SimCom SIM548C HD V1.01, Проектирование аппаратного обеспечения

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SIM548C Hardware Design
antenna with a reasonably uniform hemispherical gain pattern of >-4dBi. Use of an antenna with
lower gain than this will give less than desirable results. Please note that a RHCP antenna with a
gain of -3dBic, equates to a linear polarized antenna of -0dBi. Proper ground plane sizing is a
critical consideration for small GPS antennas. Proper placement of the GPS antenna should always
be the FIRST consideration in integrating the GPS Module.
If the customer’s design will allow for a ceramic RHCP patch antenna with an appropriately sized
ground plane, and the patch is normally oriented towards the sky, then that particular solution
usually works the best. Please note that if the patch antenna ground plane is less than 60x60mm,
then compromises to the beam width and gain pattern could result. Usually the gain becomes very
directional, and looses several dB of performance. Since results can vary, measuring the antenna
radiation pattern in the final housing in an appropriate anechoic chamber will be required.
Some customers do not have the size availability to implement a patch antenna approach. In that
instance, use of a Linear Polarized (LP) antenna is the next best alternative. There are new ceramic
LP antennas on the market that exhibit reasonable gain characteristics once properly mounted in
the housing, and when mated to an appropriate sized ground. That is the key point to consider here.
“When mated to an appropriate sized ground”.
Usually, the ground plane requirements are smaller for a LP antenna when compared to a patch,
but once again, proper testing in an anechoic chamber is a mandatory requirement. These ceramic
elements will need to be located near the end of the ground plane, and will require several mm of
clearance between the closest component.
It is important to note that use of a LP antenna will result in a minimum of 3dB of gain loss when
compared to a RHCP antenna at a defined elevation. This is due to the right hand gain rule of
antenna propagation.
Use of PIFA antenna is another LP possibility, but the PIFA usually exhibits a considerable
amount of gain nulls, or “holes” in the radiation pattern. This will be undesirable for obtaining a
low circular error probability (CEP), since the antenna may not allow the receiver to capture the
desired satellite at the ideal orientation due to these noted gain nulls.
Once again, careful testing in an appropriate anechoic chamber is required.
If the customer’s design is for automotive applications, then an active antenna can be used and
located on top of the car in order for guarantee the best signal quality.
SIM548C_HD_V1.01 09.05.2008
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