by Simon Kingsley, Antenova
Wednesday 11 April 2007

The incorporation of satellite navigation capability into mobile handsets introduces a new set of challenges, not least the issue of how to fit yet another antenna and associated radio front end into a compact and functional handset design without making it cumbersome. The correct orientation of the antenna is a problem in a unit that may either be used in the hand or next to the user’s head.

Existing designs for GPS antennas have generally been optimised for PDA-style units or for mounting on a vehicle dashboard, where it is simple to orientate the antenna to point skywards towards the satellites. In a mobile handset the narrowest edge of the case points upwards when in use, which means that the orientation of maximum field strength would be entirely wrong if the same antenna were used as for a stand-alone GPS receiver.

The proximity of the user’s head and hand will also have a significant effect on the radiation characteristics of most antennas. A wide beamwidth is necessary in order to acquire signals concurrently from four satellites for a full 3D fix, while precise filtering is required in order to eliminate interference at GSM and UMTS frequencies.

When a GPS receiver is switched on it needs to recognise and acquire data from the signals from at least four different satellites, which for a good 3D positional fix needs to be as widely spaced as possible. The time taken for this process depends on several factors, including the carrier-to-noise ratio of the incoming signals, and is known as the time to first fix - usually about 45 seconds for a ‘cold start’ where the unit is some distance from the position it was last used in.

The satellites radiate the signals with right-hand circular polarisation (RHCP), so an ideal antenna for receiving GPS in an outdoor environment without obstacles requires a broad, upward-looking beam pattern that receives RHCP. For a freestanding receiver the situation is relatively straightforward, but for a mobile phone handset the antenna will point in different directions depending on how it is being used. An antenna mounted on the front of the system PCB will be focused upwards when the handset is held horizontally, but during a phone call its ‘view’ of the sky will be obstructed by the user’s head. Even if the antenna is mounted on the rear of the phone, the sky will still be partially obscured.

If the RHCP signal from the satellite is reflected by the ground, its polarisation is reversed to left-hand circular polarisation (LHCP). In a cluttered environment such as indoors or on a city street there may be no direct signals from many satellites, and the reflected signals may have arbitrary polarisation, so a wide-beam linearly polarised antenna actually gives better performance than the antenna types conventionally used for receiving circular polarisation, such as crossed dipoles, Lindenblad arrays, quadrifilar helices, and circularly-polarised patches –all of which are difficult to fit into a handset.

Antenova has developed a miniature GPS antenna module with integral RF front end, the GPS Radionova, which has been designed for handset applications.

The antenna is designed so that both radiating arms are coplanar and meandering, one on each side of a PCB substrate, thus overcoming the problem that a half-wave dipole at the 1575MHz GPS frequency is too long at 95mm to fit comfortably into a handset.

A PTFE-glass laminate was used as a substrate to keep losses low. Although it is acceptable for the radiating arms of the dipole to lie above one another, it has been found that displacing the arms with respect to each other to follow different paths in their respective planes can improve both radiation efficiency and bandwidth. Bandwidth can also be improved if the arms are of different lengths.

Prof. Simon Kingsley is chief scientist at Antenova