A whip-style monopole antenna provides outstanding overall

performance and stability. A low-cost whip can be easily fabricated

from wire or rod, but most product designers opt for the improved

performance and cosmetic appeal of a professionally made model.

To meet this ne ed, Linx offers a wide variety of straight and

r e d u c e d - h e i g h t w h i p - s t y l e a n t e n n a s i n p e r m a n e n t a n d

connectorized mounting styles.

Th e waveleng th of t he ope rational frequency determ ines an

antenna's overall length. Since a full wavelength is often quite long,

a partial 1/4-wave antenna is nor mally employed. Its size and

natural radiation resistance make it well matched to Linx modules.

The proper length fo r a 1 /4-wave antenna can be easily found

using the formula below. It is also possible to reduce the overall

height of the antenna by using a helical winding. This decreases

the antenna's bandwidth but is an excellent way to minimize the

antenna's physical size for compact applications.

Page 9

Helical Style

Whip Style

Loop Style

1/4 wave wire lengths for

RM frequencies:

418MHz = 6.7"

433MHz = 6.5"

COMMON ANTENNA STYLES

There are literally hundreds of antennas styles that can be successfully employed with

the RM Series. Following is a brief discussion of the three styles most commonly utilized

in compact RF designs. Additional antenna information can be found in Linx application

notes #00500, #00100, #00126 and #00140.

A helical antenna is pr ecisely for med from wire o r rod. A helical

ante nna is a good choice for low-cost prod ucts requiring average

range performance and internal concealment. A helical can detune

badly in proximity to other objects and its bandwidth is quite narrow

so care must be exercised in layout and placement.

A loop- or trace-style ant enna is n ormally printed directly on a

produ ct's PCB. This makes it the most cost-effective of antenna

styles. There are a variety of shapes and layout styles which can be

utilized. The element can be made self-resonant or externally

resonated with discrete components. Despite its cost advantages,

PCB antenna styles are generally inefficient and useful only for short-

range applications. Loop-style antennas are also ver y sensitive to

changes in layout or substrate dielectr ic which can int roduce

consistency issues into the production process. In addition, printed

styles initially are difficult to engineer, requiring the use of expensive

equipment including a network analyzer. An improperly designed loop

will have a high SWR at the desired frequency which can introduce

substantial instability in the RF stages.

Linx offers a low-cost planar antenna called the “SPLATCH” which is

an exce llent alter native to the sometimes proble matic PCB trace

style. This tiny antenna mount s directly to a product's PCB and

requires no test ing or tuning. Its design is stable even in compact

applications and it provides excellent performance in light of its

compact size.

Where:

L=length in feet of quarter-wave length

F=operating frequency in megaher tz