Radiometrix Ltd., TX1 & RX1 Data Sheet

page 11

Helical antenna

R

F

Loop antenna

RF GND

RF

C

tune

C

match

capacitors may be variable or fixed

(values depend on loop dimensions)

track width = 1mm

min. area 500mm

2

Whip antenna

410mm @ 173MHz

R

F

wire, rod, PCB track

or a combination of these

length(mm) = 71250 / freq(MHz)

trim wire length or expand coil for

best results

35-40 turns wire spring

length 120mm, dia 10mm

Fig.10: integral antenna configurations

Integral antenna summary:

whip

helical

loop

Ultimate performance

***

**

*

Ease of design set-up

***

**

*

Size

*

***

**

Immunity to proximity effects

**

*

***

Tamper-proof integral antenna

Where the RX1 is used in alarm applications it may sometimes be necessary to provide a warning if any

attempt is made to remove or disable its antenna. A typical solution to this problem is as follows:

Fig.11: Tamper-proof antenna arrangement

In normal operation the output will have a resistance to ground of R1+R2. If the antenna is shorted to

ground it will show R2 only, and if the antenna is cut it will be open-circuit.

R1 and R2 may be any value over 1k

Ω. All track lengths should be kept to a minimum and the output

may need to be decoupled in some cases to avoid noise being injected into the antenna from the

following circuitry.

RX1

1 23

8 9

4 567

R2

1/4 wave

hairpin loop

antenna

R1

1n

to comparator circuit, etc.