Integral antenna summary:
whip
***
***
*
helical
Ultimate performance
Ease of design set-up
Size
**
**
***
*
Immunity to proximity effects
**
471mm @ 151MHz
wire, rod, PCB track
or a combination of these
RF
Whip antenna
length(mm) = 71250 / freq(MHz)
35-40 turns wire spring
length 120mm, dia 10mm
RF
trim wire length or expand coil
for best results
Helical antenna
Figure 8: 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
and simple to use as they come pre-tuned for a given application, but are relatively inefficient and are
best suited to shorter ranges.
Quarter-wave whip. Again similar to the integral type, the element usually consists of a stainless
steel rod or a wire contained within a semi-flexible moulded plastic jacket. Various mounting options
are available, from a simple BNC connector to wall brackets, through-panel fixings and magnetic
mounts for temporary attachment to steel surfaces.
A significant improvement in performance is obtainable if the whip is used in conjunction with a metal
ground plane. For best results this should extend all round the base of the whip out to a radius of
300mm or more (under these conditions performance approaches that of a half-wave dipole) but even
relatively small metal areas will produce a worthwhile improvement over the whip alone. The ground
plane should be electrically connected to the coax outer at the base of the whip. Magnetic mounts are
slightly different in that they rely on capacitance between the mount and the metal surface to achieve
the same result.
Radiometrix Ltd.,
BiM1H transceiver data sheet
Page 8
RADIOMETRIX [ RADIOMETRIX LTD ]
