Radiometrix Ltd.,

HX1 high power transmitter data sheet

Page 5

Applications information

Expected range

Predicting the range obtainable in any given situation is notoriously difficult since there are many

factors involved. The main ones to consider are as follows:

•

Type and location of antennas in use

•

Type of terrain and degree of obstruction of the link path

•

Sources of interference affecting the receiver

•

“Dead” spots caused by signal reflections from nearby conductive objects

•

Data rate and degree of filtering employed

The following are typical examples – but range tests should always be performed before assuming that

a particular range can be achieved in a given situation:

Data rate

Tx antenna

Rx antenna

Environment

Range

1.2kbps

half-wave

half-wave

rural/open

10-15km

10kbps

half-wave

half-wave

rural/open

3-4km

10kbps

helical

half-wave

urban/obstructed

500m-1km

10kbps

helical

helical

in-building

100-200m

Note: The figure for 1.2kbps assumes that the receiver bandwidth has been suitably reduced by

utilising an outboard sallen-key active audio filter and data slicer or similar arrangement.

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.