Several conflicting requirements, such as achieving two separate high-gain antenna patterns within a single compact module, increase the challenge of designing SDARS antennas.
Automotive satellite radios, specifically the Satellite Digital Audio Radio System (SDARS), place stringent requirements on the receiving antenna. SDARS employs a dual-transmitter broadcast format in which signals are sent from both satellite-based and terrestrial transmitters. The satellite transmission cover most areas, but are complemented by terrestrial transmitters when satellite coverage is blocked (such as by tall buildings in urban areas). Examples of the SDARS include XM Satellite radio and SIRIUS Satellite Radio in the US, providing customers with as many as 100 channels of MP3-quality digital radio service. Antennas for SDARS must be able to handle both types of transmissions with optimal receive performance.
Antenna modules for SDARS feature low-noise amplifiers (LNAs) and passive elements that receive low-power satellite signals and terrestrial signals. Currently, SDARS antennas are dual-arm antennas, consisting of two separate antennas, one optimized for terrestrial (TER) signal reception and the other optimized for satellite (SAT) signal reception. The TER element is typically a monopole, while the SAT element is a circularly polarized structure. Due to the requirements for extremely low noise figures, the LNAs are located directly below the passive antennas. The separate outputs of the two LNAs is connected to RF cables typically 15 to 20 feet in length; the cables are terminated in SMB connectors to interface with the SDARS radio equipment.
The basic electrical performance of SDARS antenna modules is summarized in the table. The SAT antenna employs left-hand circular polarization while the TER uses linear polarization. Type-approval antenna testing re- quires that the mobile antenna module be mounted at the center of a 1.0-m-diameter circular ground plane. Figure 1 shows a typical elevation pattern in two planes of an SAT antenna placed at the center of such a ground plane.1 Minimum antenna gain of +2 dBic is required for elevation angles between 20 to 60 deg. for XM, while minimum antenna gain of +3 dBic is required between 25 to 90 deg. elevation for Sirius.2 The TER antenna performance should be equivalent to that of a monopole, or −1 dBi antenna gain at an elevation angle of 0 deg. (the horizon).
Figure 2 shows the elevation pattern of a SAT antenna located on a vehicle roof, spaced 15 cm from the back roof edge.2 The pattern curves are not as smooth as the ground plane curves of Fig. 1 due to asymmetries. While on a vehicle, ideally the antenna elements must be positioned in a substantially unobstructed view of the satellites. The ideal location of a mobile antenna module is on the vehicle roof. Both SAT and TER elements of roof-mount antennas require a minimum of six inches from sheet metal edge to provide satisfactory antenna performance.3 Other antenna modules such as those for Advanced Mobile Phone Service (AMPS), personal communications services (PCS), and Global Positioning System (GPS), can be incorporated with an SDARS antenna in a common housing as long as the antennas do not interfere with each other. (For example, enough isolation should be provided between the PCS band at 1920 to 1990 MHz and the SDARS band at 2320 to 2345 MHz.)
Figure 3 shows a standard quadrifilar SAT antenna with a helix monopole TER structure located inside the quadrifilar antenna. The quadrifilar helix antenna consists of four helices spaced equally and circumferentially on a cylinder. The four helices are etched on a flexible substrate and wrapped in a cylindrical fashion. From much research,4-8 it is known that quadrifilar antenna performance is unaffected by the presence of the monopole inside. A feed network printed on a low-loss flexible substrate, along with the helix winding direction, helps achieve the left-hand circular polarization. To improve the return loss and radiation characteristics of the monopole antenna, the shield height below the antennas is much higher than that of a standard shield (typically 5 mm). This arrangement yields excellent antenna performance, nearly equal to that of a typical monopole antenna. The height of the antenna including the housing is approximately 95 mm.
Figure 4 shows a crossed dipole/monopole array combination. The assembly consists of a crossed dipole structure for receiving the circularly polarized satellite signals and an array of four monopoles for receiving linearly polarized terrestrial signals.9 The dipoles are etched on a low-loss substrate. While crossed dipoles have been around for several years and used extensively in mobile SAT communications,10,11 the novelty of this design is in the way the monopole array is arranged symmetrically about the cross dipoles. This symmetrical configuration yields good performance for both the SAT and TER antennas. Each monopole is positioned within each quadrant of the cross dipole. Each monopole is approximately 0.25 wavelengths in length. The four monopoles are connected to a standard corporate feed network. The two crossed dipoles are connected to a 90-deg. equal-power feed network etched on a low-loss substrate. This configuration yields the circular polarization required for SDARS. The SAT antenna provides excellent performance for elevation angles of 45 deg. or higher. The height of the antenna including the housing is approximately 40 mm.
Figure 5 shows that the SAT element of the quadrifilar/PIFA combination is a "folded" low-profile quadrifilar antenna while the TER element is a planar inverted-F antenna (PIFA).12,13 The "folded" quadrifilar antenna is essentially a standard quadrifilar antenna, where the helices in the top section are folded or bent back resulting in a shorter antenna. The helices are etched on a flexible substrate and then wrapped in a cylindrical fashion. The width of each line is 2 mm and each helix is matched to 50 Ω and connected to a miniature surface-mount feed network.14 The helix antenna height is 6 cm. The SAT antenna provides good performance between elevation angles 20 and 60 deg. However, the TER antenna radiation pattern exhibits a valley due to blockage by the quadrifilar antenna. The height of the antenna including the housing is approximately 70 mm.
Figure 6 shows this configuration's lens-loaded ceramic patch SAT antenna. The TER element is a quarter-wave monopole located in the center of the antenna structure and extends from the base through a hole in the patch. The lens helps increase the patch gain at low elevation angles (20 to 30 deg.) by increasing the antenna beamwidth. However, the antenna gain at high-elevation angles is reduced compared to a patch with no lens. Care must be taken in the placement of the monopole so that it does not interfere with the radiation pattern of the patch antenna, especially when high-dielectric-constant ceramic materials are used. The height of the antenna including the housing is approximately 40 mm.
Of the various antenna SDARS configurations discussed, the coupled loop/monopole combination is the most popular (Fig. 7). It is sold along with the popular XM Delphi SkyFi™ receiver. The SAT element is a coupled-loop antenna with perimeter length of approximately one-half wavelength. The TER element is a helix monopole located inside the SAT antenna without affecting the SAT design's performance. A feed network printed on a low-loss substrate helps achieve the left-hand circular polarization. It is similar to the feed network used on the quadrifilar/monopole combination design (Fig. 3). This arrangement yields good performance for both SAT and TER antennas in that the TER radiation pattern presents no asymmetries. The height of the antenna including the housing is approximately 30 mm.
Figure 8 shows an annular microstrip antenna consisting of a full-wavelength loop etched on a low-loss substrate. This type of patch antenna operates in TM21 mode and produces a conical radiation pattern. It yields good performance for elevation angles between 20 and 60 deg. but poor performance at higher angles (60 to 90 deg.), with a null at the zenith (90 deg.). For this reason, it is not suitable for Sirius applications. The TER antenna is a top-loaded monopole located at the center of the annular patch and at the same height as the patch. The SAT antenna performance is unaffected by the presence of the monopole. The TER radiation pattern is similar to that of a standard quarter-wave monopole with slightly less gain. The height of the antenna including the housing is approximately 15 mm and it is the lowest-profile XM antenna.
For applications where no ground plane exists, mast or ground-independent antennas are needed. These antennas are mounted on sedan or sport-utility-vehicle (SUV) window glass or on the mirrors of long-haul trucks. A quadrifilar helix antenna is a typical mast-type ground-independent SAT configuration for SDARS. In such a configuration, the four helices and feed network are printed on a low-loss flexible substrate and wrapped around a cylindrical tube (Fig. 9a).
The TER antenna should also be designed to be ground-independent. A natural choice for this element would be a dipole. The first SDARS mast antenna shipped was the TRK SR1 (Fig. 9b).15 It is a combination SAT and TER antenna comprising a quadrifilar helix antenna and a tubular dipole.16 The SAT coaxial cable runs substantially concentrically through the dipole without affecting the dipole's radiation characteristics. This arrangement effectively reduces coupling between the two elements and yields good performance for both antennas. The TRK SR1 design is unique in that both dc power and RF energy are transferred from the interior of the vehicle glass to the exterior surface. The coupling scheme utilizes two pairs of RF couplers (SAT and TER), and a pair of large coils which are part of the DC power (bias) circuitry. The LNAs are placed on the exterior glass surface, underneath the antennas, to maintain the low-noise-figure requirements. Technical challenges to this arrangement include oscillation due to the undesired coupling between the antennas, couplers, and LNA outputs, and interference on the AM radio caused by the DC transferring circuitry. These problems are avoided by correct LNA design, filtering, and proper antenna installation.
In an alternate implementation from the design of Fig. 9b, the dipole is replaced by a monopole that is printed on the same flexible substrate as the feed network. In this case, the monopole average gain is approximately −1 dBi at the horizon. This antenna element arrangement yields a shorter combination on-glass mast antenna and is used in both the TRK SR1X (a shorter version of the TRK SR1) and the TRK XM11 antennas (Fig. 9c).15
More recently, on-glass XM mast antennas have been introduced, incorporating RF coupling only (models XM8000F in Fig. 9d and XM8100F).15 These configurations utilize low-loss RF couplers for both SAT and TER elements. The SAT coupler loss is 0.5 dB, and this scheme avoids the potential oscillation and AM interference issues of the TRK SR1 and SR1X models. The trade-off, however, is an increase in noise figure of approximately 0.7 dB for SAT (0.5 dB coupler loss + 0.2 dB cable loss). The TER coupler is physically smaller than the SAT coupler, resulting in a 1 dB loss. The TER antenna for this design is a folded dipole located underneath the SAT antenna. Long-haul truck, recreational vehicle (RV), and marine antennas are mast antennas similar to the quad/dipole or quad/monopole combinations.15
To reduce the cost of the various antenna modules presented here, there is a need for developing single-arm antennas. These antennas consist of a single passive element and a single LNA configured such that good reception of both TER and SAT signals is achievable. Such modules exhibit vertical polarization properties along the horizon and circular polarization properties at higher elevation angles. Currently, the vast majority of all SDARS receivers have two antenna inputs (TER and SAT). Single-arm antennas can be equipped with an RF splitter in order to be connected to dual-input receivers. A good candidate for single-arm antennas is a microstrip patch antenna etched on a low-loss ceramic or substrate. The right choice of ceramic or substrate can produce acceptable TER and SAT performance. Single-arm SDARS Sirius antennas have been just released into the market. They are patch antennas etched on a low-loss substrate. These antennas yield excellent SAT performance. However, the TER performance is poor, with an average antenna gain of approximately −7 dBi.17 Single-arm XM antennas are expected to be available by the third quarter of 2003.
The aesthetic limitations of mounting large SDARS antenna structures on vehicles may be in part responsible for the slow customer acceptance of this technology. For this reason, manufacturers are investigating the use of hidden SDARS antennas, located in the interior of a vehicle. The goal is to match the performance of a single roof or on-glass antenna by using a minimum of two hidden antennas. This is a challenging task, since antennas located inside the vehicle would yield poor performance due to signal blockage from the roof, pillars, and other vehicle structures. Figure 10 shows two radiation patterns corresponding to an SDARS SAT antenna located inside a sedan (front dash and rear deck lid) for the elevation angle of 25 deg. By itself, neither of the radiation patterns is acceptable. However, when the two patterns are combined through a receiver diversity algorithm, the resulting radiation pattern can be significantly improved. A potential antenna choice for this application is a patch antenna. A pair of patch antennas can be used, one placed in the front and the other in the rear of the vehicle. A perceived disadvantage of implementing hidden antennas is the cost increase associated with each additional antenna module: the antenna element, LNA, cable, and connector.
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