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. A professionally designed
Figure 17: Linx Antennas
antenna, such as those from Linx, will
help ensure maximum performance and FCC compliance.
Linx transmitter modules typically have an output power that is slightly 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 or a T-pad attenuator. For more
details on T-pad attenuator design, please see Application Note AN-00150.
A receiver antenna should be optimized for the frequency or band in which the
receiver operates and to minimize the reception of off-frequency signals. The
efficiency of the receiver’s antenna is critical to maximizing range performance.
Unlike the transmitter antenna, where legal operation may mandate attenuation
or a reduction in antenna efficiency, the receiver’s antenna should be optimized
as much as is practical.
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. You may wish to review
Application Note AN-00500 “Antennas: Design, Application, Performance”
ANTENNA SHARING
In cases where a transmitter and receiver
module are combined to form a transceiver,
it is often advantageous to share a single
antenna. To accomplish this, an antenna
Transmitter
Module 0.1 μ F
0.1 μ F
VDD
0.1 μ F
Antenna
switch must be used to provide isolation
between the modules so that the full
transmitter output power is not put on the
Receiver
Module
GND
GND
0.1 μ F
sensitive front end of the receiver. There
are a wide variety of antenna switches that
are cost-effective and easy to use. Among
0.1 μ F
Select
Figure 18: Typical Antenna Switch
the most popular are switches from Macom and NEC. Look for an antenna
switch that has high isolation and low loss at the desired frequency of operation.
Generally, the Tx or Rx status of a switch will be controlled by a product’s
microprocessor, but the user may also make the selection manually. In some
cases, where the characteristics of the Tx and Rx antennas need to be different
or antenna switch losses are unacceptable, it may be more appropriate to utilize
two discrete antennas.
Page 14
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