A NEW OPTIMIZED DOUBLE STACKED TURNSTILE ANTENNA DESIGN

Open Access
Author:
Alkhatib, Mohamed
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 22, 2016
Committee Members:
  • Mohamed Redha Mohamed Shareef Ahme Alkhatib, Thesis Advisor
Keywords:
  • Radiation Pattern
  • Maximum Gain
  • Minimum Gain
  • Turnstile Antenna
  • VSWR
Abstract:
The Turnstile Antenna is one of the many types of antennas that have been developed to be primarily used for omnidirectional very high frequency (VHF) communication. The basic turnstile consists of two horizontal half-wave antennas (half-wave dipoles) mounted at right angles of each other on the same plane. When these dipoles are excited with equal currents that are 90 degrees out of phase, the typical figure-eight radiation pattern of the two diploes are merged into an almost circular radiation pattern. Typically, however, the gain of such an antenna is not very high in the horizontal direction, thus to increase the gain in the horizontal and to eliminate the gain in vertical direction, pairs of the same dipole antenna are stacked vertically and are separated by a distance, thus creating the Stacked Turnstile Antenna. In the original stacked turnstile antenna design, both 50 ohm and 75 ohm cables are used to connect the dipoles, using quarter wave impedance matching transformers, which leads to inaccurate radiation patterns and a higher VSWR. In this paper, we are addressing this issue by introducing a new design using only 75 ohm cables to achieve a more proper impedance matching and omnidirectional pattern. By using this approach, we are introducing a double stacked turnstile antenna that has a more circular radiation pattern, and a low VSWR at the frequencies of desired operation. Rigorous computer modeling and simulations codes were used for designing and evaluating the antenna. Furthermore, five models of this antenna were made, three computer models and two experimental models: an aluminum tubing model at 50.1 MHz which was done by Dr. James K. Breakall, a wire model at 700 MHz, and a cylindrical model at 700 MHz modeled by the author. All of these models were optimized using the Nelder-Mead Simplex method that FEKO provides. Two experimental models were also built, a scaled down antenna operating at 700 MHz (Mohamed’s Antenna) and a regular antenna operating at 50.1 MHz (Dr. James’s Antenna), where in both cases, the results agreed with the simulated models.