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.