Embedded dual-band GSM antenna design

14th January 2014
Nat Bowers

This application note illustrates how CST MICROWAVE STUDIO (CST MWS), Antenna Magus and Optenni Lab can be used in combination to improve an existing GSM tracking device application. The requirement was to replace an existing “off-the-shelf” antenna on a GSM tracking device with an embedded, integrated antenna in order to reduce manufacturing and component costs.

The new antenna had to operate inside the standard GSM 900 and 1800 frequency bands and had to use the existing substrate and metallic layers, without changing the existing circuit layout or the size of the PCB. The new integrated antenna was designed using Antenna Magus in combination with CST MWS to account for unwanted coupling, ensuring that the antenna operated within the desired frequency bands.

Why do sheep need antennas?

ETSE electronics is based in Stellenbosch, South Africa and is an expert in developing microwave and RF communication devices and systems. They have developed a product called the Celmax collar. This is a GPS tracking device which is worn by a sheep, usually part of a large herd. In the case of theft or other danger, the sheep will start running. The device will detect the acceleration and notify the farmer via an SMS text message, which is sent through the local GSM cellular network.

The image above shows the initial tracking device design, with the off the shelf GSM antenna mounted on the PCB and connected through a via to the RF chip mounted on the opposite side of the PCB. The device’s operating frequency bands are the standard GSM 900 (890-960MHz) and 1800 (1710-1879.8MHz) bands.

Antenna Magus and CST MWS will be used to replace this antenna with a printed dual band antenna, without changing the current circuit layout or size of the PCB. The substrate is made of 4 layers of FR4 with a permittivity of 4.44 and tan δ = 0.01. The maximum available distance between two metal layers is 1.203mm and the available space for the antenna is 70x10.5mm.

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