User's Manual

CT-EM250 ZigBee-Ready RF Transceiver Modules

The antenna should be kept away (> 10mm) from metallic or other conductive and

dielectric materials, and should never be used inside a metallic enclosure.

Compared to lower frequencies, operation at 2.4GHz usually has a shorter LOS.

However, the ZigBee mesh network topology provides a more flexible and reliable

network topology to end users. The farther end device can easily be reached over a

mesh network through other full function (routing) devices. Reflections from walls and

other objects may give multi-path fading resulting in dead-zones. With mesh network,

other nodes in the network can reach devices in dead-zones. Furthermore, in the

case of any failure of a single node, the system can easily reroute to other paths. The

mesh network is therefore highly recommended for increased reliability and extended

coverage.

In applications where the module must be placed in a metallic enclosure, an external

antenna must be used. The external antenna must match to 50 Ohm.

A PCB antenna can be made as a copper track where the ground plane is removed on

the back side. The rest of the PCB board should have a ground plane as large as

possible, preferably as large (in one dimension) as the antenna itself, to make it act as

a reflector mirror to the antenna. A quarter-wavelength antenna on a PCB must be

shorter than the wire antenna due to the influence of the dielectric material of the PCB.

The length reduction depends on the PCB thickness and material, as well as how

close to the edge of the board the antenna is placed. Typical reduction is to 75-90 %

but specific results may vary.

The length of a quarter-wave antenna is given in the table below.

Frequency

( MHZ )

Length of whip

antenna

( cm )

Length of PCB track

( cm )

2450 2.9 2.2---2.7

If, for space reasons, the track is made even shorter than the resonating quarter of

wavelength, the antenna should be matched to 50 ohms using a series inductor and a

shunt capacitor.