GPS RF antenna modules for mobile handset applications

The most suitable antenna for a fixed receiver provides a beam extending down to about 15deg above the horizon over a range of 360deg azimuth with some gain reduction overhead. However the situation for mobile handsets is not so clear, and in this article we discuss what the best mobile GPS antenna design strategy should be.

Hand-held navigation systems
Until recently GPS receivers have tended to be used with a fixed orientation. For example, navigation devices in vehicles are generally mounted on the dashboard with a fixed vertically oriented antenna beam. With the introduction of satellite navigation capability into mobile handsets, for E911 and location-based services, a new set of challenges has been introduced. The immediate issue concerns how to fit yet another antenna and associated radio front end into a compact and functional handset design without making it cumbersome. But a more fundamental issue is raised by the unknown orientation in which a handset may be used. Typically with mobile handsets or converged devices, the narrowest edge of the device points upwards when in use, which means that the orientation of maximum field strength would be entirely wrong if the traditional GPS antenna was in the same way as it is in a stand-alone GPS receiver. The handset may also be used in any other orientations such as lying horizontally on a table or a car seat. Proximity effects of the user's head and hand are also much more significant for GPS antennas in mobile devices than they are for other types of navigation systems. As a result of these considerations we have to re-evaluate what would be the best GPS antenna for a mobile device.

The choice of antenna
The most commonly used type of GPS antenna is the ceramic patch, which has quite a large area but a relatively low profile. When used on a large groundplane the patch antenna is stable, efficient and can have very good right hand circular polarization (RHCP) characteristics, matching the polarization characteristics of the signals radiated by the GPS satellites. An alternative to the patch is the helix antenna, which has similar electrical characteristics but is higher in profile and correspondingly smaller in area. The disadvantages of patch antennas are that they tend to be large in area, heavy and very narrow band, especially as they decrease in size, so manufacturing tolerances need to be pretty tight as even the limited sensitivity to the ground and the surrounding objects is enough to knock them out of tune. This means ceramic patches need to be customized for different applications and this increases the inventory for the manufacturers of navigation devices. Small ceramic patches, or those mounted on small groundplanes are generally less efficient, less stable and have worse RHCP characteristics than their larger counterparts.

For mobile handsets operated in a variety of possible positions, the biggest drawback of large ceramic patches is their high gain/ narrow beam antenna pattern characteristics. At first, high gain (typically 4-6 dBi) seems advantageous because the higher the antenna gain the stronger the signals that are received from the satellites in view. Unfortunately high antenna gain is synonymous with having a narrow beam and the view of the sky is correspondingly restricted. When installed in a mobile phone handset, an antenna mounted on the front of the system PCB will look upwards when the handset is held horizontally, but when in use for a phone call its view of the sky will be severely obstructed by the user's head. An antenna on the back of the handset will see only part of the sky without obstruction. In contrast, a low gain antenna will receive signals from more satellites and the system will be less sensitive to orientation. We must consider whether it is better to receive signals from a few satellites with a good carrier-to-noise ratio (C/N₀) or to receive data from more satellites at lower signal strength, provided of course that the GPS receiving chip and software can process and make use of the data from more satellites in its calculation of position. One way to answer this question is empirically by experimenting with low and high gain antenna systems connected to the same radio receiver.

Drive tests
Figure 1 shows the results of a drive test round London using low and high gain antennas. The high gain antenna is a 25x25 mm active patch and the low gain antenna is an Antenova design with radiation pattern similar to a dipole. The route taken was selected to be difficult for satellite signal reception because of narrow streets lined by tall buildings, overhead bridges, overhanging trees, etc. Despite the high gain antenna receiving some signals with a higher C/N₀ ratio than the lower gain antenna, it can be seen that the low gain antenna actually gave a better tracking performance. A single experiment like this cannot be considered as giving a scientifically valid result because there are many potential sources of error; for example the position and number of satellites visible will vary between the drive test with one antenna and the following run with the other. However, the experiment has been repeated many times in some of the most difficult cities in the world " Chicago, Seoul, Taipei, etc. " and the same results have consistently been obtained, demonstrating that a low gain antenna is not a drawback in a mobile system and may even be an advantage.

Designing GPS RF antenna module
Armed with the drive test results, Antenova set out to go one step further and design a complete GPS navigation module suitable for use in a mobile phone handset. One of the major problems was to keep GSM and UMTS transmissions from the handset out of the GPS receiver because the signals may only be 10% apart in frequency (GPS signals are received at 1575 MHz and the handset may transmit on 1710 MHz). The handset transmissions may be of the order of 10¹⁶ times (160 dB) more powerful than the faint satellite signals that the GPS system is trying to receive. Introducing two SAW filters into the system and carefully designing the two antennas (cellular and GPS) for minimum coupling between them achieved this signal separation. Figure 2 shows the transfer function of the RF front end. [It is worth noting that the isolation does not include the antenna rejection, which can add a good 20dB or more to the isolation.]

The navigation module is a complete unit containing a "quasi-symmetric antenna," a pair of SAW filters, low noise amplifier, temperature controlled crystal oscillator and a GPS RF plus baseband integrated circuit such as the SiRFstarIII chip. The term 'quasi-symmetrical antenna' needs some elaboration. Conventional antenna wisdom is that there are two types of antenna; the simplest type is the symmetrical, or balanced, dipole that has two radiating elements, one driven against the other. Dipoles work well in free space but when an antenna must work close to a conducting plane, such as a PCB, then it is more common to use an unbalanced monopole type of antenna; this is a sort of half-antenna driven against the PCB ground, which acts as the other half. Besides the monopole, the most common unbalanced antenna is the planar inverted F antenna or PIFA (which is really just a fat monopole bent over to be parallel to the groundplane and fed at the appropriate 50 ohm point). However, there exists an in-between state that we call a 'quasi-symmetrical antenna' that I should explain. Whilst there must always be two parts to the antenna, they do not necessarily have to be 180 degrees apart, as in a conventional dipole. The phase relationship between the two parts of the antenna may be varied and the effect of changing this phase relationship, and that between it and the currents flowing on the main PCB, creates benefits such as:

• An antenna with improved RHCP reception (compared to strictly linear polarization)
• Wide beamwidth to receive signals from as many satellites as possible
• Optimum matching between the antenna and the radio
• A beam direction that can be adjusted to 'see' the maximum number of satellites in any given application.