Shipboard shortwave direction finders

Different types of antennas for shipboard installation are discussed. One of the main obstacles is the permissable size of the antenna and the impact from the ships structure. The sensitivity of some different types of commercial antennas is studied and related to the path loss at sea and expected signal levels. It is noticed that small antennas such as crossed loops have approximately 20 dB less sensitivity than large circular arrays. DF antennas of the type crossed loops are useful for vertical polarization but will give severe errors for horizontal polarization with elevation i.e. skywave. Vertical polarization is dominant at sea since ground wave signals with horizontal polarization will suffer heavy attenuation. Nevertheless skywave with horizontal polarization will be present even at short distance, which was verified. DF errors as a result from instrumental errors, reradiation and coastal refraction are briefly discussed.