Radiometrix Ltd

TDH2 Data Sheet

page 9

Antenna requirements

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

A) Whip

This is a wire, rod ,PCB track or combination connected directly to RF pin of the

module. Optimum total length is 16cm (1/4 wave @ 433MHz). 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

B) Helical

Wire coil, connected directly to RF pin, 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.

C) Loop

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

fed from RF pin at a point 20% from the ground end. Loops have high immunity to

proximity de-tuning.

A

B

C

whip

helical

loop

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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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. Helical antenna

B. Loop antenna

RF

C1

C2

C4

C3

RF-GND

C. Whip antenna

16.4cm

RF

0.5 mm enameled copper wire

close wound on 3.2 mm diameter former

433 MHz = 24 turns

Feed point 15% to 25% of total loop length

track width = 1mm

4 to 10 cm inside area

wire, rod, PCB-track or a combination

of these three

433 MHz = 16.4 cm total from RF pin.

2

RF

Figure 8: Antenna types