NRX1 Datasheet, PDF (6/8 Pages) Radiometrix Ltd – Low cost VHF Narrow Band FM receiver

410mm @ 173MHz
Whip antenna
wire, rod, PCB track
RF _{or a combination of these}
length(mm) = 71250 / freq(MHz)
Helical antenna
35-40 turns wire spring
RF
length 120mm, dia 10mm
trim wire length or expand coil
for best results
C_tune
Loop antenna
RF
C _match
track width = 1mm
min. area 500mm ²
capacitors may be variable or fixed
(values depend on loop dimensions)
RF GND
Figure 5: 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.
Ground plane
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.
As shown on figure 9, 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.
Radiometrix Ltd.
NRX1 receiver data sheet
Page 6