Ublox TIM-5H Hardware Integration Manual

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Ublox TIM-5H Hardware Integration Manual Manual pdf 43 page image

TIM-5H - Hardware Integration Manual AppendixGPS.G5-MS5-07015-A-1 u-blox proprietary Page 43your position is our focusAnother area of optimization is accurate determination of the phase center of the antenna. For precisionGPS/GALILEO applications with position resolution in the millimeter range it is important that signals fromsatellites at all elevations virtually meet at exactly the same point inside the antenna. For these types ofapplications receivers with multiple antenna inputs are often required.At the low end of the spectrum of possible antenna solutions - if the user is willing to accept significant signallosses - a simple linear polarized whip or strip antenna will work. Compared to a circular polarized antenna, aminimum of 3 dB of signal to noise ratio will be lost.B.2 Active and Passive AntennasPassive antennas contain only the radiating element, e.g. the ceramic patch or the helix structure. Sometimesthey also contain a passive matching network to match the electrical connection to 50 Ohms impedance.Active antennas have an integrated low-noise amplifier. This is beneficial in two respects. Firstly, the losses of thecable no longer affect the overall noise figure of the GPS/GALILEO receiver system. Secondly, the receiver noisefigure can be much higher without sacrificing performance. Therefore, some receivers will only work with activeantennas. Active antennas require a power supply that contributes to total GPS/GALILEO system powerconsumption, typically in the region of 5 to 20 mA. Usually, the supply voltage is fed to the antenna through thecoaxial RF cable. Inside the antenna, the DC component on the inner conductor will be separated from the RFsignal and routed to the supply pin of the LNA.The use of an active antenna is always advisable if the RF-cable length between receiver and antenna exceedsapproximately 10 cm. Care should be taken that the gain of the LNA inside the antenna does not lead to anoverload condition at the receiver. For receivers that also work with passive antennas, an antenna LNA gain of15 dB is usually sufficient, even for cable lengths up to 5 m. There’s no need for the antenna LNA gain to exceed26 dB for use with u-blox receivers. With shorter cables and a gain above 25 dB, an overload condition mightoccur on some receivers.When comparing gain measures of active and passive antennas one has to keep in mind that the gain of anactive antenna is composed of two components, the antenna gain of the passive radiator, given in dBic, and theLNA power gain given in dB. A low antenna gain cannot be compensated by high LNA gain. If a manufacturerprovides one total gain figure, this is not sufficient to judge the quality of the antenna. One would needinformation on antenna gain (in dBic), amplifier gain, and amplifier noise figure.B.3 Patch AntennasPatch antennas are ideal for an application where the antenna sits on a flat surface, e.g. the roof of a car. Patchantennas can demonstrate a very high gain, especially if they are mounted on top of a large ground plane.Ceramic patch antennas are very popular because of their small size, typically measuring 25 x 25 mm 2down to12 x 12 mm 2. Very cheap construction techniques might use ordinary circuit board material like FR-4 or even airas a dielectric, but this will result in a much larger size, typically in the order of 10 x 10 cm 2. Figure 27 shows atypical example of the radiation pattern of a 16 x 16 mm 2ceramic patch antenna. This measurement only showsthe upper sphere of the radiation pattern. Depending on ground plane size there will also be a prominent backlobe present.