by Peter Parker VK3YE - first appeared in Amateur Radio, August 1997
Repeater operating is one of the most
popular facets of amateur radio. For the Novice Limited licensee, restricted to
30 watts on 2m and 70cm, repeaters offer a means to make longer distance
VHF/UHF contacts, especially when operating mobile and away from home. This
article looks at the technical aspects of amateur voice repeaters; other topics
such as equipment and operating procedure were covered in October 1995's Novice
Notes.
Voice repeaters extend a station's
transmitting range by receiving incoming transmission and simultaneously
retransmitting them on another frequency. They consist of a receiver,
transmitter, filter, antennas and timing/control circuitry. Repeaters are
normally situated on hill tops or high buildings for best coverage of a
particular area.
Repeaters use two frequencies; an input (or
receive) frequency and an output (or transmit) frequency. These frequencies are
600 kHz apart for two metre repeaters and 5 MHz apart for repeaters operating
on 70 centimetres.
For a repeater to transmit, it needs to be
activated by a sufficiently strong signal on its input frequency. The presence
of such a signal activates the repeater's squelch or mute circuit. This causes
the repeater to start transmitting. The audio from the receiver is fed direct
to the transmitter. The result is that for as long as there is a strong enough
signal on the repeater's input, the transmitter is activated. Those listening
to the repeater's output frequency will then hear a stronger, retransmitted
version of the signal on the input.
Ex-commercial VHF FM transceivers such as
the Philips FM828 are often used in amateur repeaters due to their low cost,
good performance, and easy availability. Depending on the availability of power
at a repeater site, wind, solar or 240 volt mains may be used to run the
repeater. Rechargeable lead acid batteries are often used to provide backup
during power failures. Because their receivers are operating 24 hours per day,
even very quiet repeaters draw appreciable currents. Antennas used in repeaters
must be built to withstand adverse weather. This is because access to a site
may be dificult or time-consuming, particularly if it is shared with other
users or is in a remote location.
The main difficulty when designing a voice
repeater is the closeness of the transmit and receive frequencies. This small
difference makes it very easy for the repeater's receiver to be overpowered
('desensed') by the strong signal from the repeater's own transmitter. Failure
to cure this problem makes the repeater unable to receive weak input signals.
Repeater builders separate the transmit and receive antennas and install banks
of cavity filters before the repeater's receiver and after the transmitter to
eliminate desensing. Cavity filters are simply very sharp (high Q) tuned
circuits. Depending on how they are connected, they can be made to act like
band pass or notch filters. The notch filter connection is used when you want
to deeply attenuate signals on a particular frequency, but want little attenuation
of signals on other frequencies. This would be useful at a repeater's receiver,
where it is important to attenuate the signal transmitted by the repeater's
transmitter (to prevent desensing) but allow good sensitivity at the repeater's
input frequency.
As well as transmitting, receiving and
filtering equipment, repeaters include timing and control circuitry. The
sophistication of this varies between repeaters. The following are some
functions performed by these circuits:-
Modern repeaters use a control board
featuring an EPROM microprocessor IC to perform many of these functions. By
programming this chip, repeater builders can customise the functions provided
to suit their needs.
Most repeaters are open access. This means
that a carrier signal on the right input frequency is all that is required to
operate the device. However, a few repeaters are 'closed', ie they require
special tones to be transmitted before they will operate.
For example, a UHF repeater may include a
user-controlled crossband link to a repeater on 29 MHz FM. By transmitting a
suitable tone to activate the link, the user can enjoy contacts on ten metres
FM, even though they themselves do not posses HF equipment. Signals with no
access tone will be retransmitted on the UHF repeater output only.
Where a repeater is installed near
high-powered VHF or UHF transmitters, it may be continually being triggered by
spurious signals or mixing products. Making the repeater's mute tone-activated
means that only amateurs sending the required tone can open the repeater. This
makes monitoring the repeater more pleasant - an important consideration given
that far more people listen to repeaters than actually talk on them!
Repeater builders may have other reasons for
making their repeater tone access only. The frequencies of the tones used by
these repeaters are given in the repeater section of the Australian Callbook.
So what are the main ways to control access
to a repeater or repeater link? There are at least three methods. These are as
follows:-
As mentioned before, two or more repeaters
may be linked together so that users of one repeater can talk to users of
another. This is done for the following reasons:-
In places where activity is sparse (eg
country areas), repeater sponsors usually want to have the link operational at
all times. Where there is more activity or links are used for special purposes
only, users may wish to switch links on and off. This can be done by installing
special tone decoding circuitry at the repeater. It is then up to the operator
to decide whether to activate the link by transmitting the correct CTCSS or
DTMF control tone.
Linked repeaters must have better than
average receive and transmit quality. This is because each transmitter and
receiver in the chain between the transmitting and receiving station degrades
the quality of the signal slightly. Generally speaking, the simpler the link,
the greater its reliability and the better the recovered audio.
In repeater linking there is considerable
scope for ingenuity and experimentation. The following are examples of the
types of links that are possible:-
An off-air link does not have to be between
two repeaters; a remote link comprising a UHF transceiver connected to a two
metre rig on a nearby hill could provide access to a 70 cm repeater from two
metres without the need for a full 2 metre repeater. Such installations are
particularly attractive for coverage of small geographic areas, or where
activity is insufficient to justify the installation of a stand-alone repeater.
Simplex gateways. A simplex gateway is a
means to allow access to a simplex frequency by users of a repeater, or vice
versa. In its simplest form, it consists of a single frequency FM transceiver
wired to a conventional repeater. A typical application is to provide access to
ten metres FM to UHF repeater users. This allows UHF operators to enjoy
interstate and overseas contacts given favourable conditions on 29 MHz.
All repeaters listed in the Australian
Callbook use at least two frequencies and operate on FM. However, not all
repeaters operate like this. This section introduces the reader to some
lesser-known types of repeaters.
A parrot repeater differs from a
conventional repeater in that it uses one frequency only. Its name is very apt.
Incoming signals are recorded on a digital voice recorder inside the repeater.
When the user stops transmitting, the repeater switches to transmit and plays
back the recording. This retransmission is heard by all stations monitoring the
frequency. In other words, whereas conventional repeaters retransmit the
incoming signal on a different frequency while the user is talking, parrot
repeaters retransmit a recording of the signal immediately after the user has
finished. Thus, unlike a normal repeater which transmits as it receives, the
parrot repeater is either receiving or transmitting and does not do both
simultaneously.
This need for repetition makes communicating
via parrot repeaters slower than through conventional repeaters. As well, there
is a risk of users accidentally transmitting over one another. For this reason,
parrot repeaters are most useful during emergency-type communication exercises
where transmissions are normally kept short.
Parrot repeaters are very simple to build -
no cavity filters or separate antennas are required. Those with some
constructional expertise may wish to build their own by adding a digital voice
recorder and associated control circuitry to a standard amateur mobile or hand-held
transceiver.
Conventional repeaters transmit and receive
FM only and relay only one transmission at a time. Linear translators, in
contrast, can relay many signals at once. They work by receiving a segment of
frequencies in one amateur band and retransmitting it in another band. Any
signal, whatever its mode, appearing within the linear translator's receive
passband is retransmitted. This means that a linear translator with a
sufficiently wide passband (say 50kHz or more) can relay several Morse, voice
and data transmissions simultaneously. Tuning across the output of a linear
translator is a lot like tuning across an amateur HF band, where signals of
several different modes can be heard.
Unlike FM repeaters, which demodulate the incoming
signal and use the resulting audio to operate the transmitter section, no
demodulation takes place within linear translators. Instead, linear translators
use a mixing, filtering and conversion process, similar to that which operates
in a superhetrodyne radio receiver. Again like superhet receivers, linear
translators incorporate automatic gain control (AGC) circuits to prevent strong
signals from overloading the system.
Most amateur satellites incorporate linear
translators. However, their use on land is limited, and none are licensed in
Australia. Because they can relay SSB and CW signals alongside FM
transmissions, a terrestrial linear translator could have better coverage than
a conventional FM repeater at the same site. As well, they make possible
activities such as full-duplex voice operation (similar to speaking on a
telephone) and transmitting slow-scan television images simultaneously with a
voice commentary. For the experimenter, a linear translator would be an
advanced project calling for a high level of expertise and access to test
equipment.
This article has given the reader a quick
tour of the various types of voice repeaters, with special emphasis given to
repeater linking and control. More information about all aspects of repeaters
can be obtained from your local repeater committee or group. Also, see the
Repeater Link column elsewhere in this issue. I would like to thank Will McGhie
VK6UU for his assistance in the preparation of this article.
back to Gateway Branching
Out Page
This page was produced by Peter Parker VK3YE parkerp@NOSPAMalphalink.com.au. Material may be copied for personal or non-profit use only.