AD490 and AD491 combined L1/L2 GPS and DGPS antenna with interdigital filters

 

Introduction

The AD490 antenna (figure 1) is a robust, lower cost alternative to existing GPS L1/L2 models from other manufacturers, which incorporates DGPS reception and interdigital filters for use in areas where general purpose antennae are inoperable due to L-band interference. The AD491 is an externally identical derivative of the AD490 enabling GLONASS L1/L2 reception in addition to GPS and DGPS services.

 

 

 

 

 

 

 

Figure 1. AD 490 (AD491) dual frequency L1, L2 antenna with interdigital filters. The overall diameter of the base is 210 mm (8.5").

 

Electrical Specification

Patch antenna elements are used for both L1 and L2 reception. The patches are stacked with L1 topmost, consequently the phase centres are 59 mm and 51mm above the base of the antenna for L1 and L2 respectively. The radiation pattern is RH circular (+/- 0.5 dB at zenith) and omni-directional in azimuth, whilst good reception can be achieved down to 5° in elevation.

Interdigital filters are used throughout with pass-bands selected for the frequencies required. A typical application is GPS L1 (1575.42 +/-2 MHz) and L2  (1227.6 +/- 2MHz) with DGPS reception within the band 1531 to 1559 MHz. In this case, the dual interdigital filter with responses shown in figures 2 and 3 is appropriate. For P1/P2 reception the pass-bands can be broadened- an example for P2 is shown in figure 3 (right), which is also used for GLONASS reception.

A triple interdigital filtered option is also available, with pass-bands centred at 1227.6,1536, and 1575.42 MHz- the L1 section is shown in figure 4. A typical application would be to eliminate interference occurring at approximately 1560 MHz  in order to permit  DGPS reception at 1535 MHz and L1/P1 GPS at 1575.42 MHz.

For combined DGPS/GPS/GLONASS reception in the L1 band, a 5th order interdigital filter is the recommended option for use in the AD491, with the response shown in figure 5.

 

 

Figure 2. Gain and noise figure of L1 filter and amplifier. DGPS serivces are typically available in the frequency range 1531 to 1559MHz. The filter passband incorporates these frequencies and also L1/P1 GPS (1575.42 +/-10.23MHz).

 

 
 

 

Figure 3.  Top: Gain and noise figure of a L2 filter and amplifier, centred on 1227.6  MHz. Bottom: The response of a broad-band interdigital filter for L2/P2 and GLONASS reception.

 

 

 

Figure 4. Gain and noise figure of L1 filter and amplifier with frequency rejection at 1560 MHz.

 

 

Figure 5.  L1 filter response for an AD491 antenna, with passband tuned for reception of DGPS (1530 -1558MHz), GPS (1575.42MHz) and GLONASS (1597 -1611 MHz).

 

A single LNA is used for both L1 and L2 reception. It employs a low noise GAsFet input stage followed by a silicon bipolar MMIC. The amplifier’s gain is factory preset usually to 45+/-2 dB for both L1 and L2, although other gains are available on request. A noise matching input circuit gives a total noise figure for amplifier and filter of <2.5 dB across each pass-band, figures 2 and 3. Supply voltage to the antenna is in the range 5V to 20Vdc with a supply current of typically 50mA. A single output is provided for connection to the receiver. Dual outputs for separate GPS and DGPS signals are available as an option.

The antenna complies with EMC requirements and is CE marked accordingly.

Mechanical Specification

The antenna case is a two part construction. Machined from solid aluminium blocks, both parts are hard anodised and dichromate and nickel acetate sealed resulting in a 21cm diameter base with an attractive green finish that is both very tough and extremely resistant to salt water corrosion.

The radome is a 4 mm thick GRP pressure molding, which can be painted to any required colour.

The antenna is sealed to a minimum of IP67 with an ‘O’ ring preventing water ingress between the case halves; the limiting factor is the seal between the type N connector on the antenna and the corresponding connector on the coaxial cable.

Provision is made for an optional ground-plane extension.

Operating temperature range is -30°C to +70°C and storage temperature range is -40°C to +100°C.

The weight of the antenna is typically 3.2kg. Outline dimensions are shown in the drawings.

 

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