K. K. Belostotskaya – Ph.D. (Eng.), Leading Research Scientist, AO «OKB MEI» (Moscow)
I. V. Bel’kovich – Post-graduate Student, Moscow Power Engineering Institute; Head of Laboratory of Onboard Antennas and Antenna Arrays, AO «OKB MEI» (Moscow)
V. N. Seleznyov – Post-graduate Student, Moscow Power Engineering Institute; Head of Department of Terrestrial Antenna Systems, AO «OKB MEI» (Moscow)
P. G. Chuenkov – Senior Research Scientist, AO «OKB MEI» (Moscow)
It is often true that when designing space radio links for reliable communications with a low-orbit spacecraft, there is a problem of development of omnidirectional antennas with circular polarization. At the same time rigorous requirements can be set for the dimen-sions of an antenna. There are several antenna types for such systems. The simplest design which has good characteristics and flexible tuning capacity is based on a dipole turnstile antenna. However, a traditional dipole antenna above a screen usually has a large size. We research the potential of turnstile antennas for which the height above the screen is significantly smaller than the wavelength, with a high ellipticity coefficient, which is almost constant in the entire solid angle.
We investigate antenna designs based on a dipole turnstile emitter with curved arms. The first antenna type is a dipole turnstile antenna above a screen with curved arms. The radiation pattern was expanded by lifting the arms of the dipole above the screen up to 0,35λ and by optimizing the geometry of the dipole arms. The half-power beam width is equal to 130 degrees; the radiation patterns in the E- and H-planes are identical. The antenna was matched by introducing a quarter-wave transformer and a short-circuiter. Excitation is performed using a balun device.
The second design is a dipole turnstile antenna on a dielectric hemisphere. The idea is to lay dipole shoulders on a hemisphere with a twist of shoulders. Due to the special twisted shape of the dipole shoulders it is possible to select a necessary length of the shoulders in order to obtain a required radiation pattern. The first step is resolving the matching problem. It was done by applying an original method of introducing additional reactivity by cutting the slot in the dipole shoulder. Also the diameter of the dielectric hemisphere is optimized according to the requirements of matching and the beam width of the radiation pattern. This antenna was manufactured and the characteristics were measured. The measured radiation pattern matches the calculated one. The great advantages of the turnstile antenna on the dielectric hemisphere is a small size (max. 0,16λ) and high values of the ellipticity coefficient; however, compared to the first dipole turnstile antenna with the curved shoulders, we lose the ability of radiation pattern customization.
Thus, if minimization of the dimensions is prior, then the best design is the turnstile antenna on the hemisphere. The height of the antenna above the screen is h = λ /6; the half-power beam width is 110 degrees. If some freedom is allowed for the screen and antenna sizes, the half-power beam width can be increased to 130 degrees when h = 0,35λ (the turnstile antenna with the curved dipole shoulders). In both designs, the ellipticity coefficient is not smaller than 0,8 in a half-power solid angle. Both designs are optimized and matched using electrodynamic modeling.