VOX in Ham Radio: Gain, Delay and Anti-VOX Settings

Pressing a push-to-talk button for every over is a habit that most operators accept without thinking, until the day their hands are busy with a logging program, a paddle, a mobile steering wheel or a pair of gloves in a cold field. Then VOX ham radio operation becomes attractive: the transmitter switches itself on when you speak and off when you stop. Done well, VOX gives a relaxed, hands-free way of operating that also suits digital modes and contest work. Done badly, it clips the first syllable of every sentence, keys the transmitter on the noise of the room and sends the speaker audio back into the microphone. This guide explains what VOX does, what its three controls are for, how to adjust them in a few minutes, and when a footswitch or a computer-controlled push-to-talk is the better choice.

What VOX Actually Does

The abbreviation comes from the Latin word for voice, and a voice-operated switch is a circuit that monitors the microphone signal. When the level exceeds a threshold, the circuit activates the transmit relay or the electronic changeover, and the radio goes from receive to transmit. When the level drops below the threshold for a chosen time, the circuit releases and the radio returns to receive. The idea is simple, but it needs time to act. The relay and the transmitter (see push-to-talk for the usual alternative) take a few milliseconds to come on, so the beginning of your first word can be lost if the circuit reacts too late.

VOX is meant for voice modes. In SSB and FM it saves you from pressing a key, and many people use it for long ragchews. It is also used on digital modes through a sound card: the radio is keyed by the audio from the computer, so no serial or USB control line is needed. In CW the corresponding function is called break-in and works on the keying, not on audio, as explained in the article on full break-in CW.

The Three Controls

Almost every transceiver with VOX has three adjustments, though the names vary. The first is the VOX gain, sometimes called the threshold or the sensitivity. It sets how loud you must speak before the radio goes to transmit. A high gain makes the circuit sensitive and keys on a whisper, and also on the noise of a fan or a dog barking. A low gain needs a loud voice and may cut off the quiet ends of words.

The second is the VOX delay, also called the hang time. This is how long the radio stays in transmit after the last sound. A short delay lets the radio return to receive quickly, which is good for a lively QSO, but it may drop out in every pause between words and make the transmitter chatter. A long delay keeps the transmitter on through natural pauses but makes you wait before you can hear the other station. A typical value is between a quarter and three quarters of a second.

The third is the anti-VOX. It prevents the sound from the loudspeaker, which enters the microphone, from keying the transmitter again. The circuit takes a sample of the receiver audio and subtracts it from the microphone signal before the comparator. A high anti-VOX setting resists the loudspeaker better, but makes the transmitter less responsive to your own voice. A headset eliminates the problem and allows a much lower anti-VOX setting.

A Five-Minute Adjustment

Begin with the microphone gain, since the VOX circuit often uses the same signal. Set the mic gain for normal modulation on voice peaks, as described in the guide to SSB audio settings, so that the ALC meter just touches its zone. Use a dummy load or a free frequency, and switch VOX on. Start with the VOX gain at about one third of the range, the delay at the middle and the anti-VOX at the minimum.

Now say a short sentence in your normal voice and watch whether the transmit light comes on at the first word. If the first syllable is late, increase the gain a little. Then stay silent and make normal room noises, such as typing and moving the chair, and verify that the radio does not key. If it does, reduce the gain or move the microphone closer to your mouth, because a smaller distance allows a lower gain. Set the delay so that the radio stays in transmit for the pauses in your sentences and drops back quickly at the end. Finally raise the receiver volume to the normal listening level and increase the anti-VOX until the loudspeaker no longer keys the transmitter while the other station is speaking.

Record a short transmission and listen to it on a second receiver. This is the real test: it shows whether the first syllable is clipped, whether the end of your sentence is cut and whether the changeover noise is audible. Make small changes and test again. Once the values are right, write them down, because many radios keep VOX settings per mode and a firmware reset removes them.

Why the First Syllable Disappears

Clipped beginnings are the most common complaint. Several causes are possible and it is useful to separate them. The first is a low VOX gain, because the circuit waits until your voice is loud enough. The second is that the sound of the beginning of a word is weak: words that start with a soft consonant such as f, h or th are quiet, and the circuit does not notice them until the vowel arrives. The third is the relay. A mechanical relay takes 5 to 10 milliseconds to close, and some radios add a small delay to protect an amplifier. If an external linear amplifier is used, it adds its own changeover time, often longer than the radio’s.

Many transceivers cope with this by delaying the audio slightly, so the circuit has time to switch before the voice reaches the modulator. In others, the cure is a habit: begin each over with a short sound or a breath before the first word. If you use an amplifier, the sequence of the relays in the chain is critical, and the guide to HF linear amplifiers explains why hot switching of the amplifier relay is harmful.

False Keying and Feedback

Two situations call for a different approach. In a shack with a loud receiver, the loudspeaker sound reaches the microphone and keys the transmitter, and the transmitter then sends your own receiver audio. The solution is a headset, which is a better choice than an elaborate setting of the anti-VOX, and it also helps in a contest hall with several stations. The article on the contest headset and footswitch describes the connections.

The other situation is a noisy place: a car, a campsite, a shack with a computer fan. The microphone picks up the background and keys the transmitter when you did not speak. A noise-cancelling headset microphone placed close to the mouth gives the best signal-to-noise ratio, and the VOX gain can then be reduced. Remember also that a transmitter that is keyed by noise sends that noise to everyone on the frequency and blocks you from hearing replies. On a busy band this is a poor habit.

VOX With Digital Modes and Sound Cards

When you work FT8, RTTY or PSK31 through a sound card, the computer’s audio can key the radio by VOX. The method has an attraction: no cable except the audio interface. The drawbacks are that the delay turns the transmitter on a little after the first tone, which is usually harmless, and that the VOX gain must be set from the audio level of the program, which may be tuned for a very low drive to avoid distortion. The system sound of the computer, such as an alert or the sound of an email, may key the transmitter at any time and send an unwanted signal on the air, a risk that people only find once.

For this reason most operators prefer a push-to-talk line, a CAT command or a serial control line (DTR or RTS) for digital modes, and keep VOX for casual voice use. Do not forget to disable computer sounds on the audio device used for the radio, and read about the audio side in the article on the virtual audio cable for digital modes.

VOX in the Car, in the Field and at a Special Event

Hands-free operation is most valuable away from the desk. In a mobile installation, a VOX headset or a microphone clipped to the sun visor lets you keep both hands on the wheel, although local traffic rules about headsets differ, and the safest practice is to talk only when the driving situation allows it. In a portable station on a hill or at a park, VOX allows you to hold a logbook or a tablet in one hand and a drink in the other, and the cold fingers of a winter activation do not have to find a small button. At a special event station, a casual visitor who has never held a microphone often finds VOX easier than a push-to-talk switch, because speaking is all that is needed.

In every one of these situations the surrounding noise is the enemy. Wind across a microphone is a loud, low-frequency rumble that keys the circuit, so a foam windscreen is cheap protection. A headset with a boom microphone placed a finger’s width from the corner of the mouth is the best arrangement, and it lets you whisper to keep a conversation private in a crowded place. After any change of microphone, repeat the five-minute adjustment, since the new level will not match the old settings.

Operators who use a handheld on FM should note that many handhelds have a simplified VOX with only a sensitivity level and a fixed delay. The same principles apply, but there is less to adjust, so a good headset becomes even more important. Test the handheld in the real place with the real noise, and be ready to switch to the push-to-talk button if the conditions change.

When Not to Use VOX

VOX is not the best tool in every situation. In a pileup or a contest, the rhythm of calling and listening is quick, and a push-to-talk switch, a footswitch or the computer’s voice keyer gives exact control. In a noisy environment, or when you share a room with other people, accidental keying is a risk, and a licence holder is responsible for every transmission. Always verify that the radio is not transmitting when you leave the desk, and consider switching off VOX when you are not at the radio. Some operators also prefer a manual switch on a long ragchew, because it gives a natural break to think.

Finally, a point of operating practice. VOX makes it easier to talk over the other station, since you do not hear the click of a relay or notice the moment you transmit. Listen between overs, give the other operator a clear pause to break in, and pass the turn with a clear closing. A tool that frees your hands should not make you a worse listener.

The Bottom Line

VOX ham radio operation switches the transmitter on by your voice and relies on three controls: VOX gain for the threshold, delay for the hang time and anti-VOX to keep the loudspeaker out of the microphone. Set the microphone gain first, then adjust the gain until the first syllable is not lost and the room noise does not key the radio, set a delay of about half a second and raise the anti-VOX only as far as needed. A headset solves most of the feedback problems. Keep VOX for relaxed voice operation and prefer a footswitch, CAT or a serial line for contests and digital modes.

Frequently Asked Questions

What is VOX in ham radio?

VOX is a voice-operated switch that puts the transmitter on the air when the microphone detects speech and returns to receive when you stop talking.

What does anti-VOX do?

It stops the sound of the loudspeaker, picked up by the microphone, from keying the transmitter, by subtracting a sample of the receiver audio from the microphone signal.

How long should the VOX delay be?

Usually between 0.3 and 0.7 seconds. It should bridge the pauses between words but not make you wait long to hear the reply.

Why does my VOX cut off the first word?

The gain may be too low, the relay and the amplifier need a few milliseconds to switch, or the first sound is quiet. Increase the gain slightly or begin each over with a short sound.

Is VOX good for FT8 and other digital modes?

It works, but computer sounds can key the transmitter by accident. CAT control or a serial PTT line is the safer choice for digital modes.

Can I use VOX with a linear amplifier?

Yes, but the amplifier relay adds its own switching delay, so the radio must wait for the amplifier before it sends power. Check the amplifier sequencing in the manual.