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Amateur Radio Info & Exams - VHF FM - Repeater and Simplex Operation

Repeater Operation

Repeaters are automatic stations which receive a signal, and simultaneously retransmit it on a second frequency. Most are in the 10 metre to 23 cm range. TV ones may receive on a higher band, and retransmit on 70 cm. For voice 2 metres and 70 centimetres are popular for repeaters.

Repeaters can be located on hill-tops / mountains, communications towers, tall buildings, and grain silos. These can be stand-alone, or integrated into a commercial site, even sharing antennas through combiners. Small cross-band repeaters (below) are sometimes flown on re-purposed weather balloons.

A variation of the repeater theme is the "digipeater", a device which receives packets of data, including location information, and retransmits them.

Repeaters transmits on the advertised frequency, such as 147.250 MHz (Hornsby), or 146.650 MHz (Mt Bindo). Australian repeaters use 25 kHz spacing, European may be 12.5 kHz, as the band has been restricted to 144 to 146 MHz. US systems use a 30 kHz spacing, sometimes 15 kHz. The input has to be offset from the output, and for 2 metres this is traditionally by 600 kHz. Below 147 MHz this is a negative offset, so for Bindo, you transmit at 146.050 MHz; and above 147 MHz, a positive offset, so for Hornsby it is 147.850 MHz. In some places 147.000 goes up, to 147.600 MHz; some down, to 146.400 MHz. Dural is negative. As there are stupidly large numbers of repeaters in small US states, there are also systems in the 144 - 145 MHz area, some using 600 kHz offsets, and some (including DMR ones) using larger offsets into the 147.4 to 147.6 MHz segment normally used for simplex.

On 70 cm the offset is typically 5.00 MHz. This can be upward or downward, depending whether the repeater is at the bottom or top of the 440 to 450 MHz. In Australia repeater outputs are in the 438-440 MHz segment, with downward offsets, including 5 MHz. However, as idiot bureaucrats here not only allowed low duty cycle devices , such as car door openers, and wireless door-bells, like in Europe, but small walkie-talkies, wireless headphones, and even crane controllers to be used in the 433.050 - 434.790 segment, repeaters are now using -5.4 and -7 MHz offsets. Some remain at -5 MHz, especially if the input is outside this segment.

Many radios have a "Reverse" function, used to listen to the repeaters input frequency. This can be used to check for interference, or to see if you can hear the person you are having a conversation with directly, before suggesting a change to a simplex channel. On many radios it also lets you transmit on the output frequency, which may or may not be a good idea.

For technical reasons, there is an upwards trend in the offset with increasing frequency. These include the special filtering devices called cavities, used to isolate transmitter and receiver signals. Cavity filters can be around 30 cm in diameter, and 2 metre high for the lowest bands; down to maybe 10cm by 40 cm, and several are needed.

Some repeaters are part of a network, or are linked on-site to repeaters on different bands. These function as, say, a normal 2m repeater, and a normal 70 cm repeater, although there is an option to transmit on one band, and listen on the other. These are different to a purely cross-band device. On these one might transmit on 70 cm, and listen on 2m. A benefit of such systems is that there is usually no need for heavy cavity filters, so they can be set up in a vehicle, or even operate on a space station. Some hand-helds and mobile radios have a cross-band repeat mode.

Some repeaters have an Internet linking, where it is possible to dial into other repeaters or conference servers. Many DMR repeaters are also linked into networks.

VHF  folded dipoleThis is a (rather grotty) folded dipole for VHF High band, which covers 2 metres. These are mounted vertically, using the bar. The spacing from the mast determines the radiation pattern of the antenna. Also, important, they must NOT be mounted at the top of a pole, but with maybe 10 cm above the top of the antenna, otherwise the pattern will also be distorted vertically, probably with a high angle of radiation, in at least some directions.
These antennas are often used on properly engineered repeaters, including because they are structurally strong, and are at DC ground.

FM Simplex

This is what computer type call "half duplex", as one station transmits, then the other. However the term Simplex is used when the station is transmitting and receiving on the same frequency. This is how most HF and SSB VHF operation occurs. It is often a good idea when two stations can hear each other directly. Many transceivers have a "Reverse" button, which allows a quick check on the repeater input frequency, to see if this is possible.

In the US 146.520 MHz is the "National Simplex Calling Frequency". Once a contact is made, you should generally move off this to a working frequency. There are two spaces between repeater inputs and outputs: 146.400 to just below 146.600, and from 147.400 to just below 147.600.

In Australia the calling frequency is 146.500 MHz. Frequencies from 146.425 and 146.575 inclusive are the simplex working frequencies. WICEN uses simplex at or around 147.500 MHz, as 147.400 to 147.600 MHZ is free of either repeater inputs or outputs. These can be used when there is not a WICEN event.

In communities without a local repeater, a specific frequency might be selected by the locals. 147.475 MHz is/was used in the upper Hunter Valley, for example. 147.525 is also fairly common.

on 70 cm 446.00 MHz is the US National simplex frequency; in Australia it is 439.000 MHz.

For 6 metres 52.525 MHz is used for calling in most countries, and it is possible for such signals to travel significant distances.

CTCSS, DCS, Toneburst

Repeaters inputs can suffer interference from several sources. These include the so-called Low Interference Potential Devices (LIPDs) on 70 cm, inter-modulation products, where a rusty wire against a galvanised pole creates a diode which mixes two signals together, generating an interfering signal, and distant Ham stations using a different repeater. Pre-dating SMS, high powered VHF pager transmitters used to be a problem in some areas.

To stop unwanted signals triggering repeaters, several advances tone systems used to activate the repeater.

CTCSS, continuous tone-coded squelch system, uses a a range of low frequency tones to activate the repeater. In some cases, such as the system in Hornsby, where Optus pay TV signals leaked out of the cables at poor joints, a tone is sent to operate the users radio. In this case, its use is optional, as carrier squelch still works. The operator sending a tone can also be used to turn on linking from one band to another band not permitted for some classes of licence, say 2m to 6m, an Australia-only issue.

DCS, digital coded squelch uses a low speed data stream, consisting of 3 digits, and some ancillary numerals to open the repeater.

Tone-burst is primarily a European system, where an audible tone, often of 1750 kHz is transmitted at the beginning of the over, and the repeater then stays open until the carrier ceases. If someone had this on by accident, you will hear something like "BEEEEEP! VK2DOH listening".

Systems which don't use these are said to use "carrier squelch".

DTMF, dual tone multi-frequency signalling is NOT used for accessing repeaters, but can be used for dialling between Echolink and IRLP nodes. Similar 5-tone, and other "Selcall" systems do use voice-frequency tones to signal radios, generally in commercial systems.


SSB (Single sideband) is legally permitted in all bands above 50 MHz, except in segments where Morse only or data only are mandated. Operation should, however be restricted to segments listed as SSB or All-modes in the ARRL bandplan, or those of the country you are in. Being above 10 MHz, USB is the normal sideband.

SSB is used during contests, and by stations trying to set distance records, or make contact with stations in as many gridsquares a possible; as the bandwidth is narrower. You can also use it for chatting, as long as you avoid the various calling channels.


Not on the exam, but AM can also be used in voice segments on VHF and UHF, ideally the all-mode parts, but no one should get upset if you use it in the 147.500 segment. Most modern multi-mode transceivers have an AM mode. The Yaesu VX-7R hand-held has an AM mode on 6 metres; and the air-band plus 2 meters Vertex-Standard VXA-700 Spirit, which sometimes pops up on eBay has AM on 2 metres.

The Fredbox is a home-brew or DIY miniature hand-held radio, for AM on 2 metres. Despite its 10 mW output, conversations have been held between England and France. See: www.amateurradio.com/fredbox-schematic/


A number of three letter Q-codes feature in the exam. Originally used in Morse code, these usually function as both questions and statements. "QRM?" thus asks, "Are you suffering from man-made interference?", and without the question make, indicates "I am suffering interference". "QSY?" asks "Shall I change frequency?".

Examples are:

QRM - Man-made interference
QRN - Natural interference, such as lightning crashes, or atmospheric Noise.
QSY - Change frequency, can be a frequency, repeater name, or suggesting to move up by, say 10 kHz, from the call channel.
QSB - Your signal is fading
QTH - Location
QRZ - Who is calling me?
QRU - Have you anything for me? Used on manual / Morse message forwarding networks.
QSL - Can you confirm reception? Relates to a message in formal and emergency communications support, but also to QSL cards.


People in New York probably don't want to hear two stations down the road from each other in California chatting, so only use the power necessary for the communications, within general and band-specific power limits, answer D.

DMR - Digital Mobile Radio

This is a spectrum efficient digital radio system, often used with repeaters. It is a fairly open European standard, which allows radios from a range of manufacturers to communicate with each other. It is a commercial standard taken into Amateur use.

There are modes of operation which allow local communications, but it also allows widespead communications through Internet Protocol based linking.

Talk groups are for interests, such as languages, physical areas, emergency support, and a few test modes. If you connect to TG 17, and speak Norwegian to the users in that world-wide group, Spanish speaking users with their radio set to the world-wide Spanish language TG 14 won't hear you. In Australia, each state, plus WICEN, have talk-groups, with 3802 being for VK2 (NSW) users. That said, if you are visiting interstate, and regularly chat on 3802, you can still do so. It also supports person-to-person calls through the network. It supports two timeslots, so two people can hold independent communications on the one device. Note also that more than one user can be using the same talk-group on a local repeater, just as a group can have a round-table discussion on an FM repeater.

Colour Codes act like CTCSS tones (or P25 NACs) to control access to a repeater, and it would be possible to group users by CC, if a repeater was set up this way.

Relevant Questions

Actual exam questions, from the published NCVEC Technician pool.

What is a common repeater frequency offset in the 2 meter band?
A. Plus or minus 5 MHz
B. Plus or minus 600 kHz
C. Plus or minus 500 kHz
D. Plus or minus 1 MHz

On 2 metres, the most popular offset is plus or minus 600 kHz, answer B.

The first option is bigger than the whole band, so nope!

What is the national calling frequency for FM simplex operations in the 2 meter band?
A. 146.520 MHz
B. 145.000 MHz
C. 432.100 MHz
D. 446.000 MHz

The last two are not 2 metres, so chuck them, and 432.100 MHz, is SSB anyway. In the US, the FM calling frequency is 146.520 MHz, answer A.

What is a common repeater frequency offset in the 70 cm band?
A. Plus or minus 5 MHz
B. Plus or minus 600 kHz
C. . Plus or minus 500 kHz
D. Plus or minus 1 MHz

On 70 cm, plus or minus 5 MHz is a common offset, answer A.

What is an appropriate way to call another station on a repeater if you know the other station's call sign?
A. Say "break, break" then say the station's call sign
B. Say the station's call sign, then identify with your call sign
C. Say "CQ" three times then the other station's call sign
D. Wait for the station to call CQ, then answer

"Hi Fred, this is Bill from the radio club." works on the 'phone, so how about saying Fred's callsign, followed by yours? "VK4FRED, this is VK2BIL.", yep, answer B.

You can just use "VK4FRED VK2BIL". Or K3TOM W6BOB.

How should you respond to a station calling CQ?
A. Transmit "CQ" followed by the other station’s call sign
B. Transmit your call sign followed by the other station’s call sign
C. Transmit the other station’s call sign followed by your call sign
D. Transmit a signal report followed by your call sign

Transmit THEIR call, followed by YOURS, answer C. Hi Fred, I'm John.

Which of the following is required when making on-the-air test transmissions?
A. Identify the transmitting station
B. Conduct tests only between 10 p.m. and 6 a.m. local time
C. Notify the FCC of the transmissions
D. All of these choices are correct

What is the rule for any transmission? IDENTIFY! Answer A, "AJ9ZD testing".

What is meant by "repeater offset?"
A. The difference between a repeater’s transmit frequency and its receive frequency
B. The repeater has a time delay to prevent interference
C. The repeater station identification is done on a separate frequency
D. The number of simultaneous transmit frequencies used by a repeater

Say you are listening to a repeater on 146.950 MHz, and you transmit at an appropriate time. The radio, either automatically, or due to memory settings, transmits on 146.350 MHz. This difference, 600 kHz, or specifically -600 kHz is the offset. The answer is A.

What is the meaning of the procedural signal “CQ”?
A. Call on the quarter hour
B. Test transmission, no reply expected
C. Only the called station should transmit
D. Calling any station

CQ means you are calling for any station, answer D.

Which of the following indicates that a station is listening on a repeater and looking for a contact?
A. “CQ CQ” followed by the station’s call sign
B. The station’s call sign followed by the word "monitoring"
C. The repeater call sign followed by the station’s call sign
D. “QSY” followed by your call sign

On a repeater just saying your callsign is usually sufficient, possibly followed by "monitoring", answer B.

The callsign of the repeater tends not to be used on air.

What is a band plan, beyond the privileges established by the FCC?
A. A voluntary guideline for using different modes or activities within an amateur band
B. A list of operating schedules
C. A list of available net frequencies
D. A plan devised by a club to indicate frequency band usage

There are voluntary guides for modes and activities within the various bands, answer A.

For example, 50.400 MHz is the 6 m AM calling frequency, 28.885 MHZ USB in the 10 m band is the liaison frequency for DX operations on 6 metres. National associations such as the ARRL and WIA publish these.

What term describes an amateur station that is transmitting and receiving on the same frequency?
A. Full duplex
B. Diplex
C. Simplex
D. Multiplex

The common term for this simplex, answer C.

This describes most HF communications, and some general and emergency VHF+ communications.

What should you do before calling CQ?
A. Listen first to be sure that no one else is using the frequency
B. Ask if the frequency is in use
C. Make sure you are authorized to use that frequency
D. All these choices are correct

You should do all of these, answer D.

These apply especially to HF, as a nearby station may be talking to a distant station, and one is having a long over. Sometimes it is only the local station you can hear, and sometimes it is only the distant station you can hear, as the local one, maybe a few hundred kilometres away is in the "skip zone" for your station.

How is a VHF/UHF transceiver’s "reverse" function used?
A. To reduce power output
B. To increase power output
C. To listen on a repeater's input frequency
D. To listen on a repeater's output frequency

This allows you to listen to the repeater input channel, for example, to see if the station you are talking to is in range for a simplex chat, answer C.

It can also be a quick check to see of an interfering signal is coming from near your location.

What is the term describes the use of a sub-audible tone transmitted with normal voice audio to open the squelch of a receiver?
A. Carrier squelch
B. Tone burst

CTCSS signals are continuous low frequency audio frequency tones, such as 91.5 MHz, which open the squelch on a receiver, such is that on a repeater. Some (also) transmit to the user. Answer D.

Radios generally filter these out, and small speakers have a poor response here anyway. I did notice listening on headphones using an older 2 metre HT, I could hear the 123 Hz tone, similar to power supply hum.

Which of the following describes a linked repeater network?
A. A network of repeaters in which signals received by one repeater are transmitted by all the repeaters in the network
B. A single repeater with more than one receiver
C. Multiple repeaters with the same control operator
D. A system of repeaters linked by APRS

A group of repeaters can be linked, to provide wide area coverage, and sometimes also link between different bands, answer A.

A repeater can have more than one receiver, called a voting receiver, to give spatial diversity, but this is not a network.

Which of the following could be the reason you are unable to access a repeater whose output you can hear?
A. Improper transceiver offset
B. You are using the wrong CTCSS tone
C. You are using the wrong DCS code
D. All these choices are correct

You need to ensure the offset is correct, as some systems do use unusual ones (including to avoid "LIPD" interference). Depending on the system, different CTCSS tones or DCS codes are needed. Thus, all are correct, answer D.

What would cause your FM transmission audio to be distorted on voice peaks? A. Your repeater offset is inverted
B. You need to talk louder
C. You are talking too loudly
D. Your transmit power is too high

The louder the signal on FM, the greater the deviation, and if you are speaking to loudly, answer C, this can cause either distortion, or the transmitter to close down momentarily, to avoid transmitting a signal in an adjacent channel.

However, any properly designed transmitter would limit the audio level internally, and the modulation in a sensible way.

What type of signaling uses pairs of audio tones?

DTMF stands for Dual-Tone Muli-Frequency, and is used in many home telephones to send the called number. Each column, and each row corresponds to a particular tone. A 7 consists of row tone 852 Hz and column tone 1209 Hz. Answer A.

Where a repeater is connected to IRLP (Internet Repeater Linking Project) or Echolink, a repeater used can dial a short sequence of digits to connect to other repeaters. Tones can also control various functions of a repeater.

How can you join a digital repeater’s talk group"?
A. Register your radio with the local FCC office
B. Join the repeater owner’s club
C. Program your radio with the group’s ID or code
D. Sign your call after the courtesy tone

Digital repeaters use codes for access, and this can allow either local conversations, or can allow you to talk across a network of repeaters, linked either via the Internet, or via microwave based IP networks. The answer is C, program your radio with the group’s ID or code.

DMR (Tier II or Tier III) allows two simultaneous conversations on one repeater, using timeslots, say one for local or regional contacts, the other for global special interest groups.

Which of the following applies when two stations transmitting on the same frequency interfere with each other?
A. The stations should negotiate continued use of the frequency
B. Both stations should choose another frequency to avoid conflict
C. Interference is inevitable, so no action is required
D. Use subaudible tones so both stations can share the frequency

The stations should negotiate, answer A.

Note that you should make way for things like scheduled nets, unless handling emergency traffic. The previous version of this question indicated that common courtesy should prevail. The idea that subaudible tones somehow multiplies the number of channels available is something from the realms of marketing for FRS or LIPD type radios, having "22 channels, and 99 sub-channels", bunkum.

In order to run a new 80 metre net, our club publishes a net frequency of 3608.5 kHz, as this fits between existing nets.

Why are simplex channels designated in the VHF/UHF band plans?
A. So that stations within mutual communications range can communicate without tying up a repeater
B. For contest operation
C. For working DX only
D. So that stations with simple transmitters can access the repeater without automated offset

Note the "Why": These are to allow stations, generally in close range, to communicate directly, keeping the repeater free, answer A.

You can use the simplex channels for contests, home-brew experiments, or DX, but that isn't the "Why".

Which Q signal indicates that you are receiving interference from other stations?

It is QRM, answer A.

M is said to stand for Man-made.

Which Q signal indicates that you are changing frequency? A. QRU

It is QSY, answer B.

What is the purpose of the color code used on DMR repeater systems?
A. Establishes groups of users
B. Defines the frequency pair to use
C. Identifies the codec used
D. Defines the minimum signal level required for access

The answer is that it "establishes groups of users", answer A.

It behaves a little more like a CTCSS tone, controlling access to a repeater. In Australia all repeaters use CC 1. It is perhaps that in a commercial system this might be used by different parts of a company, or different clients of a communications provider.

What is the purpose of a squelch function?
A. Reduce a CW transmitter's key clicks
B. Mute the receiver audio when a signal is not present
C. Eliminate parasitic oscillations in an RF amplifier
D. Reduce interference from impulse noise

This mutes the noise which would otherwise be heard when a signal is not present, answer B.

On to: Public Service

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Written by Julian Sortland, VK2YJS & AG6LE, February 2022.

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