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ANTENNA CONSIDERATIONS

The range of the RF link is widely variable and depends upon the type of antenna employed

and the operating environment. Proper design and matching of an antenna is a complex

task requiring sophisticated test equipment and a strong background in principles of RF

propagation. While adequate antenna performance can often be obtained by trial and error

methods, you may also want to consider utilizing a premade antenna from Linx. Our low-cost

antenna line is designed to ensure maximum performance and Part 15 compliance.

It is usually best to utilize a basic quarter-wave whip for your initial concept evaluation.This

can easily be made from a piece of wire as shown on the next page. Once the prototype

product is operating satisfactorily, a production antenna should be selected to meet the cost,

size and cosmetic requirements of the product. It is important to recognize that the antenna

plays a significant role in determining the performance and legality of your end product. In

order to gain a better understanding of the considerations involved in the design and

selection of antennas, please review Linx applications note #00500 “Antennas: Design,

Application, Performance”.

The following notes should help in achieving optimum 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 perform ance will be obtaine d from a 1/4- or 1/2- wave straight whip

mounted at a right angle to the ground-plane. 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.

3.

It is always a good practice to include a T-attenuation pad as describ ed under

"Board Layout Considerations". This allows your product's output power to be adjusted

for cer tification purposes without change or compromise to the antenna.

4.

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

particularly large items like transformers, batteries, and PCB tracks and ground-planes.

In many cases, the space around the antenna is as important as the antenna itself.

5.

In many antenna designs, particularly 1/4-wave whips, the ground-plane acts as a

counterpoise, forming, in essence, a 1/2-wave dipole. For this reason adequate ground-

plane area is essential. As a general rule the ground-plane to be used as counterpoise

should have a surface area

≥ the overall length of the 1/4-wave radiating element.

6.

Remove the antenna as far as possible from potential internal interference sources.

Switching power supplies, oscillators, 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 all potential sources of noise. Shield

noisy board areas whenever practical.

7.

In some applications it is advantageous to place the transmitter and its antenna

away from the main equipment. This avoids interference problems an d allows the

antenna to be oriented for optimum RF performance. Always use 50

Ω coax such as

RG-174 for the remote feed.

PARAMETER

LOOP

HELICAL

WHIP

Ultimate performance

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Ease of design setup

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Size

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Immunity to proximity effects

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Antenna Selection Chart