TRM-915-R250 datasheet(37/39 Pages) LINX

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Common Antenna Styles

There are hundreds of antenna styles and variations that can be employed

with Linx RF modules. Following is a brief discussion of the styles most

commonly utilized. Additional antenna information can be found in Linx

Application Notes AN-00100, AN-00140, AN-00500 and AN-00501. Linx

antennas and connectors offer outstanding performance at a low price.

Whip Style

A whip style antenna (Figure 86) provides

outstanding overall performance and stability.

A low-cost whip can be easily fabricated from

a wire or rod, but most designers opt for the

consistent performance and cosmetic appeal of

a professionally-made model. To meet this need,

Linx offers a wide variety of straight and reduced

height whip style antennas in permanent and

connectorized mounting styles.

The wavelength of the operational frequency determines

an antenna’s overall length. Since a full wavelength

is often quite long, a partial ½- or ¼-wave antenna

is normally employed. Its size and natural radiation

resistance make it well matched to Linx modules.

The proper length for a straight ¼-wave can be easily

determined using the formula in Figure 87. It is also

possible to reduce the overall height of the antenna by

using a helical winding. This reduces the antenna’s bandwidth but is a great

way to minimize the antenna’s physical size for compact applications. This

also means that the physical appearance is not always an indicator of the

antenna’s frequency.

Specialty Styles

Linx offers a wide variety of specialized antenna

styles (Figure 88). Many of these styles utilize helical

elements to reduce the overall antenna size while

maintaining reasonable performance. A helical

antenna’s bandwidth is often quite narrow and the

antenna can detune in proximity to other objects, so

care must be exercised in layout and placement.

L =

234

F MHz

Figure 86: Whip Style Antennas

Figure 87:

L = length in feet of

quarter-wave length

F = operating frequency

in megahertz

Figure 88: Specialty Style

Antennas

Loop Style

A loop or trace style antenna is normally printed

directly on a product’s PCB (Figure 89). This

makes it the most cost-effective of antenna

styles. The element can be made self-resonant or

externally resonated with discrete components,

but its actual layout is usually product specific.

Despite the cost advantages, loop style antennas

are generally inefficient and useful only for short

range applications. They are also very sensitive to changes in layout and

PCB dielectric, which can cause consistency issues during production.

In addition, printed styles are difficult to engineer, requiring the use of

expensive equipment including a network analyzer. An improperly designed

loop will have a high VSWR at the desired frequency which can cause

instability in the RF stage.

Linx offers low-cost planar (Figure 90) and chip

antennas that mount directly to a product’s PCB.

These tiny antennas do not require testing and

provide excellent performance despite their small

size. They offer a preferable alternative to the often

problematic “printed” antenna.

Figure 89: Loop or Trace Antenna

Figure 90: SP Series

“Splatch” Antenna

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