AD420 and AD422 DGPS antennae

Robust antennae recommended for reception of DGPS spot beam services (with options for DGPS + L1 GPS, and L1 GPS + GLONASS reception) in areas where general purpose antennae are inoperable due to L-band interference.

Figure 1. AD420 (top) with a crossed dipole receiving element, and the AD422 (bottom) with a quadrafilar helical element for use at high latitudes

Mechanical Specification

The antenna body is machined in two parts from aluminium (dimensions shown in figure 1) and hard anodised resulting in a rugged grey finish resistant to salt water corrosion. An ‘O’ ring ensures a waterproof seal between the case halves to a minimum of IP67, the limiting factor being the seal between the N-type coaxial connectors (TNC optional) on the antenna and its cable.

A 4mm thick GRP pressure moulding, which can be painted to any colour required, provides a strong, but electrically low-loss radome. The mass of the antenna is 1.4kg.

Electrical Specification

The AD420 antenna element is a shaped dipole with RHCP and an omni-directional radiation pattern in azimuth. The AD422 is a variant with an RHCP quadrifilar receiving element designed to provide enhanced reception of geo-stationary satellites close to the horizon at polar latitudes. Radiation patterns are shown in figure 2.

Figure 2. Antenna element elevation gain (0 degrees is at zenith). AD420 dipole – blue line, AD422 quadrafilar element – purple line at 1545MHz.

The element is connected to a built in GaAs FET amplifier via a comb-line filter. The amplifier’s gain is factory preset within the range 25dB to 46dB (42dB typical) with a total noise figure of less than 2.5 dB including the filter. In order to suppress L band interference, a low loss comb lined filter is used, figure 3.

The amplifier is phantom powered via the coaxial output lead with a +5V to +20Vdc supply at typically 45mA. Either TNC or N-type connectors are available on request. The antenna complies with EMC requirements and is CE marked.

Operating temperature range for the antenna is -55°C to +70°C and storage temperature range is -55° C to +100° C.

Figure 3. Frequency responses of two comb filters with different bandwidths for DGPS reception (in red), and DGPS and GPS reception (in green).  The measurements were obtained over the frequency range 1500 to 1600 MHz with an Agilent HP8714ET network analyser. Comb filters are light-weight, very robust, and have low pass-band losses.
Figure 4. Gain and noise figure of AD420/AD422 low noise amplifier with a comb filter tuned for a requested bandwidth of 1519 to 1559.5 GHz. The response is characterised by a low filter loss and hence low noise figure (<2.5dB) in the required pass-band, with rapid ‘roll-off’ either side. The measurements were obtained with an HP 8970B noise figure meter.