Radiometrix Ltd, TX3A & RX3A Data Sheet

page 10

Depending on the application and bearing in mind applicable legal requirements (see p.11), a

variety of antenna types may be used with the TX3A and RX3A.

Integral antennas generally do not perform as well as externally mounted types, however

they result in physically compact equipment and are the preferred choice for portable

applications. The following can be recommended:

Whip (¼-wave). This consists simply of a piece of wire or rod connected to the module at one

end. The lengths given below are from module pin to antenna tip including any

interconnecting wire or tracking. This antenna is quite simple and performs well, especially if

used in conjunction with a ground plane. This will often be provided by the PCB on which the

module is mounted, or by a metal case.

Base-loaded whip. This is a shortened whip, tuned by means of a coil inserted at the base.

This coil may be air-wound for maximum efficiency, or a small SMT inductor can be used if

space is at a premium. The value must be carefully chosen to tune the particular length of

whip in use, making this antenna more difficult to set up than a ¼-wave whip.

Helical. This is a compact but slightly less effective antenna formed from a coil of wire. It is

very efficient for its size, but has high Q and tends to suffer badly from detuning caused by

proximity to nearby conductive objects. It needs to be carefully trimmed for best performance

in a given situation and the required dimensional tolerances can be difficult to achieve

repeatably, nevertheless it can provide a very compact solution.

Loop. A loop of PCB track, tuned and matched with 2 capacitors. Loops are relatively

inefficient but have good immunity to proximity detuning, so may be preferred in shorter

range applications where very high component packing density is necessary.

1/4-wave whip

RF

Loop

L

RF

Base-loaded whip

Helical

RF

RF-GND

RF

C

tune

C

match

trim wire length, number of turns

and turns spacing for best results

wire spring, dia 3mm approx.

wire, rod, PCB track

or a combination of these

L (mm) = 71250 / freq(MHz)

shortened wire, rod, PCB track etc.

with loading coil.

SMT inductor may be used if reqd.

capacitors 0.5-5pF variable or fixed

(values depend on loop dimensions)

track width = 1mm

area 100 - 400 mm

2

Fig.8: Integral antenna configurations