DS000022

Rev D

July ‘99

©1999 REG No 277 4001, ENGLAND.

Page 7

FM-TX2-XXX

FM-RX2-XXX

Antenna requirements

Three types of integral antenna are recommended and approved for use with the module:

Helical

Wire coil, connected directly to pin 2, open circuit at other end. This antenna is

very efficient given it's small size (20mm x 4mm dia.). The helical is a high Q

antenna, trim the wire length or expand the coil for optimum results. The helical

de-tunes badly with proximity to other conductive objects.

Loop

A loop of PCB track tuned by a fixed or variable capacitor to ground at the 'hot'

end and fed from pin 2 at a point 20% from the ground end. Loops have high

immunity to proximity de-tuning.

Whip

This is a wire, rod, PCB track or combination connected directly to pin 2

of the module. Optimum total length is 17cm (1/4 wave @ 418MHz) Keep

the open circuit (hot) end well away from metal components to prevent

serious de-tuning. Whips are ground plane sensitive and will benefit from

internal 1/4 wave earthed radial(s) if the product is small and plastic

cased

helical

loop

whip

Ultimate performance

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Easy of design set-up

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Size

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Immunity proximity effects

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Range open ground to similar antenna

(for TX2-433-A & RX2-433-A)

200m

100m

300m

The antenna choice and position directly controls the system range. Keep it clear of other metal

in the system, particularly the 'hot' end. The best position by far, is sticking out the top of the

product. This is often not desirable for practical/ergonomic reasons thus a compromise may need

to be reached. If an internal antenna must be used try to keep it away from other metal

components, particularly large ones like transformers, batteries and PCB tracks/earth plane. The

space around the antenna is as important as the antenna itself.

A

F