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P3-D Antenna Design

Fabrication Responsibility:
Stan Wood, WA4NFY (USA)

AMSAT Phase 3D Antenna Design Review

Table of Contents

Introduction

This is a review of the antenna systems used on the AMSAT P3-D satellite. This satellite is the latest addition to the amateur satellite program. The satellite has 11 antenna arrays covering frequencies from 4 MHz to 24 GHz. The satellite has receivers and transmitters on 12 different frequency bands with transmitter power levels as high as 300 watts. All the antennas are mounted on a satellite less than eight feet in diameter. Multiple receivers and transmitters are expected to operate at the same time. A major effort has been required by AMSAT team members both in Europe and the US to design, build and test the antenna systems for P3-D.

Antenna Design

At apogee (48000 km high) the earth is 13 deg wide, all stations in view of P3-D will be within 6.5 deg of the center of the satellites antenna pattern. With an antenna gain of 15 dBic the gain for all stations in view should not vary more than 2 dB. At perigee (5000 km high) the earth is 68 deg wide and stations with low elevation would have squint angles of 39 deg. This means that stations with low elevation would see a loss of satellite antenna gain of more than 15 dB. With the 20 dB decrease in path loss as the spacecraft descends stations will see an increase of 5 to 29 dB depending on their quint angle. The real test is to design antennas that will maintain the same or better signal levels for all stations at perigee. This requires antennas with slightly lower gain (15 dBic) and very smooth patterns at wide pointing angles.

Spacecraft Limitations

All the high gain antennas are placed on the top of the spacecraft. The principal radiation direction is along the +Z axis, the center line of the spacecraft, toward the top. This top area amounts to 3.7 sq M. Antenna height limitations on the top of the spacecraft were increased from 75mm to 330mm. This was done by moving the 400 N. motor from the bottom to the top of the spacecraft. While this increased our selection of possible High Gain antenna types, it placed the motor right in the center of the antenna arrays. This change also raised concerns of the motor's effect on antenna patterns and the heat radiated during motor firing. The 145 MHz and 437 MHz antennas were redesigned to fit around the motor and to resist the radiant heat from the motor. Computer antenna models show little effect from the motor.

The bottom or -Z surface of the spacecraft is now clean except for the Arc-jet motor and the three separation studs. This is where we are mounting the Omni Antennas. We have a large flat groundplane with a height limitation of 140mm. We are presently testing an array of flexible groundplane antennas. Omni antennas will installed for 146 MHz, 435 MHz, and 1269 MHz.

The launch of AMSAT P3D into an initial transfer orbit and it's final placement into a 16 hour Molnia orbit places multiple requirements on the antenna system. Initially P3D will be spun on the Z axis to maintain stability. The Z axis will be oriented 90 degrees to the major axis of the orbit and in line with the plane of the orbit. This means that the Omni antennas must be used for the initial orbits. Table 1 shows the link variables encountered in this orbit.

         Table 1, AMSAT P3D Orbit Parameters

 Orbit hr   Phase      Range       Path Loss    Earth <
  8 hr       128      49800 km     -185.5 dB     13.0 degrees
  7 hr       112      48900 km     -185.4 dB     13.5 degrees
  6 hr        96      46600 km     -185.0 dB     14.5 degrees
  5 hr        80      43000 km     -184.3 dB     15.0 degrees
  4 hr        64      38000 km     -183.2 dB     17.0 degrees
  3 hr        48      31500 km     -182.6 dB     20.0 degrees
  2 hr        32      23700 km     -180.2 dB     25.5 degrees
  1 hr        16      15800 km     -177.9 dB     38.5 degrees
  0 hr         0       5000 km     -165.5 dB     68.0 degrees
This table shows the orbital Phase, Range, Path Loss at 1269 MHz, and the width of the Earth as seen from the spacecraft (Earth <). The maximum pathloss is -185.5 dB at apogee (Phase 128) with a minimum pathloss of -165.5 dB at perigee (Phase 0). The path loss varies 20 dB on all bands over an entire orbit.

When the spacecraft reaches the final orbit it will be despun and three axis stabilized using reaction wheels and magnetic torquing. The solar panels will be deployed increasing the power available by a factor of three. The spacecraft will rotate to point the +Z surface at the Earth for the entire orbit. All high gain antennas are fixed in the +Z direction. Antenna pointing is done by positioning the entire spacecraft during the orbit so that the high gain antennas are Earth pointing. The omni antennas will be used when off-pointing is required for firing the arc-jet motor and in case of loss of attitude control.

Omni Antennas

The Omni Antennas are becoming more important on P3-D as their use during critical maneuvers of the spacecraft is studied. All of the critical motor firings of both the 400 N. motor and the Arc-Jet motor require use of the Omni antennas. A review of (Fig 1.) shows that all 400 N. motor firings are made with the +Z surface P3-D off pointed 90 degs to the major axis of the orbit. To increase the inclination or change the orbit period with the the Arc-Jet motor also requires the reorientation 90 degs from Nadir Pointing. This means the Omni antennas are Mission Critical for the entire life of P3-D.

Omni antennas are provided for 146 MHz, 437 MHz, 1269 GHz. The Omni antennas are mounted on the -Z surface and are mounted 250mm behind the Arc-Jet motor. The Arc-Jet motor is mounted 78mm forward of the center of the bottom of the satellite and protrudes about 140mm from the this surface. This was done to place the Arc-Jet motor on the center of gravity with the solar panels deployed. The three 8mm separation bolts are located 50mm in from three corners of the spacecraft.

The Omni antennas are all ground mounted 1/4 wavelength stubs. They are arranged to form a 5 element tri-band omni-directional vertically polarized array. The array consists of a center 1/4 wavelength stub for 146 MHz and a pair of stubs for both 437 MHz and 1269 MHz. The 437 MHz stubs are spaced 28mm from the center element and the 1269 MHz stubs are 22mm from the center element. The 437 MHz elements are mounted on opposite sides of the center element and the 1269 elements are mounted in front and behind the center element.

Hi-Gain Antennas

For 29.4 MHz the antenna (fig 3.) is a two element ground mounted Yagi. This "ZL Special" is the same type of antenna used on Oscar-13 for 145 MHz. The antenna consists of a 1/4 wave whip with a single director. The director is 2100mm long and mounted on the -X edge of the top +Z surface points in the -X direction and is canted up +Z 30 deg. The driven element is 2670mm long and mounted on the -X edge of the bottom -Z surface and points in the -X direction. Both elements are constructed of 13mm flexible Tape Measure Stock. The driven element feed impedance is 50 ohms and is fed with 50 ohm coax and fed against ground. No matching system is required. This antenna gives a gain of >4 dBi and its pattern (figs 4,5.) is close to ideal at perigee.

For 145 MHz a 3 element 750mm diameter circular array of folded dipoles mounted 150mm above the +Z surface was selected. The elements are 980mm long and constructed of 10mm diameter .4mm wall silver plated brass tubing. The max gain is 12.2 dBic with 12.0 dBic at 13 deg beam width and 8.7 dBic at 68 deg beam width. At apogee the ripple in the pattern at the lem (edge) of the earth is < 1 dB and at perigee it is < 2 dB. A trimmer tab is located on the support post under the ends of each element for fine tuning after installation.

The 145 MHz feed system consists of 3/1 power divider feeding three 50 ohm delay lines cut to give 120 deg phasing between elements. The 1/4 wave power divider is constructed of 14mm dia .4mm wall silver plated brass tubing and an 8mm dia .4mm wall inner conductor. The impedance is 29 ohms and will match the three feed lines to a single 50 ohm line.

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 0.115wavelength 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 8.5Ÿ8.8dBic. With this level of element performance, useful arrays can be formed with six or seven elements, providing overall gains of 15Ÿ19dBic, 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.

The 1.269 GHz antenna is a Short Back Fire (SBF). This antenna is two wavelengths in diameter and has a 1/4 wavelength high outer ring with a 1/2 wavelength high post in it's center supporting a turnstile at 1/4 wavelength high and a 1/2 wave circular reflector. The antenna has a gain of 15 dBic and has a very smooth pattern. The antenna fits well on the spacecraft and is within the Negotiable volume set by ESA. This antenna also had the maximum gain per unit of area for any antenna tested.

The 2.401 GHz antenna is a 500mm dish with a gain of 18 dBic. The first prototype was built by K5SXK and weighs in at 1.3 Kg or 3 pounds for the rest of us. The feed is a turnstile backed by a reflector. K5SXK builds space qualified antennas in his line of work and expects to deliver a space qualified antenna ready for our coax connector. Nice Work.

The 5.6 GHz antenna could be a 250mm dish or a multi-element array. W3TMZ is working on a 5.6 GHz receiver and antenna array. The antenna would be a low profile design be 250mm in diameter. AMSAT has received a 250mm spun aluminum dish from a group in Belgium. It weighs 175 grams an would have a gain of 20 dBic.

The 10 GHz antenna is now a single 20 dBic circular horn. OH7JP and his group from Finland are well along on their design. The original design using four horns with a separate amp on each horn has been changed to a single horn with multiple amps and a waveguide feed.

Analytic Studies

In this effort, all orbital parameters have been determined using Franklin Antonio's InstantTrack and antenna parameters using Brian Beezley's (K6STI) MNC 4.0 and Antenna Optimizer 5.03 antenna analysis software. InstantTrack provided the orbital RF range, path losses and Earth size information. We have found that the MNC and AO analyses to be very useful for this effort, although the package was designed analyzing for wire antennas, and is definitely not designed to evaluate slot antennas, per se. "Substitute" equivalent wire antennas were used with MNC and AO to achieve our analytic goals. All antennas were evaluated over an infinite ground plane, which is less of an approximation with the higher frequencies. We feel that despite all of these analytic limitations for analyzing our highly specialized satellite antennas, the Beezley software has performed superbly and permitted us to achieve given some very good understandings of our Phase 3D needs.

Antenna Testing

Antenna testing is now being done at the AMSAT Integration Facility in Orlando Florida. A full scale and 1/3 scale model of the Phase 3D spacecraft have been constructed to permit the testing of the antennas. Antenna patterns of the six element 435 MHz array have been plotted using 1.2 GHz test range. A 1/15 scale model was used to evaluate the 29.4 MHz antenna. A 1/5 scale model is being used to test the Omni antennas and Patch Array.
Last updated: Dec 26, 1995
by Ralf Zimmermann, DL1FDT