M2M devices in cars and trucks have sometimes a need for two antennas. If the external antenna will be demounted or damaged then the M2M device will switch to the internal antennas. Other reason for two antennas could be that the internal antennas on dashboard works fine, but the M2M device will be mounted on a place in a car, where the signal strength is less. The mobile control rooms for police or fire-fighters are a good example for M2M devices in a bus with decreasing the signal field strength by metal roof and metallised windows. Metallised windows we often find in new cars as protection against solar radiation as well.
Heavy machine on railway
Be careful by switching the GNSS signal. In year 1999 I ago had to detect where is to front and the back of a heavy machines (20 to 800 meter long) working on railways. Such machines has can work in one direction only. Worst case the machine has to be moved on a long distance to a turntable at railways station. Moving machines on railway costs money. You have to pay per kilometre. The way out was one GPS module with two antennas. The GPS Selective Availability of the US was in this time active. The front and the back were jumping +/- 30 degree on the screen. Railways are long straight lines on a map. Even with the +/- 30 degree fault it was possible to detect the direction of the machine. The only drawback was that the GPS module was completely lost by switching the antenna. It started with downloading the almanac again and made a complete new calculation of the position. If you want two GPS positions in close the same time, then you will run in trouble.
Test your M2M device at laboratory detailed before you move it to the field test.
GaAS switch
If you search on Google for GaAS switch, then you will find a lot of examples for such RF switches.
By selection of the GaAS switch just take care on
- Frequency range cellular (e.g. LTE 698, 800, 2100, 3500 MHz GSM EU 900 and 1800 MHz)
- Frequency range GNSS (GPS 1575,42 MHz and Glonass up to 1610 MHz)
- Maximum current (GSM transmits in peaks with 2 Ampere, this will force TX power peaks as well)
- Low insertion loss
- Low harmonic distortion
- Good isolation
If you are still not sure which RF switch to select then just drop an email to harald.naumann (at) gsm-modem.de
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