Every digital mode program has to answer one question before it can put a signal on the air: how does it tell the radio to transmit? That answer is your push-to-talk (PTT) method. Get it right and you can forget about it. Get it wrong and you may see clipped transmissions, a radio that keys at odd moments, or a transmitter that will not let go. This guide compares the three common choices and helps you pick one.
Why the PTT method matters
Digital modes such as FT8, FT4, PSK31 and RTTY are usually generated by your computer as audio tones, which go to the radio's audio input. The radio must be in transmit for those tones to reach the antenna. Timed modes are especially unforgiving because they expect transmissions to start and stop at set moments. A PTT method that is slow, unpredictable or easy to trigger by accident causes problems that are easy to mistake for audio or software faults.
CAT control: the radio is told to transmit
CAT (Computer Aided Transceiver) control is a data connection, usually USB or serial, that lets software read and set the radio's frequency and mode. The same link can carry a command telling the radio to switch to transmit and back. Many current radios expose CAT over a single USB cable that also carries audio, so CAT PTT can need no extra wiring. It also keeps the software and the radio in agreement about what is happening.
The trade-offs are that each radio family has its own command set, so the software needs to support your model, either directly or through a library such as Hamlib. On Windows, only one program can normally open a given serial port at a time. If two programs both want the port, you need a rig-sharing tool or a different arrangement.
RTS and DTR: keying with a control line
A serial port has control lines, including RTS and DTR, that software can switch on and off. Many radios and interfaces can treat one of these lines as PTT. The software raises the line to transmit and drops it to receive. This is simple, fast and independent of the radio's command set, so it is a common choice when CAT PTT is awkward or unsupported.
The main caution is that these lines can change state when a program opens the port. Depending on the hardware and settings, that can key the transmitter briefly at start-up, or leave it keyed. You also have to choose the correct line, RTS or DTR, and the correct polarity. If your radio has a menu option for which line acts as PTT, it must match the software.
VOX: the radio listens for audio
VOX (voice-operated switch) keys the transmitter when it detects audio at its input. It needs no control connection at all, only the audio path. That makes it attractive for a radio with no CAT support or for a quick test.
It is also the least predictable method for digital work. VOX has to hear the audio before it keys, so the first part of a transmission can be clipped. After the tones stop, it waits for its delay before returning to receive. Worst of all, any sound your computer plays, such as an alert or notification, can key the radio and be transmitted. Treat VOX as a fallback, not a first choice.
Which one is most reliable for your rig?
- Modern radio with USB CAT and built-in audio: start with CAT PTT. If your radio needs a menu setting so that CAT PTT uses the USB audio input, set that too.
- Radio with USB, where CAT PTT does not work as expected: try RTS or DTR, whichever the radio's menu offers for PTT.
- Older radio with a sound card interface: use the interface's PTT method, usually a control line or CAT.
- No usable control connection: VOX, with the levels set carefully and computer sounds silenced.
Your radio's manual and your software's documentation are the final word. Where they disagree with a general guide, follow them.
Set it up step by step
- Connect the radio and computer, and connect a dummy load in place of the antenna.
- Note the port name the computer assigns to the radio. Radios with several ports may list a CAT port and a separate one for other functions, so check the manual.
- In the radio's menu, set the baud rate to match the software, and note any PTT or data-mode options.
- In your software's radio settings, choose your rig, the port and the same baud rate. If it offers a test button, use it to confirm the software can talk to the radio.
- Select the PTT method, starting with CAT, then use the PTT test or a very short transmission to check it.
Test safely before you go on the air
Always test into a dummy load at low power first. A method that sticks in transmit, or keys unexpectedly, is much less of a problem when there is no antenna radiating. Watch the radio's transmit indicator. It should light only when you expect it to, and should go out when the software says it has stopped. Close and reopen the program, and reboot the computer, to see whether the radio keys at start-up.
Keep antennas away from people and pets while transmitting. Transmitting also requires the appropriate amateur licence, and the rules differ between Canada and the USA, so follow those that apply to you, including identification requirements.
Common mistakes
- Choosing the wrong control line: selecting RTS in the software when the radio expects DTR, or the reverse, means no keying or a permanently keyed radio.
- Two programs fighting over one port: a logging program, a digital program and a rig-control utility cannot all open the same serial port at once.
- Forgetting the audio routing: the radio is keyed but the tones do not reach it, because it is listening to the microphone rather than the USB audio input.
- Leaving VOX on with PC sounds active: notifications can be transmitted.
- Fixing audio problems with PTT settings: if the radio keys correctly but the signal looks wrong, the cause is usually the drive level. Our guide to digital mode audio levels covers that.
A quick pre-flight checklist
- Dummy load connected for the first test.
- Correct port, baud rate and radio model in the software.
- PTT method chosen and matching the radio's menu.
- Radio's transmit light behaves correctly when the program starts and closes.
- Audio input set to the USB or data source, not the microphone.
- Computer sounds muted if you use VOX.
- Computer clock synced for timed modes; see how to sync your PC clock.
Once that is done, note the settings somewhere safe. Then you can check current band conditions and make a first call, and if you are new to the mode, FT8 for beginners walks through it.
Remote and unattended operation
PTT reliability matters more when you are not in the room. A stuck transmitter at a distance is harder to notice and fix. Many radios include a transmit time-out timer, and it is worth enabling it. If you operate over the internet, software such as Station Bridge sits between the radio, audio interface and computer, and Remote Rig lets you control the radio and its push-to-talk audio from other devices. A PTT setup that behaves well on your desk is the best foundation for that.
Wrap-up
For most operators with a modern radio, CAT PTT is the tidiest choice, RTS/DTR is a dependable alternative when CAT is awkward, and VOX is a fallback for radios with no control connection. Whatever you choose, set it up into a dummy load, test start-up behaviour, and keep audio routing separate in your mind from keying. When the transmit light and the software agree every time, you are done, and you can log your contacts in the free online logbook.
How HamRadioList can help
Once your PTT is sorted, log your digital contacts in the free HamRadioList logbook and export them as ADIF whenever you like. Check the live HF propagation page before you start calling, and if you want to run your radio from somewhere else, take a look at Remote Rig and Station Bridge.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- Live HF propagation - see current band conditions before you call CQ.
- Remote Rig - operate your own radio from anywhere on Android, iPhone, Windows or Linux, with live push-to-talk audio and CAT control.
- Station Bridge - the software that connects your radio, audio interface and computer to Remote Rig.
- Ham radio FAQ - plain answers to the questions new operators ask most.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.