The following types of integral antenna are in common use:
Quarter-wave whip.
This consists simply of a piece of wire or rod connected to the module at one end.
At 169MHz the total length should be 421mm from module pin to antenna tip including any
interconnecting wire or tracking. Because of the length of this antenna it is almost always external to
the product casing.
Helical.
This is a more compact but slightly less effective antenna formed from a coil of wire. It is very
efficient for its size, but because of its high Q it suffers badly from detuning caused by proximity to
nearby conductive objects and needs to be carefully trimmed for best performance in a given situation.
The size shown is about the maximum commonly used at 169MHz and appropriate scaling of length,
diameter and number of turns can make individual designs much smaller.
Loop.
A loop of PCB track having an inside area as large as possible (minimum about 5cm
2
), 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 high component packing density is
necessary.
Integral antenna summary:
whip
***
***
*
**
helical
**
**
***
*
loop
*
*
**
***
Ultimate performance
Ease of design set-up
Size
Immunity to proximity effects
421mm @ 169MHz
RF
Whip antenna
wire, rod, PCB track
or a combination of these
length(mm) = 71250 / freq(MHz)
RF
Helical antenna
35-40 turns wire spring
length 120mm, dia 10mm
trim wire length or expand coil
for best results
RF
C
tune
C
match
RF GND
track width = 1mm
2
min. area 500mm
capacitors may be variable or fixed
(values depend on loop dimensions)
Loop antenna
Figure 4: integral antenna configurations
External antennas
These have several advantages if portability is not an issue, and are essential for long range links.
External antennas can be optimised for individual circumstances and may be mounted in relatively
good RF locations away from sources of interference, being connected to the equipment by coax feeder.
Helical.
Of similar dimensions and performance to the integral type mentioned above, commercially-
available helical antennas normally have the coil element protected by a plastic moulding or sleeve and
incorporate a coax connector at one end (usually a straight or right-angle BNC type). These are compact
Radiometrix Ltd.,
HX1 high power transmitter data sheet
Page 6
RADIOMETRIX [ RADIOMETRIX LTD ]
