V. A. Yatskevich, A. N. Mikirev

In this paper the log-periodic antenna (LPA), сonsisting of metallic strip on a dielectric substrate was studied. The substrate consists of tow layers, and between the layers there is the strip which together with two other strips, located on external sides of a base sheet, forms a three-wire transmission line. Electromagnetic simulation has shown, that printed LPA of small electrical sizes provides a small return loss and radiation patterns of a stable shape in frequency band 6:1. Antenna of such kind has the following advantages as compare to usual printed LPA: - lack of a cable which is usually built along one of strips, enables to manufacture the antenna by means of printed technology even for centimeter wave band; - high input impedance of the line to which the arms of dipoles are connected, ensures lack of parasitic resonances, inherent to usual LPA; - the principle of operation of the antenna is such, that a high input impedance of the line, is recalculated on a coaxial antenna port with a transformation ratio 4:1, that results in an average value of an input impedance of 50 Ohm. The main distinction between LPA and Vivaldi antenna is that for the last the gain increases with increase of frequency, the beamwidth in both planes decreases, side and back radiation levels increase. For LPA these parameters depend on frequency a little. At the same time, LPA has the important design advantage: it does not require a matching device which designing usually hampers the creation of Vivaldi antennas. There are, at least, two justified engineering solutions, permitting after some modification to use the log-periodic antenna for ultra-wideband signals radiation.