W-Linx Technology TXE-315-KH Antenna Considerations, Antenna Sharing, General Antenna Rules

Figure 17: Linx Antennas

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 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,		VDD
it is often advantageous to share a single	Transmitter	0.1μF
	Module	0.1μF
antenna. To accomplish this, an antenna		Antenna
		0.1μF
switch must be used to provide isolation		GND
		0.1μF
between the modules so that the full
	Receiver	GND
transmitter output power is not put on the
	Module
sensitive front end of the receiver. There		0.1μF
are a wide variety of antenna switches that		Select
	Figure 18: Typical Antenna Switch
are cost-effective and easy to use. Among

the most popular are switches from Macom and NEC. Look for an antenna

GENERAL ANTENNA RULES

The following general rules should help in maximizing antenna performance.

1.Proximity to objects such as a user’s hand, 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 performance will be obtained

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,	OPTIMUM
	NOT RECOMMENDED
an alternative antenna style such as a	USEABLE

helical, loop, or patch may be utilized Figure 19: Ground Plane Orientation and the corresponding sacrifice in performance accepted.

3.If an internal antenna is to be used, keep it away from other metal components, particularly large items like transformers, batteries, PCB tracks, and ground planes. In many cases, the space around the antenna is as important as the antenna itself. Objects in close proximity to the antenna can cause direct detuning, while those farther away will alter the antenna’s symmetry.

4. In many antenna designs, particularly 1/4-wave
	VERTICAL λ/4 GROUNDED
whips, the ground plane acts as a counterpoise,
	ANTENNA (MARCONI)
forming, in essence, a 1/2-wave dipole. For this
	E	DIPOLE
		ELEMENT
reason, adequate ground plane area is essential.			λ/4
The ground plane can be a metal case or ground-fill	I
areas on a circuit board. Ideally, it should have a
surface area > the overall length of the 1/4-wave	GROUND
	PLANE
radiating element. This is often not practical due to	VIRTUAL λ/4		λ/4
	DIPOLE
size and configuration constraints. In these
instances, a designer must make the best use of the
	Figure 20: Dipole Antenna
area available to create as much ground plane as

possible in proximity to the base of the antenna. In cases where the antenna is remotely located or the antenna is not in close proximity to a circuit board, ground plane, or grounded metal case, a metal plate may be used to maximize the antenna’s performance.

5.Remove the antenna as far as possible from potential interference sources. Any frequency of sufficient amplitude to enter the receiver’s front end will reduce system range and can even prevent reception entirely. Switching power supplies, oscillators, or 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 potential sources of noise to shunt noise to ground and prevent it from coupling to the RF stage. Shield noisy board areas whenever practical.

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.

6.In some applications, it is advantageous to place the module and antenna away from the main equipment. This can avoid interference problems and allows the antenna to be

oriented for optimum performance. Always use 50Ω coax, like RG-174, for the remote feed.

	CASE
NUT	GROUND PLANE
	(MAY BE NEEDED)

Figure 21: Remote Ground Plane

Page 14 Page 15