Masters Superturnstile antennas consisting of stacked batwing element bays are the most popular horizontally polarized omni-directional antennas used by VHF television broadcasters. When properly constructed, such antennas can achieve a VSWR of 1.1:1 or less over a 30 percent bandwidth (Jasik's Antenna Engineering Handbook reports 35 percent), along with substantial amounts of gain. With the advent of HDTV, superturnstiles are starting to become popular with UHF broadcasters who must share antennas with several different transmitters simultaneously. The wide bandwith and outstanding performance possible with this type of antenna make it attractive for ATV repeater use.
The following graphic shows a batwing element with dimensions for a center frequency of 434.5 MHz (which should cover 370 to 500 MHz with a VSWR of 1:1.1 or less). All dimensions shown are in inches. Multiply by 2.54 to obtain dimensions centimeters:
A balanced 70 ohm feedpoint is available in the center of the element (Jasik reports approximately 75 ohms). The vertical member of the element runs parallel with the support mast at a distance of 0.68 inches. For an omni-directional pattern, two batwing elements are mounted perpendicular to one another, and fed 90 degrees out of phase. In order to achieve greater vertical directivity than that possible with only a single bay, multiple bays may be stacked vertically at a center-to-center distance of approximately one wavelength (27.17 inches):
The total length of the batwing structure is about 17.5 feet, and theoretical gain is close to 10 dBd.
Mr. L. B. Cebik, W4RNL, noted expert in the field of antennas and antenna modeling, has recently published an extensive analysis of the performance and basic properties of Batwing antennas. It is a must read for anyone interested in Batwing antenna construction and operation. It can be found at Mr. Cebik's web site.
Although batwing elements are typically mounted through the vertical support mast, one idea that would eliminate drilling (and weakening) the support mast is to mount batwing elements individually on separate masts that are clamped to the main support mast. A commercial antenna that uses this type of construction is the Scala (now Kathrein) TVO VHF-TV turnstile array:
Our mission is to construct a commercial-quality, 8-bay Masters Superturnstile worthy of being top or side mounted on a commercial radio tower. Design goals include:
Since it will be impractical (and highly undesirable) to service this antenna once it has been installed, it must be designed properly to last many years from the start. Questions still remain about the following issues, and any assistance would greatly be appreciated.
Incidentially, commercial antennas meeting these specifications easily cost tens of thousands of dollars! Our current working budget is considerably less than that. :-)
Help, suggestions, catalogs, engineering data, and advice are welcome and should be directed to John, KD2BD via e-mail (kd2bd AT amsat DOT org). Thanks to all those who provided valuable input thus far.
Dimensions and design information for the Masters Superturnstile were obtained from Antennas, by John Kraus, W8JK, Second Edition. Some additional information was obtained from a Dielectric Communications catalog, and Jasik's Antenna Engineering Handbook.