5.2.2  FRPA  Types

There are many types of GPS FRPA antenna.  The simplest is a resonant monopole

approximately 5 cms in length.  However, as the monopole has a toroidal radiation

pattern and is vertically polarized, it is not optimum for use with the circularly polarized

GPS transmissions. Gain is very low,  40 dBic on boresight and peaking to

approximately 0 dBic at 70  from boresight depending on the conductivity of the ground

plane. 

Spiral Helix antennas are useful for several receiver applications where a small

antenna is required that is generally unaffected by the presence or absence of a

ground plane.  The antenna can be configured to be low profile, but is not conformal

and is therefore not suitable for fast aircraft.  The antenna is less sensitive to the

influence of the ground plane than some other FRPAs and is capable of being mounted

on non conducting surfaces, making it suitable for a variety of applications from

vehicles to handheld receivers.  Typically the gain is better than   4 dBic from boresight

to 80 .  The antenna s mechanical layout and typical dimensions are shown in Figure 5 

1.

The FRPA Bifilar Helix is designed for hand held applications and is capable of being

integrated into a broad category of ground vehicles in addition to its main application

on the Precise Lightweight GPS Receiver (PLGR).  The antenna is insensitive to

ground plane and installation location.  Streamlined outer shell can be added to enable

the device to be used in medium dynamic, for instance helicopter applications.  It

provides a gain of not less than    3 dBic over 80  angle from boresight.  The antenna  s

mechanical layout and typical dimensions are shown in Figure 5 2.

To produce a conformal design for aircraft applications where minimal drag is required,

a crossed slot or patch antenna can be used.  The crossed slot is  effectively four

monopoles laid out at right angles with a suitable separation above the ground plane. 

Patches can take many formats.  These antennas rely on the aircraft skin acting as a

ground plane to achieve the required antenna performance.  Gains of +2 dBic are

typically achieved on boresight and, although the gain to circularly polarized radiation

falls to  5 dBic at 90  (from boresight), the gain is sufficient to allow satellites to be

tracked through medium dynamic aircraft maneuvers. Antennas can be made that are

sensitive to L1 and L2 GPS frequencies.  The mechanical layout and the dimensions of

an example antenna are shown in Figure 5 3.

A special derivative of a FRPA crossed monopoles antenna is the FRPA Ground

Plane. This special FRPA assembly (see Figure 5 4) is intended  for shipborne mast

applications where there is no ground plane.  The assembly consists of a ground

plane/mounting surface for the FRPA plus an environmentally sealed enclosure

containing an integrated preamplifier. A derivation of the FRPA Ground Plane is

employed for GPS Reference Stations in a differential system.  In these

applications a special choke ring is added to the antenna to reduce the gain in the

direction of likely sources of multipath.

5 2