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.
A ground plane can also be simulated by using 3 or 4 quarter-wave radials equally spaced around the
base of the whip, connected at their inner ends to the outer of the coax feed. A better match to a 50Ω
coax feed can be achieved if the elements are angled downwards at approximately 30-40° to the
horizontal.
1/4-wave whip
1/4-wave whip
Metal ground plane
3
0
-
4
0
d
e
g
.
50
Ω
coax feed
50 Ω coax feed
Fig.12: Quarter wave antenna / ground plane configurations
Radiometrix Ltd., TX1 & RX1 Data Sheet
page 12
RADIOMETRIX [ RADIOMETRIX LTD ]
