TRM-915-R250 datasheet(36/39 Pages) LINX

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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, 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 is obtained from a ¼- or ½-wave straight whip

mounted at a right angle to the ground plane (Figure 83). In many

cases, this isn’t desirable for practical or ergonomic reasons, thus,

an alternative antenna style such as a helical, loop or patch may be

utilized and the corresponding sacrifice in performance accepted.

3. If an internal antenna is to be used, keep it away from other metal

components, particularly large items like transformers, batteries,

PCB tracks and ground planes. In many cases, the space around the

antenna is as important as the antenna itself. Objects in close proximity

to the antenna can cause direct detuning, while those farther away will

alter the antenna’s symmetry.

4. In many antenna designs, particularly ¼-wave whips, the ground plane

acts as a counterpoise, forming, in essence,

a ½-wave dipole (Figure 84). For this reason,

adequate ground plane area is essential.

The ground plane can be a metal case or

ground-fill areas on a circuit board. Ideally, it

should have a surface area less than or equal

to the overall length of the ¼-wave radiating

element. This 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 ground

OPTIMUM

USABLE

NOT RECOMMENDED

NUT

GROUND PLANE

(MAY BE NEEDED)

CASE

Figure 83: Ground Plane Orientation

DIPOLE

ELEMENT

GROUND

PLANE

VIRTUAL

λ/4

DIPOLE

λ/4

VERTICAL

λ/4 GROUNDED

ANTENNA (MARCONI)

Figure 84: Dipole Antenna

plane as possible in proximity to the base of the antenna. In cases

where the antenna is remotely located or the antenna is not in close

proximity to a circuit board, ground plane or grounded metal case, a

metal plate may be used to maximize the antenna’s performance.

5. Remove the antenna as far as possible from potential interference

sources. Any frequency of sufficient amplitude to enter the receiver’s

front end will reduce system range and can even prevent reception

entirely. Switching power supplies, oscillators or even relays can also

be significant sources of potential interference. 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 ground plane under potential sources of noise to shunt noise

to ground and prevent it from coupling to the RF stage. Shield noisy

board areas whenever practical.

6. In some applications, it is advantageous to place the module and

antenna away from the main equipment (Figure 85). This can avoid

interference problems and allows the antenna to be oriented for

optimum performance. Always use 50

Ω coax, like RG-174, for the

remote feed.

OPTIMUM

USABLE

NOT RECOMMENDED

NUT

GROUND PLANE

(MAY BE NEEDED)

CASE

Figure 85: Remote Ground Plane

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