Specifications

XT65/XT75 Hardware Interface Description

6.2 GPS Antenna

XT65_XT75_HD_v01.001 Page 88 of 133 2007-1-8

Confidential / Released

The use of an active antenna is always advisable, if the RF-cable length between receiver and antenna exceeds

about 10 cm.

For more information on GPS antenna design see [13].

Note: If you are not an expert in RF designs, it is recommended to implement an active antenna setup and place

the antenna away from any emitting circuits.

Table 21: GPS antenna: Active versus Passive

Active Antenna Passive Antenna

Active antenna connected to the GPS module. Passive patch antennas or quadrifilar dipole antennas con-

nected with a microcoax to the GPS module.

• A wide range of active patch or quadrifilar

dipole antennas is available in the market.

They differ in size, gain, selectivity and

power consumption.

• Less sensitive to jamming than a passive

antenna, if the placement of the active

antenna is some distance away of other

noise or signal radiating devices.

• Needs DC current.

• Easier to handle.

• More freedom to place the antenna

• Some low noise amplifier (LNA) circuits in an

active antenna may be sensitiv to GSM Tx

interference. GPS reception can therefore be

distorted or the LNA may be damaged.

Some cars for instance have a metallic coating on the windshield. GPS reception may be degraded in such a

car. There is usually a small section, typically behind the rear view mirror without the coating for mobile phone

and GPS antennas. The antenna has to be placed with optimal sky visibility. An external antenna (e.g. with a

magnetic base) is easier to use and usually allows a better positioning.

• Passive patch antennas or helical antennas are avail-

able in different form factors and sensitivity.

• Antenna must be connected with a low insertion loss

line to the GPS module to ensure a good GPS sensitiv-

ity.

• The PCB design with a passive antenna must consider

the sensitivity of the GPS antenna to other radiating cir-

cuits or general signal jamming.

• Due to the proximity of the GPS antenna to other elec-

tronic circuits, in-band jamming may become a critical

issue.

• Requires more experience in RF design.

• Requires more effort to optimise the circuit design to

minimize jamming into the antenna and the antenna

signal routing.