Every time a pilot keys the microphone and contacts approach control or unicom, the transmission rides on a very-high-frequency (VHF) radio wave. Understanding why aviation uses VHF, how channels are organized, and what the squelch control actually does will make you a more confident and precise communicator — and it will keep you from being tripped up on the FAA knowledge test. This article walks through the hardware basics, the frequency band, channel spacing, and practical squelch technique, all grounded in FAA guidance.
The VHF Aviation Band
Aviation voice communication in the United States takes place in the VHF band between 118.000 MHz and 136.975 MHz. The Federal Communications Commission (FCC) and the FAA coordinate use of this spectrum; the FAA's Aeronautical Information Manual (AIM) describes this range as the standard airborne voice communication band. Frequencies below 118 MHz are reserved for other services (the VOR navigation band runs from 108.0 to 117.95 MHz, which is why your nav radio and comm radio share similar-looking numbers but are distinctly separate systems). Frequencies above 136.975 MHz belong to other users as well.
VHF signals travel in essentially straight lines — they are line-of-sight transmissions. This is both a limitation and a safety feature. Because VHF does not bounce off the ionosphere the way HF (high-frequency) signals do, there is very little risk of an unintended distant station stepping on your local transmission. The practical consequence is that your usable range increases with altitude: a pilot at 5,000 feet AGL can generally reach a ground station many more miles away than a pilot at 500 feet AGL. The Pilot's Handbook of Aeronautical Knowledge (PHAK, FAA-H-8083-25) explains this line-of-sight characteristic in the context of radio communications and navigation systems.
Channel Spacing: 25 kHz and 8.33 kHz
The 118–136.975 MHz band must be shared among hundreds of frequencies: tower, ground, approach, departure, center, unicom, multicom, CTAF, ATIS, and more. To fit as many usable channels as possible into the band without adjacent channels interfering with each other, the band is divided into channels separated by a fixed spacing.
25 kHz Spacing (the U.S. Standard)
In the United States, the standard channel spacing is 25 kHz. This divides the 118–136.975 MHz band into 760 channels. Frequencies increment in steps of 0.025 MHz, so valid channels include 118.000, 118.025, 118.050, 118.075, 118.100, and so on up to 136.975 MHz. A common student mistake is trying to dial in a frequency like 118.02 or 125.3 — these are not valid 25 kHz channels. On a typical U.S. comm radio with 25 kHz spacing, the last two digits of the kilohertz portion will always be 000, 025, 050, or 075 (displayed on most radios as .00, .02 or .025, .05, .07 or .075 depending on the display format). Some older radios display only three decimal places and round, so 118.025 may appear as 118.02 — be aware of your specific avionics display format.
8.33 kHz Spacing (European Context)
Europe and certain high-density airspaces have moved to 8.33 kHz channel spacing, tripling the number of available channels in the band to approximately 2,280 channels. While U.S. student pilots will rarely fly with 8.33 kHz-capable radios for domestic operations, the FAA knowledge test and some avionics documentation reference this spacing, and pilots considering international operations should be aware that European airspace above certain flight levels mandates 8.33 kHz-capable equipment. A radio with 8.33 kHz spacing can also tune the older 25 kHz channels, making it backward compatible.
How a VHF Comm Radio Works — The Basics
A VHF comm radio is a transceiver — a combined transmitter and receiver in one box. In receive mode, the radio listens on the selected frequency continuously. When you press the push-to-talk (PTT) switch, the radio switches to transmit mode; the receiver is disabled while you transmit, which is why you cannot hear incoming transmissions while you are talking. This is known as half-duplex operation. Proper radio technique — listening before transmitting and keeping calls concise — flows directly from this hardware reality.
Most general aviation aircraft carry at least one comm radio, and many carry two. Having two independent comm radios (COM1 and COM2) gives a pilot the ability to monitor a second frequency such as ATIS or CTAF while actively working with ATC on the primary radio. Many modern avionics allow you to set a standby frequency and swap it to active with a single button — a technique called flip-flopping that reduces the chance of misdialing a frequency at a critical moment.
Squelch: What It Is and How to Set It
A VHF receiver left fully open (no squelch) produces a loud, harsh static roar whenever no signal is present. This happens because the antenna picks up random electromagnetic noise from the atmosphere and electrical systems. The squelch circuit solves this by muting the audio output whenever the received signal falls below a set threshold. In plain terms: squelch silences the speaker or headset when no one is transmitting, and opens the audio path when a valid signal — someone talking on frequency — arrives.
Manual Squelch
Older and simpler comm radios use a manual squelch knob. Rotating the knob counterclockwise (or to its minimum setting) defeats the squelch entirely and allows full static through — useful for checking that the radio is functioning. Rotating it clockwise increases the squelch threshold. The correct technique is to turn the squelch clockwise until the static just disappears, then stop. Setting it too high risks missing weak but valid transmissions from distant aircraft or ground stations. The PHAK and most avionics manuals describe this break-squelch-then-set technique.
Automatic or Fixed Squelch
Many modern panel-mount and portable radios use automatic squelch (sometimes called carrier squelch or tone squelch), which adjusts internally without a pilot-operated knob. These radios open the squelch whenever a signal of sufficient strength is detected on the active frequency. Some avionics also use a squelch defeat button that temporarily opens the squelch — handy when listening for a very weak or distant station that might not break the automatic threshold.
Why This Matters for Safety and Situational Awareness
Correct frequency management is not a formality — it is a primary layer of airspace safety. ATC separation, traffic advisories, ATIS updates, and collision avoidance calls all depend on every aircraft being on the right frequency with a properly functioning radio. A misdialed frequency (e.g., 121.9 instead of 121.90 — actually the same, but confusion between 121.900 and 121.925 is real) can result in an aircraft operating in controlled airspace without two-way radio contact, which has regulatory consequences under 14 CFR Part 91. The emergency frequency 121.500 MHz (sometimes called Guard) is continuously monitored by many ATC facilities and military assets; knowing it is part of every pilot's basic radio literacy.
Squelch set incorrectly — particularly too high — creates a false sense of a quiet frequency. A pilot who cannot hear weak transmissions from opposite-direction traffic in a traffic pattern may have no warning before a potential conflict. Proper squelch technique is therefore directly linked to the see-and-avoid and hear-and-avoid responsibilities outlined in 14 CFR 91.113.
Key Numbers and Rules
- VHF comm band: 118.000 MHz to 136.975 MHz
- Standard U.S. channel spacing: 25 kHz (increments of .000, .025, .050, .075 MHz)
- European / high-density spacing: 8.33 kHz
- Total 25 kHz channels in band: 760
- Emergency / Guard frequency: 121.500 MHz
- Line-of-sight propagation: range increases with altitude
- Squelch setting rule: turn up until static just disappears — no higher
- Half-duplex operation: you cannot transmit and receive simultaneously
- VOR nav band (adjacent, not comm): 108.0–117.95 MHz — do not confuse with comm band
Common Test Traps
- Invalid frequency entries: The FAA may present a frequency like 123.45 (valid — it ends in .025 equivalent: 123.450) versus 123.42 (invalid on a 25 kHz system). Know that the last digit in kilohertz must be 0 or 5 in the tenths-of-kHz place, resolving to multiples of 25.
- Confusing nav and comm bands: VORs operate 108.0–117.95 MHz; voice comm starts at 118.000 MHz. Test questions sometimes blur this line to see if you know where one band ends and the other begins.
- Squelch defeating a weak station: Questions may describe a scenario where a pilot hears nothing on a frequency and ask what to check — one correct answer is to verify the squelch is not set too high (defeating valid but weak signals).
- Guard frequency: 121.500 MHz is the international distress frequency, not a general-purpose frequency. Knowing it and its purpose is commonly tested.
- Transmit vs. receive lockout: Because comm radios are half-duplex, pressing PTT while ATC is talking means you hear nothing and ATC hears a garbled squeal. The test may ask why a pilot missed a transmission — PTT inadvertently depressed is a classic distractor.