bandwidth is determined then by its impedance characteristics and is suitable for receiving application over a 2 to 1 frequency band. This slot arrangement offers a figure eight type pattern for both polarizations of interest. The nulls of these figure eight patterns are sufficiently well defined and aligned to offer excellent direction finding capability, but with an inherent 180 degree ambiguity. This ambiguity is resolved by making use of the unidirectional properties of a cylindrical antenna containing a single slot. The antenna is transformed from the double slot array to the single slot configuration by means of a simple R.F. switching arrangement. The unidirectional pattern thus achieved from a slotted cylinder of limited size is not in itself adequate for accurate direction finding. However, when used as a sense pattern in conjunction with the aforementioned figure eight pattern quite accurate direction finding capability is achieved. The antenna possesses several features advantageous from a mechanical point of view. Among these are the low weight and the relatively small moment of inertia of the rotating members. Thus the use of a fractional horsepower drive motor and light-weight drive mechanism is permissible. The theory of the antenna's operation is discussed as well as the effect environment on its electrical characteristics. Theoretical and experimental data is also presented.