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.9: Quarter wave antenna / ground plane configurations
Half-wave. There are two main variants of this antenna, both of which are very effective and are
recommended where long range and all-round coverage are required:
1. The half-wave dipole consists of two quarter-wave whips mounted in line vertically and fed in the
centre with coaxial cable. The bottom whip takes the place of the ground plane described previously.
A variant is available using a helical instead of a whip for the lower element, giving similar
performance with reduced overall length. This antenna is suitable for mounting on walls etc. but for
best results should be kept well clear of surrounding conductive objects and structures (ideally >1m
separation).
2. The end-fed half wave is the same length as the dipole but consists of a single rod or whip fed at the
bottom via a matching network. Mounting options are similar to those for the quarter-wave whip. A
ground plane is sometimes used but is not essential. The end-fed arrangement is often preferred
over the centre-fed dipole because it is easier to mount in the clear and above surrounding
obstructions.
Yagi. This antenna consists of two or more elements mounted parallel to each other on a central boom.
It is directional and exhibits gain but tends to be large and unwieldy – for these reasons the yagi is the
ideal choice for links over fixed paths where maximum range is desired.
Radiometrix Ltd.,
BiM1H transceiver data sheet
Page 9
RADIOMETRIX [ RADIOMETRIX LTD ]
