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Circular Polarized Patch antennas

Some good information on the 435MHz Circular Polarized Patch antennas on AO40 from http://www.amsat.org/amsat/sats/phase3d/antennas.html

For 435 MHz an array of 6 circular polarized patch antennas has been selected. The antennas are mounted directly on the top cover panels of P3-D. The elements are bonded to a 13mm thick Kevlar Honeycomb which is bonded to the top skin of the spacecraft. There are 6 skin panels with a separate antenna element on each panel. The patch elements serve as a structural stiffener for the top panels. The spacing of the array was reduced to fit on the new spacecraft structure. This reduced the gain to 15 dBic but allowed the removal of the old center element. With wider spacing the center element was required to maintain a clean antenna pattern. This allowed the 400 N. motor to be mounted on the top of the spacecraft.

Patch antennas can be described as a thin square of conductor material, approximately 1/2 wavelength on a side, closely spaced above a larger reflector plane. The center point of this active element may be grounded using a vertical conductor. While not exact, this patch radiator can be thought of as a pair of stacked dipoles, spaced by 1/2 wavelength and backed by a reflector. The more correct description of the antenna is that it is a pair of 1/2 wavelength slot antennas, spaced 1/2 wavelength. Active element feed points can be from the center of an edge, or inward with corresponding impedance variations. Patch antenna active elements may be of almost any shape, including round, rather than square. When properly feed, patch antennas operate with good circular polarization (CP) radiation performance. In the CP operation, all edges of the patch are actively included in the performance "equation".

When antenna users come in contact with the concept of the patch antenna, it is often in the context of those versions constructed with printed circuit board (PCB) materials and employing microstrip feed techniques. While such methods permit some rather impressive arrays to be constructed, the efficiencies encountered are often as low as 50 percent, principally due to dielectric losses and reduced element size caused by the material dielectric constant. Patch antennas do not need to employ high dielectric materials in their construction, hence the references herein will be to "patch" rather than "microstrip" antennas, and will use air (or space vacuum) as the dielectric.

Our tests have shown that the construction of these patch antennas requires some careful attention to fabrication methods and dimensional details. We have found that square patch active elements need to be approx. 0.47 wavelength on each side, while the round versions are 0.540 wavelength in diameter. Close control of element size is important. Another important dimension is that of the spacing of the element from the reflector, >0.01 wavelength. Spacings of less than 0.01 wavelength result in reduced efficiency.

With the construction methods being employed, the feed impedance characteristics are somewhat different than those of the PCB microstrip antennas. 50 ohm feed points have been found to be located at 0.078 wavelength from the center, while a 100 ohm feed point is located at 0.115ÿwavelength from the center. These are good dimensions to know, as a simple quarter wavelength coupling line of UG141 coaxial cable connected between quadrature 50 Ohm points, and with the main feed located at a 100 Ohm point will result in an overall 50 Ohm feed impedance and a circular radiation pattern for the patch. All coaxial cable feeds are terminated with the outer conductor connected to the reflector plane and the center conductor connected to the active element. The ends of the coaxial cable are located perpendicular to the reflector plane, and the outer conductor should be extended to within a close proximity to the active element.

The antenna gain of single element patch antennas, constructed as described, have been measured to be in the range of 8.5Ä8.8dBic. With this level of element performance, useful arrays can be formed with six or seven elements, providing overall gains of 15Ä19dBic, depending upon element spacing.

Rules for Patch Antennas
1. Use only air dielectric. Air (or space vacuum) has the lowest loss and a dielectric value of unity. A dielectric constant, E=1.0, makes the patch element full size which gives maximum gain. Teflon with a E=2.45 reduces the size to 64 percent and with no loss reduces the maximum gain by 3 dB. This is caused by the wider beam width of the smaller patch. 
2. Mount patch higher not lower. The height of the patch above the groundplane should be approximately two percent of the width of the patch. With air dielectric a half wave square patch should be a minimum 0.01 wavelengths above the groundplane. Lower heights result in higher Q and high currents resulting in higher losses. 
3. Design for maximum bandwidth. The bandwidth of a patch antenna is direct function of it's height. The limiting factor is mutual H-plane coupling in a close spaced planner array. The higher the elements the greater the spacing required between elements. Minimum edge spacing for 20 dB isolation between elements is 0.12 wavelength for a height of 0.04 wavelength. 
4. Use coaxial not stripline feed. Patch antennas and striplines are not compatible on the same dielectric material. Strip lines prefer a high dielectric substrate and minimum height to work properly. 50 ohm strip lines also require a 1/4 wave transformer to match the edge of a patch. 

With these rules in mind a 0.435 GHz six element circular patch array was designed for the Phase 3D Spacecraft. They are supported by a central grounding post and a dielectric honeycomb under each element. Each element is operated in a RHCP mode. Element center-center spacing is set at 0.69 wavelengths (470mm) and is limited by the size of the available top plate area of the spacecraft. The original, and most basic of these six element arrays is a hexagonal pattern. With equal power to all elements, the array is set for maximum gain. All elements are fed in phase and no phase changes are required.

Don Woodward
AMSAT 33535


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