As more electronics are stuffed into ever decreasing sized enclosures and at a lower cost, it looks like corners are starting to be cut with respect to good engineering practice at the design stage. Below are some pointers with respect to designing in GPS antennas into handheld equipment. The experience is gained from being involved with the design of GPS equipment since 1994. The most important item to consider is the best siting for the antenna. Even with today's latest generation chipsets, a GPS antenna still needs a clear view of the sky especially during first acquisition when it needs to download the latest ephemeris data or on some occasions, a complete almanac. This almanac and ephemeris data provides a virtual map to the GPS receiver allowing it to know the current and future locations and status of the satellites, relevant for that particular location. The almanac is a long-term picture of what the whole GPS network is doing whilst the ephemeris is for a short-term picture. The delay when the GPS receiver is first switched on is due to the latest ephemeris data being downloaded,golf gps, which is essential for the GPS receiver. The data is valid for around 4 hours and takes approximately 20 seconds with a clear line of site to a satellite. It takes longer if using multiple satellites, for instance if the GPS receiver is moving during first acquisition. The antenna needs to be facing the sky through a non-metallic enclosure. A metallic enclosure will prevent the RF signal being received by the antenna and cause a Faraday cage effect. Likewise, positioning the antenna near to obstructions such as laptop screen or where hands will be over the antenna should also be avoided, as the shielded will have a negative performance. Try to avoid routing cables across other electronic equipment or backs of screens, instead try to route around the perimeter of active electronic components. Whilst it is a good idea to keep the cable relatively short, (thin RF coaxial cable used on electronic equipment has quite a high loss at 1575.42 MHz) and is susceptible to interference. However, length consideration is not essential as most GPS antennas have a 1 or 2 stage pre amplifier depending on GPS receiver used. When designing in a GPS antenna, do not always look for the smallest possible antenna. Although this can seem attractive, particularly in hand held equipment when size is everything, the effect of a smaller GPS antenna means it is more sensitive to de-tuning. Why? GPS works on a spot frequency of 1575.42MHz rather than a frequency range. This means that when other materials cover it - human tissue, laptop screens, it detunes the antenna away from this frequency. The smaller the antenna, the more susceptible it is to the detuning effect,navigator, which results in lack of GPS signal received. The feedback I have received whilst helping engineers with the design in is that 25mm x 25mm patch antennas are too large but work well, whilst the 10x10mm patch antennas tick the size requirements box, however take at least 20 minutes to get into navigation. Similarly, helical antennas sound good on paper, but during field tests are poor versus a patch antenna with a comparable pre amplifier. The sweet spots seems to be around 17x17mm. The thickness of the antenna with amplifier underneath is also now being overcome with 2mm thick ceramics now commonplace ?which now gives an overall thickness of around 6mm. As old GPS users may recall, selective availability was switched off on 1st May 2000 by the US Department of Defence. This immediately gave an improved GPS navigation performance from 100m down to 10-15 metres on civilian acquired equipment. At the same time the power output from the satellites was increased. Over 10 years on, this has resulted in a variety of GPS receiver and antenna combinations being required depending on environment the equipment will be used in. From the antenna perspective, 1 stage Low Noise Amplifiers (LNA's) or 2 stage LNA's are now available on embedded GPS antennas to optimise the final design. 1 stage designs provide a gain of around 16-20dB and are reliant on either the GPS receiver having its own antenna or a very short cable run. A 2 stage LNA provides a gain of between 25-30dB and are more popular on antennas where the GPS receiver does not have an amplifier or is unknown. Although the preconception of the higher the gain the better, this is not always true. If the gain is too high constantly, this can have an oversaturation effect on the GPS receiver, desensitising the RF front end over time and causing poor navigation. Liken it to shouting in someone's ear. It will be very loud and the message may still not get across!
2013年11月6日星期三
Gps Antenna Selection For Embedded Applications
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As more electronics are stuffed into ever decreasing sized enclosures and at a lower cost, it looks like corners are starting to be cut with respect to good engineering practice at the design stage. Below are some pointers with respect to designing in GPS antennas into handheld equipment. The experience is gained from being involved with the design of GPS equipment since 1994. The most important item to consider is the best siting for the antenna. Even with today's latest generation chipsets, a GPS antenna still needs a clear view of the sky especially during first acquisition when it needs to download the latest ephemeris data or on some occasions, a complete almanac. This almanac and ephemeris data provides a virtual map to the GPS receiver allowing it to know the current and future locations and status of the satellites, relevant for that particular location. The almanac is a long-term picture of what the whole GPS network is doing whilst the ephemeris is for a short-term picture. The delay when the GPS receiver is first switched on is due to the latest ephemeris data being downloaded,golf gps, which is essential for the GPS receiver. The data is valid for around 4 hours and takes approximately 20 seconds with a clear line of site to a satellite. It takes longer if using multiple satellites, for instance if the GPS receiver is moving during first acquisition. The antenna needs to be facing the sky through a non-metallic enclosure. A metallic enclosure will prevent the RF signal being received by the antenna and cause a Faraday cage effect. Likewise, positioning the antenna near to obstructions such as laptop screen or where hands will be over the antenna should also be avoided, as the shielded will have a negative performance. Try to avoid routing cables across other electronic equipment or backs of screens, instead try to route around the perimeter of active electronic components. Whilst it is a good idea to keep the cable relatively short, (thin RF coaxial cable used on electronic equipment has quite a high loss at 1575.42 MHz) and is susceptible to interference. However, length consideration is not essential as most GPS antennas have a 1 or 2 stage pre amplifier depending on GPS receiver used. When designing in a GPS antenna, do not always look for the smallest possible antenna. Although this can seem attractive, particularly in hand held equipment when size is everything, the effect of a smaller GPS antenna means it is more sensitive to de-tuning. Why? GPS works on a spot frequency of 1575.42MHz rather than a frequency range. This means that when other materials cover it - human tissue, laptop screens, it detunes the antenna away from this frequency. The smaller the antenna, the more susceptible it is to the detuning effect,navigator, which results in lack of GPS signal received. The feedback I have received whilst helping engineers with the design in is that 25mm x 25mm patch antennas are too large but work well, whilst the 10x10mm patch antennas tick the size requirements box, however take at least 20 minutes to get into navigation. Similarly, helical antennas sound good on paper, but during field tests are poor versus a patch antenna with a comparable pre amplifier. The sweet spots seems to be around 17x17mm. The thickness of the antenna with amplifier underneath is also now being overcome with 2mm thick ceramics now commonplace ?which now gives an overall thickness of around 6mm. As old GPS users may recall, selective availability was switched off on 1st May 2000 by the US Department of Defence. This immediately gave an improved GPS navigation performance from 100m down to 10-15 metres on civilian acquired equipment. At the same time the power output from the satellites was increased. Over 10 years on, this has resulted in a variety of GPS receiver and antenna combinations being required depending on environment the equipment will be used in. From the antenna perspective, 1 stage Low Noise Amplifiers (LNA's) or 2 stage LNA's are now available on embedded GPS antennas to optimise the final design. 1 stage designs provide a gain of around 16-20dB and are reliant on either the GPS receiver having its own antenna or a very short cable run. A 2 stage LNA provides a gain of between 25-30dB and are more popular on antennas where the GPS receiver does not have an amplifier or is unknown. Although the preconception of the higher the gain the better, this is not always true. If the gain is too high constantly, this can have an oversaturation effect on the GPS receiver, desensitising the RF front end over time and causing poor navigation. Liken it to shouting in someone's ear. It will be very loud and the message may still not get across!
As more electronics are stuffed into ever decreasing sized enclosures and at a lower cost, it looks like corners are starting to be cut with respect to good engineering practice at the design stage. Below are some pointers with respect to designing in GPS antennas into handheld equipment. The experience is gained from being involved with the design of GPS equipment since 1994. The most important item to consider is the best siting for the antenna. Even with today's latest generation chipsets, a GPS antenna still needs a clear view of the sky especially during first acquisition when it needs to download the latest ephemeris data or on some occasions, a complete almanac. This almanac and ephemeris data provides a virtual map to the GPS receiver allowing it to know the current and future locations and status of the satellites, relevant for that particular location. The almanac is a long-term picture of what the whole GPS network is doing whilst the ephemeris is for a short-term picture. The delay when the GPS receiver is first switched on is due to the latest ephemeris data being downloaded,golf gps, which is essential for the GPS receiver. The data is valid for around 4 hours and takes approximately 20 seconds with a clear line of site to a satellite. It takes longer if using multiple satellites, for instance if the GPS receiver is moving during first acquisition. The antenna needs to be facing the sky through a non-metallic enclosure. A metallic enclosure will prevent the RF signal being received by the antenna and cause a Faraday cage effect. Likewise, positioning the antenna near to obstructions such as laptop screen or where hands will be over the antenna should also be avoided, as the shielded will have a negative performance. Try to avoid routing cables across other electronic equipment or backs of screens, instead try to route around the perimeter of active electronic components. Whilst it is a good idea to keep the cable relatively short, (thin RF coaxial cable used on electronic equipment has quite a high loss at 1575.42 MHz) and is susceptible to interference. However, length consideration is not essential as most GPS antennas have a 1 or 2 stage pre amplifier depending on GPS receiver used. When designing in a GPS antenna, do not always look for the smallest possible antenna. Although this can seem attractive, particularly in hand held equipment when size is everything, the effect of a smaller GPS antenna means it is more sensitive to de-tuning. Why? GPS works on a spot frequency of 1575.42MHz rather than a frequency range. This means that when other materials cover it - human tissue, laptop screens, it detunes the antenna away from this frequency. The smaller the antenna, the more susceptible it is to the detuning effect,navigator, which results in lack of GPS signal received. The feedback I have received whilst helping engineers with the design in is that 25mm x 25mm patch antennas are too large but work well, whilst the 10x10mm patch antennas tick the size requirements box, however take at least 20 minutes to get into navigation. Similarly, helical antennas sound good on paper, but during field tests are poor versus a patch antenna with a comparable pre amplifier. The sweet spots seems to be around 17x17mm. The thickness of the antenna with amplifier underneath is also now being overcome with 2mm thick ceramics now commonplace ?which now gives an overall thickness of around 6mm. As old GPS users may recall, selective availability was switched off on 1st May 2000 by the US Department of Defence. This immediately gave an improved GPS navigation performance from 100m down to 10-15 metres on civilian acquired equipment. At the same time the power output from the satellites was increased. Over 10 years on, this has resulted in a variety of GPS receiver and antenna combinations being required depending on environment the equipment will be used in. From the antenna perspective, 1 stage Low Noise Amplifiers (LNA's) or 2 stage LNA's are now available on embedded GPS antennas to optimise the final design. 1 stage designs provide a gain of around 16-20dB and are reliant on either the GPS receiver having its own antenna or a very short cable run. A 2 stage LNA provides a gain of between 25-30dB and are more popular on antennas where the GPS receiver does not have an amplifier or is unknown. Although the preconception of the higher the gain the better, this is not always true. If the gain is too high constantly, this can have an oversaturation effect on the GPS receiver, desensitising the RF front end over time and causing poor navigation. Liken it to shouting in someone's ear. It will be very loud and the message may still not get across!
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