Smart Machine Smart Decision
SIM808_Hardware Design_V1.00 2014.03.27
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Proper ground plane sizing is a critical consideration for small GPS antennas.
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Proper placement of the GPS antenna should always be the FIRST consideration in integrating the SIM808
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. 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 is 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 matched to an appropriate
sized ground. Generally 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 millimeters 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 to achieve the best signal quality. GPS antenna choice should be based on the designing product
and other conditions.
For detailed Antenna designing consideration, please refer to related antenna vendor’s design recommendation.
The antenna vendor will offer further technical support and tune their antenna characteristic to achieve successful
GPS reception performance depending on the customer’s design.
6
Electrical, Reliability and Radio Characteristics
6.1
Absolute Maximum Ratings
The absolute maximum ratings stated in following table are stress ratings under non-operating conditions. Stresses
beyond any of these limits will cause permanent damage to SIM808.
Table 38: Absolute maximum ratings
Symbol
Min
Typ
Max
Unit
Содержание SIM808
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