This invention relates to serially connected loop and helical radiating elements placed over a conducting screen or sheet to form panel antennas and more particularly to such antennas for FM and TV circularly polarized broadcast applications.
As used herein, circularly polarized antennas refer to the general class of elliptically polarized antennas with low axial ratio.
Loop antennas with standing wave current distribution have been used for several decades for direction finding applications. The polarization of these antennas is usually vertical with respect to the earth's surface and the azimuthal radiation pattern is a "figure eight."
Other types of loop antennas are those employing traveling waves. It is known that a loop with a traveling wave and one wavelength circumference will produce circular polarization with good patterns. However, in order to achieve a traveling wave in practical manner the loop must be properly terminated as, for example, in a resistor. This results in low efficiency. U.S. Pat. No. 2,247,743 describes a balanced loop having a transmission line connected to one side of the loop and a resistive termination connected at the other side. The traveling wave voltage or current distribution in the loop at mirror image points on each side of the loop have equal magnitudes but are 180 degrees out of phase. The azimuthal pattern of the antenna is unidirectional in the back-fire direction. This type of antenna is for reception of horizontally polarized television signals.
U.S. Pat. No. 2,501,778 describes a traveling wave loop antenna with a resistive termination equal to the surge impedance placed over the earth for the transmission or reception of vertically polarized waves. The diameter of the loop is several wavelengths. For this type of antenna there is little radiation in the broadside direction and the polarization is not circular or elliptical.
Helical antennas have also been used for several decades. Kraus, Proceedings of the IRE, page 263, 1949, describes a single wire helix which can be operated in the circularly polarized normal mode or axial mode. In the axial mode, the turn length is considerably less than a wavelength at the operating frequency so that it produces a doughnut shaped radiation pattern. In the beam mode, the turn length is about one wavelength at the operating frequency and the radiation pattern is endfire.