T2B: VHF/UHF Operating Practices

VHF and UHF operation involves specific technologies and conventions that go beyond simply picking a frequency and transmitting. Modern repeaters use access tones to control who can use them. Digital systems like DMR use color codes and talkgroups to route communications. Knowing how these systems work — and what to do when they do not — is part of everyday operation on these bands.

T2B covers the full range of VHF and UHF operating practices: CTCSS and DTMF access tones, the reverse function for checking repeater inputs, linked repeater networks, DMR talkgroup programming and color codes, troubleshooting repeater access failures, why FM audio distorts, simplex channel purposes, Q signals, how to resolve frequency conflicts, and the squelch function.

CTCSS: Sub-Audible Access Tones

CTCSS stands for Continuous Tone-Coded Squelch System. It is a method of encoding a continuous sub-audible tone — a tone below the normal range of human hearing — into a transmitted FM signal alongside the voice audio. The receiving device, typically a repeater, is configured to open its squelch only when it detects the specific tone it is programmed to recognize. Without the correct tone, the repeater ignores the transmission even if the RF signal is strong enough to be received.

CTCSS serves two main purposes: it prevents the repeater from being activated by interference or unintended signals from other systems, and it allows multiple groups to share frequency space by filtering each other's transmissions. From the transmitting operator's perspective, CTCSS is a setting that must be programmed into the radio — matching the tone frequency required by the specific repeater being used.

DTMF: Dual-Tone Signaling

DTMF stands for Dual-Tone Multi-Frequency. It uses pairs of audio tones — two simultaneous tones from different frequency groups — to represent the digits 0 through 9 and several control characters. The same system is used in telephone keypads, and it works the same way in amateur radio. DTMF is used to send control commands to repeaters and linked systems, to access autopatch telephone interconnects on repeaters that have them, and to interact with various remote control systems.

CTCSS and DTMF are different technologies used for different purposes. CTCSS is continuous and sub-audible; DTMF is short, audible, and consists of specific tone pairs. Knowing which is which is essential for the exam.

The Reverse Function

Most VHF and UHF transceivers include a reverse function that temporarily swaps the transmit and receive frequencies. In normal repeater operation, you listen on the repeater's output frequency and transmit on its input frequency (the offset frequency). When you engage the reverse function, your radio listens on the input frequency instead of the output.

This is useful for checking whether a signal is reaching the repeater's input — if you can hear a station directly on the input frequency without the repeater's help, you know your signal is likely getting through. It is also used to determine whether interference on the repeater output is originating at the repeater or coming from another source on that frequency.

Linked Repeater Networks

A linked repeater network connects multiple repeaters so that a signal received by any one of them is simultaneously retransmitted by all of them. This dramatically extends geographic coverage — a single transmission can reach across hundreds of miles through a chain of linked repeaters. Linked networks are used for regional coordination, emergency communications, and long-distance contacts on VHF and UHF where normal propagation would limit range to line of sight.

Why You Cannot Access a Repeater

If you can hear a repeater's output clearly but your transmissions are not activating it, there are three common causes — and any one of them could be the problem:

• Improper transceiver offset — if your radio is transmitting on the wrong frequency, the repeater will never receive your signal regardless of how strong it is
• Wrong CTCSS tone — if the repeater requires a specific sub-audible tone and your radio is transmitting a different tone or none at all, the repeater's squelch will not open
• Wrong DCS code — some repeaters use Digital Coded Squelch instead of CTCSS; if the DCS code does not match, access will be denied

All three of these causes are correct answers on the exam — the question is asking which of these could be the reason, and all of them could be.

FM Audio Distortion

When your FM transmission audio sounds distorted on voice peaks — particularly when your voice is loudest — the most common cause is speaking too loudly into the microphone. FM transmitters have a maximum deviation limit, and when the audio input level is too high, the modulation exceeds that limit and the signal distorts. This is sometimes called overmodulation.

The fix is simple: speak at a normal conversational level and hold the microphone at a consistent distance. Moving closer to the microphone or speaking louder will make the problem worse, not better. Transmit power level has no effect on this type of distortion — it is purely an audio input level issue.

DMR: Digital Mobile Radio

DMR (Digital Mobile Radio) is a digital voice mode used on many modern VHF and UHF repeaters. DMR uses time-division multiplexing to allow two conversations to share the same repeater channel simultaneously. Two DMR-specific concepts are tested:

Color codes function similarly to CTCSS tones — your radio must be programmed with the same color code as the repeater to access it. Without a matching color code, the repeater will not pass your transmission. Color codes do not define frequency pairs, codec settings, or minimum signal levels.

Talkgroups are virtual channels within the DMR system that route conversations to specific groups of users. To join a talkgroup, you program your radio with the talkgroup's ID number or code. You do not register with the FCC, join a club, or wait for a courtesy tone — you simply program the correct talkgroup identifier into the radio.

Resolving Frequency Conflicts

When two stations are transmitting on the same frequency and causing interference to each other, the correct approach is for the stations to negotiate which one will continue using the frequency. Amateur radio frequencies are not privately owned, and there is no automatic priority based on who arrived first in every situation. The stations should communicate with each other and work out an arrangement — one may move to a different frequency, or they may agree to share the frequency by taking turns.

Simply switching to another frequency without attempting to resolve the situation, ignoring the interference, or using CTCSS to mask the problem are not the correct approaches under the rules.

Simplex Channels in Band Plans

VHF and UHF band plans designate specific simplex channels — frequencies where stations communicate directly without using a repeater. These channels exist primarily so that stations within range of each other can communicate without occupying repeater resources. When two nearby operators are close enough for direct contact, using a simplex frequency is more efficient and courteous than tying up a repeater that may serve a much larger area.

Simplex channels are not set aside exclusively for contests or DX work — they are for any direct station-to-station communication where the stations can reach each other without relay assistance.

Q Signals: QRM and QSY

Q signals are three-letter codes that originated in Morse code operation to convey common situations efficiently. They are still used in both voice and digital operation today. Two Q signals appear in the T2B questions:

QRN refers to natural atmospheric noise (not other stations). QTH means location or address. QSB means signal fading. QRZ means "who is calling me?" These are worth knowing, but QRM and QSY are the ones tested in T2B.

The Squelch Function

The squelch function mutes the receiver's audio output when no signal is present. Without squelch, a radio with its volume up would constantly produce loud noise from the receiver — squelch suppresses that background noise and only opens the audio path when a signal above the squelch threshold is detected. Setting the squelch too high can block weak signals; setting it too low can cause constant noise. Squelch does not reduce key clicks, eliminate oscillations, or filter impulse noise — its sole purpose is to mute audio in the absence of a signal.

T2B Practice Questions