Digital Modes
Digital modes in amateur radio are amazingly popular. We’re starting our review with a basic digital term, and then getting into modulation modes.
When you think of many digital modes, you think of radios connected to computers. Winlink is a service that you can use to send email via digital amateur radio. If you are running a Winlink server, it needs to listen for incoming connections over RF. It might use Automatic Link Establishment, ALE, to make these connections. ALE constantly scans a list of frequencies, activating the radio when the designated call sign is received.
If you want to transfer binary files over HF digitally, you could use the PACTOR mode. If you are all about getting the highest data throughput, you want to use PACTOR IV. Assuming clear communications conditions of course. PACTOR is one of several digital modes that does not support keyboard-to-keyboard operation. Another is WSPR or “Weak Signal Propagation Reporter.” WSPR is the only correct option when presented with a question about the lack of keyboard-to-keyboard operations.
One of the earliest digital modes, outside of CW, was radio teletype. That’s known as RTTY or R-T-T-Y. RTTY refers to several different frequency-shift keying modes. Frequency-shift keying is abbreviated FSK. FSK modulation is a common data emission below 30 MHz. There are two ways to transmit FSK, directly or over audio. The audio version is referred to as AFSK. The difference is modulation. Direct FSK applies the data signal to the transmitter VFO, while AFSK transmits tones via phone modulation.
Another digital mode that has long term popularity is PSK. That stands for phase-shift keying. One flavor of this HF mode is PSK31. PSK31 uses variable-length coding for bandwidth efficiency. The “31” indicates the bits per second. Faster modes include PSK63.
There are several modes found in the WSJT-X suite of software. This encompasses many modes optimized for weak-signal communications. WSJT-X has elements focused on HF and VHF-and-above operation. One of the interesting elements of the WSJT-X modes is that they have time based windows to transmit. That requires good synchronization of computer clocks to manage transmit and receive timing.
A very popular WSJT mode is FT8. The timing of an FT8 transmission cycle is 15 seconds. For contesters, FT4 was developed to improve the speed of contacts. 4 is half of 8, so its alternating transmission windows are at 7.5-second intervals.
Some details you’ll need to know on these modes. The 4 in FT4 refers to Four-tone continuous-phase frequency shift keying. When you are looking for the digital mode that has the narrowest bandwidth, it’s FT8.
When making a casual QSO using these modes your exchange will include a signal-to-noise ratio report. In VHF contests using FT8 or FT4 modes that gets replaced by your grid square info.
Some hams are working on a revision of FT8 called FST4. It’s designed to increase reception sensitivity. FST4 mode characteristics include:
- Four-tone Gaussian frequency shift keying
- Variable transmit/receive periods and
- Seven different tone spacings
All these choices are correct when asked about FST4 on the exam.
Let’s start shifting to modes that are used more on 50 MHz and above, and we’ll stay within that WSJT-X suite. How does bouncing signals off objects in space sound?
Earth-Moon-Earth, or EME communications, typically are called “moonbounce.” It’s just what it sounds like. Radio-waves from an earth-based transmitter are directed via reflection from the moon’s surface. An earth-based receiver picks them up. Ta-da! Bouncing radio signals off the moon!
The optimal mode for EME is currently called Q65. Q65 is optimized for EME because multiple receive cycles are averaged, which is different from JT65.
Previously JT65 was recommended for moonbounce because it decodes signals with a very low signal-to-noise ratio. JT65 modulation is multi-tone AFSK.
A method of establishing EME contacts is time synchronous transmissions alternating between stations. That’s similar to the other WSJT-X modes.
Do you want a bigger challenge than bouncing signals off the moon? MSK144 is a digital mode that has been designated for meteor-scatter communications. Meteor-scatter communications allow amateurs to make contacts up to 1,200 miles on VHF. This form of propagation relies on the many meteors that enter the earth’s atmosphere on a daily basis. Some of these meteors are very small — the size of a pebble or even a grain of sand. The number of meteors increase during meteor showers. These times of the year are when hams are able to make more contacts than normal using meteor scatter.
