Radiometrix Ltd., TX1 & RX1 Data Sheet

page 10

Antennas

The choice and positioning of transmitter and receiver antennas is of the utmost importance and is the

single most significant factor in determining system range. The following notes are intended to assist

the user in choosing the most effective antenna type for any given application.

Integral antennas

These are relatively inefficient compared to the larger externally-mounted types and hence tend to be

effective only over limited ranges. They do however result in physically compact equipment and for this

reason are often preferred for portable applications. Particular care is required with this type of

antenna to achieve optimum results and the following should be taken into account:

1. Nearby conducting objects such as a PCB or battery can cause detuning or screening of the antenna

which severely reduces efficiency. Ideally the antenna should stick out from the top of the product

and be entirely in the clear, however this is often not desirable for practical/ergonomic reasons and

a compromise may need to be reached. If an internal antenna must be used try to keep it away

from other metal components and pay particular attention to the “hot” end (i.e. the far end) as this

is generally the most susceptible to detuning. The space around the antenna is as important as the

antenna itself.

2. Microprocessors and microcontrollers tend to radiate significant amounts of radio frequency hash

which can cause desensitisation of the receiver if its antenna is in close proximity. The problem

becomes worse as logic speeds increase, because fast logic edges generate harmonics across the VHF

range which are then radiated effectively by the PCB tracking. In extreme cases system range may

be reduced by a factor of 5 or more. To minimise any adverse effects situate antenna and module as

far as possible from any such circuitry and keep PCB track lengths to the minimum possible. A

ground plane can be highly effective in cutting radiated interference and its use is strongly

recommended.

A simple test for interference is to monitor the receiver RSSI output voltage, which should be the same

regardless of whether the microcontroller or other logic circuitry is running or in reset.

The following types of integral antenna are in common use:

Quarter-wave whip. This consists simply of a piece of wire or rod connected to the module at one end.

At 173MHz the total length should be 410mm from module pin to antenna tip including any

interconnecting wire or tracking. Because of the length of this antenna it is almost always external to

the product casing.

Helical. This is a more compact but slightly less effective antenna formed from a coil of wire. It is very

efficient for its size, but because of its high Q it suffers badly from detuning caused by proximity to

nearby conductive objects and needs to be carefully trimmed for best performance in a given situation.

The size shown is about the maximum commonly used at 173MHz and appropriate scaling of length,

diameter and number of turns can make individual designs much smaller.

matched with 2 capacitors. Loops are relatively inefficient but have good immunity to proximity

detuning, so may be preferred in shorter range applications where high component packing density is

necessary.