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GENERAL ANTENNA RULES
The following general rules should help in maximizing antenna performance:
1. Proximity to objects such as a user's hand or body, or metal objects will cause
an antenna to detune. For this reason the antenna shaft and tip should be
positioned as far away from such objects as possible.
2. Optimum performance will be obtained from a 1/4- or 1/2-wave straight whip
mounted at a right angle to the groundplane. In many cases, this isn't desirable
for practical or ergonomic reasons; thus, an alternative antenna style such as
a helical, loop, patch, or base-loaded whip may be utilized and the
corresponding sacrifice in performance accepted.
3. If an internal antenna is used, keep it away from other metal components,
particularly large items like transformers, batteries, and PCB tracks and
groundplanes. In many cases, the space around the antenna is as important
as the antenna itself.
4. In many antenna designs, particularly 1/4-wave whips,
the groundplane acts as a counterpoise, forming, in
essence, a 1/2-wave dipole. For this reason adequate
groundplane area is essential. The groundplane can be a
metal case or ground-fill on the circuit board. Ideally, the
groundplane to be used as counterpoise should have a
surface area ≥ the overall length of the 1/4-wave
radiating element and be oriented at a 90° angle. Such
an orientation is often not practical due to size and
configuration constraints. In these
instances a designer must make the
best use of the area available to create
as much groundplane in proximity to the
base of the antenna as possible. In
instances where the antenna is
remotely located or the antenna is not in
close proximity to a circuit board plane
or grounded metal case, a small metal
plate may be fabricated to maximize
antenna performance.
5. Remove the antenna as far as possible
from potential interference sources such as switching power supplies,
oscillators, motors and relays. Remember, the single best weapon against
such problems is attention to placement and layout. Filter the module's power
supply with a high-frequency bypass capacitor. Place adequate groundplane
under all potential sources of noise. Shield noisy board areas whenever
practical.
6. In some applications it is advantageous to place the receiver and its antenna
away from the main equipment. This avoids interference problems and allows
the antenna to be oriented for optimum RF performance. Always use 50Ω
coax, such as RG-174, for the remote feed.
I
E
DIPOLE
ELEMENT
GROUND
PLANE
VIRTUAL
λ
/4
DIPOLE
λ
/4
λ
/4
VERTICAL
λ
/4 GROUNDED
ANTENNA (MARCONI)
OPTIMUM
USEABLE
NOT RECOMMENDED
NUT
GROUNDPLANE
(MAY BE NEEDED)
CASE
Figure 25: Antenna Orientations
