Linx Technologies TRM-xxx-LT - Manual (Page 12)

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Antenna Considerations
The choice of antennas is a
critical and often overlooked
design consideration. The range,
performance and legality of an RF
link are critically dependent upon the
antenna. While adequate antenna
performance can often be obtained
by trial and error methods, antenna
design and matching is a complex
task. Professionally designed antennas such as those from Linx (Figure
24) help ensure maximum performance and FCC and other regulatory
compliance.
Linx transmitter modules typically have an output power that is higher
than the legal limits. This allows the designer to use an inefficient antenna
such as a loop trace or helical to meet size, cost or cosmetic requirements
and still achieve full legal output power for maximum range. If an efficient
antenna is used, then some attenuation of the output power will likely be
needed. This can easily be accomplished by using the LADJ line.
It is usually best to utilize a basic quarter-wave whip until your prototype
product is operating satisfactorily. Other antennas can then be evaluated
based on the cost, size and cosmetic requirements of the product.
Additional details are in Application Note AN-00500.
Figure 24: Linx Antennas
Helpful Application Notes from Linx
It is not the intention of this manual to address in depth many of the issues
that should be considered to ensure that the modules function correctly
and deliver the maximum possible performance. We recommend reading
the application notes listed in Figure 25 which address in depth key areas
of RF design and application of Linx products. These applications notes are
available online at www.linxtechnologies.com or by contacting Linx.
Protocol Guidelines
While many RF solutions impose data formatting and balancing
requirements, Linx RF modules do not encode or packetize the signal
content in any manner. The received signal will be affected by such factors
as noise, edge jitter and interference, but it is not purposefully manipulated
or altered by the modules. This gives the designer tremendous flexibility for
protocol design and interface.
Despite this transparency and ease of use, it must be recognized that there
are distinct differences between a wired and a wireless environment. Issues
such as interference and contention must be understood and allowed for in
the design process. To learn more about protocol considerations, read Linx
Application Note AN-00160.
Interference or changing signal conditions can corrupt the data packet,
so it is generally wise to structure the data being sent into small packets.
This allows errors to be managed without affecting large amounts of data.
A simple checksum or CRC could be used for basic error detection. Once
an error is detected, the protocol designer may wish to simply discard the
corrupt data or implement a more sophisticated scheme to correct it.
Helpful Application Note Titles
Note Number Note Title
AN-00100 RF 101: Information for the RF Challenged
AN-00125 Considerations for Operation Within the 260–470MHz Band
AN-00130 Modulation Techniques for Low-Cost RF Data Links
AN-00128 Data and Bidirectional Transmissions Under Part 15.231
AN-00140 The FCC Road: Part 15 from Concept to Approval
AN-00160 Considerations for Sending Data over a Wireless Link
AN-00500 Antennas: Design, Application, Performance
AN-00501 Understanding Antenna Specifications and Operation
Figure 25: Helpful Application Note Titles