siliconchip.com.au Australia’s electronics magazine May 2019 45
433MHz Wireless Data
Range Extender
Is this repeater legal to
use without a license?
In a word, yes.
You can view the “LIPD” class li-
cense for the 433MHz “ISM” band,
which applies to everyone in Austral-
ia, at: http://www.acma.gov.au/Industry/
Spectrum/Radiocomms-licens-
ing/Class-licences/lipd-class-li-
cence-spectrum-acma
The equivalent document for New
Zealand is available at: https://
gazette.govt.nz/notice/id/2017-
go4089
Note that the New Zealand EIRP
limit of -16dBm is the same as the
Australian limit of 25mW. It is simply
specified in different units.
Neither of these documents place
any restrictions on the use of the
433/434MHz LIPD band other than
the maximum effective radiated
power. There is nothing to limit how
frequently you may transmit in that
band, or how long the bursts can be.
And there is no mention of repeaters
whatsoever.
Since our repeater uses commer-
cially available 433MHz transmitters,
which comply with the power limit,
and since it only transmits after the
original transmission has ceased, it
is entirely legal to operate in Australia
and New Zealand.
However, we do not recommend
that you use this repeater with any
signals which transmit frequently.
Typically, you would use it in con-
junction with a device that sends a
short burst (well under one second)
no more frequently than, say, once
every 30 seconds. If you used it with
a device transmitting rapidly, you
could blanket the 433MHz band with
transmissions in a 100-200m radius.
The Class License states that: “If
interference occurs, the onus is on
the user of a LIPD to take measures
to resolve that interference, for exam-
ple by retuning or ceasing to operate
the device.” (Retuning these devices
would be difficult, if not impossible,
without specialised equipment).
So keep that in mind, and use
common sense when setting up your
transmitting device and repeater(s).
cluding buildings, trees and terrain.
Other things to try first
Before building a repeater, there are
some simple ways to improve range
that may give you the range you need.
The first step is to try a better antenna.
Typically, our projects use a short
length of wire as the antenna, sized
to be one-quarter of the wavelength.
This is around 170mm for a 433MHz
transmitter or receiver.
Using a commercially-made whip
antenna for the transmitter and/or
receiver can improve the range com-
pared to the simple wire antenna, as
can a longer ½-wave antenna (340mm
for 433MHz).
But we must caution you that if your
transmitter is close to the 25mW le-
gal limit, using a better antenna (with
higher gain) may be illegal. That’s be-
cause 25mW is the effective radiated
power limit, so it takes into account
antenna gain. Each increase in antenna
gain of 3dB is equivalent to doubling
the output power.
So you cannot legally use a +3dBi
antenna with devices that exceed
12.5mW transmit power.
Antenna orientation is important
too. Having the transmitter and re-
ceiver antenna both with the same
orientation, eg, both orientated ver-
tically or both horizontally may im-
prove range.
If these changes prove to be imprac-
tical or not effective enough, then it
would make sense for you to build
this repeater.
A repeater is placed in the signal
path between the transmitter and re-
ceiver.
In this case, the repeater comprises
a UHF receiver and UHF transmitter,
plus a microcontroller, some memory
and a power supply. Once the repeater
receives a valid signal, it is stored and
With a solar panel to keep the internal battery charged, you’ll never have to
touch it once completed. Get up to double the range you had originally!
