GPS Antenna Design

Virtually every modern smartphone now has a GPS antenna. GPS antennas are somewhat unique, in that their bandwidth is fairly small. The GPS frequency is 1.575 GHz, with virtually no bandwidth. However, the GPS antennas often also support the Russian GPS service, known as GLONASS, which extends the required bandwidth to about 1.605 GHz. The GPS signals from the GPS satellites are Right Hand Circularly Polarized. However, because of difficulties in enabling RHCP polarization in small devices, GPS antennas are generally vertically polarized. Note that GPS antennas on smartphones are receive only antennas, and therefore you don't have to worry about any transmitting issues (SAR, radiated emmissions, TRP requirements, etc). The requirements for GPS antennas are therefore related to Total Isotropic Sensitivity (TIS). The sensitivity of receivers for GPS are in the (-163, -155) dBm range. Requirements for satellite acquisition tend to specify a minimum sensitivity on the order of -145 dBm. Hence, if the sensitivity of the receiver is known, along with required sensitivity, you can determine the approximate required GPS antenna efficiency. GPS antenna efficiencies are typically on the order of -3 dB to -9 dB. The GPS antenna is most often used in portrait mode, which means the mobile phone is held vertically in the hand. As a result, it is advantageous for the antenna to have a radiation pattern that is directed upwards, instead of downwards. Consequently, there are alternatives metrics that are also used besides TIS. TIS is a measure of the sensitivity when averaged "over the entire 3D sphere". UHIS (Upper Hemosphere Isotropic Sensitivity) calculates the sensitivity of a phone only over the upper hemisphere (i.e. ignoring downard directions), as shown in Figure 1: Figure 1. Illustration of UHIS (Upper Hemisphere Sensitivity Measurement) for GPS. Another common metric is PIGS, or Partial Isotropic GPS Sensitivity, which averages the sensitivity over all angles from 90 degrees above the horizon to 30 degrees below the horizon: Figure 2. Illustration of PIGS (Partial Sensitivity) for GPS. A half-wavelength at GPS frequency (1.575 GHz) is about 9.5 cm or 3.75 inches. This means that we will need to again use the ground plane of our smartphone (the chassis) as one arm of our dipole antenna. In addition, because the GPS antenna is used when the user is holding the phone vertically, they will typically have their hands on the lower end of the device. We therefore prefer to have the GPS antenna towards the top of the device. This is illustrated for the palm pre in Figure 3: Figure 3. Illustration of GPS antenna on Palm Pre. In Figure 3, it looks like Palm used a sideways IFA antenna. The location of the GPS antenna was likely chosen to maximize distance between the two cellular antennas. This increases isolation, which is useful for maximizing antenna efficiency. The isolation was likely also increased because the polarization of the GPS antenna and the cellular antennas was orthogonal (GPS here appears to be horizontally polarized, the cell antennas are vertically polarized). GPS antenna design for mobile devices would basically consist of first determining the required antenna efficiency (receiver GPS sensitivity - required TIS). Then basic IFAs or PIFA antennas can be tried (using the phone chassis as the ground plane). The design can then be optimized and tuned to take into account all of the nearby components that disturb the antenna (i.e. camera, battery, plastics, etc). In terms of design heirarchy, the parameters to optimize for are listed below in order of importance: 1. Antenna Efficiency 2. Radiation Pattern (skewed upwards is better) 3. Isolation to other antennas 4. RHCP polarization Antenna efficiency is the most important, with optmizing for polarization the least important. In the next section, we'll look at WIFI antenna design for mobile devices.