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Amateur Radio Info & Exams - Operations 3 - Digital Modes
Didn't we cover Digital modes on the Tech paper? Yes, but this section deals with modes most often used on HF, MF, and LF, although there is no reason they can't be used on VHF and up, if you can find someone to do so with.
In at least some cases the recommended digital portion in the bandplan is 70 to 100 kHz above the start of the band, for example, 3570-3600 kHz, or 14.070 to 14.100 MHz. On 40 metres it is 7.080 to 7.125 MHz.
RTTY
Often pronounced "ritty", this stands for Radio-Teletype, and originally used electro-mechanical teletype machines, with radio replacing the telegraph / telex lines. These were used by the military, diplomatic organisations, between ships, and by news agencies. You may have seen them in "Good Morning, Vietnam". TELEX was a wire based system using the same machines, where each business using one had a number, similar to a telephone number.
The original version directly keyed the transmitter to shift between two frequencies for mark and space. The modern version uses audio frequency shift keying, although for SSB this is has the same effect. RTTY usually uses Lower Sideband - LSB.
On HF the shift is usually 170 Hz. At LF is often half this, and there are certainly systems which use greater shifts. Speed is 45.45, 50, 75, or 100 baud, so fairly slow. Higher bit rates can be used on electronic systems. Baudot code, a 5 bit system, rather than ASCII, is often used. As 5 bits only gives 2⁵, or 32 characters, barely enough for upper-case characters, one of these is a "number shift" allowing the bit patterns to double-up as numbers and punctuation, a second "letter shift" character returning to letters.
RTTY is somewhat like voice, in that a station can communicate with more that one station, and take part in round-table conversations and nets. Material such as radio club news can be broadcast to many Amateurs. It is also possible for a non-ham to decode signals.
While the use of a PC, or "glass terminal" is common, restoration of electro-mechanical gear is also part of the hobby. As well as the very heavy machines, there are the SAGEM units which use a dot matrix printer, based on the 6800 processor (still a two person lift!). Many machines, modern and ancient, include a paper-tape reader, with a row of fine tractor sprocket holes, and 5 rows of holes, being the data. These allow a message to be prepared ahead of transmission, or for incoming messages to be stored. This also makes sending complex art made from alpha-numeric characters, or heading text like that below.
RRRR TTTTT TTTTT Y Y
R R T T Y Y
RRRR T T Y
R R T T Y
R R T T Y
PACTOR
A development of RTTY was AMTOR (AMateur Teleprinting Over Radio), and this was further developed with Packet Radio concepts to form PACTOR. It uses rapid switching between transmit and receive for error checking and retransmission if necessary. Its uses include inter-continent linking in packet massaging. While it is possible to listen in, only two stations can actively communicate at one time.
Another variation is WINMOR, designed to avoid the proprietary nature of some PACTOR versions, and can be used with a soundcard, rather than an overpriced modem.
PACTOR and WINMOR are (or were) used as the modulation protocol for "Winlink", a radio to email system used by hams and yacht sailors.
As of late 2022, WINMOR is being replaced by "VARA" in Winlink, although you don't need to know that if you sit your exam before the end of June 2023.
DE-9 Connectors
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The smaller connector is the DE-9 D-subminiture connector used for serial communications, for these modems, or at least the PC end of the connection. The modem may use this, or the larger DB-25 connector, as used on telephone and leased-line data communications equipment. The larger connector, invented in 1952, was adopted in the 1960s RS-232 standard. The pinout for RS-232 over the 9 pin is defined in the 1990 standard, TIA-574. Lines include ground(s), transmit data, receiver data, optional flow control lines, and modem control lines. Older PCs had these for computer control, or an interface which used an adaptor with serial at the PC end.
Some radios use USB connections which may be converted to RS-232 within the radio, including for programming.
These connectors, and versions with different numbers of pins are or were used for many things, including PC VGA (using high density DE-15)and other video connections, old external drives, printers, joysticks, barcode readers, and multiple industrial uses. They have chassis connectors in male and female; and cable mounted connectors in both genders. The panel mount versions often have nuts where the circles are on the drawing, which accept 4-40 UNC screws, to stabilise the connectors. On the DE-9 these are spaced 25 mm; and on the DB-25 it is 53 mm. The nuts are 3/16" AF (just under 5 mm).
A standard for the connector series is Mil Standard 24308. |
PSK31
This is a narrow bandwidth mode which can function with very low signal levels. The software includes a waterfall display, which allows you to see where in the band segment there are signals.
Waterfall displays
Just as water in a waterfall flows vertically over time, the information on the screen flows vertically, frequency is displayed horizontally, increasing left to right, and the colour or brightness (intensity) of the display indicates the strength of the signal. A simple signal, such as "QRSS" Morse can be seen in the waterfall. Things like harmonics of oscillators in power supplies which vary in frequency as the supply heats up will be seen as a diagonal trace. I understand that PSK31 appears as two closely spaced lines. Additional lines can be the result of over-modulation.
WSJT-X
This stands for Weak Signal by Joe Taylor, with examples being FT8, FT4, JT4, JT65, JT9, WSPR, and MSK144. These modes use AFSK (audio frequency shift keying), typically with the radio set to USB.
UI
These are "Unnumbered Information" packets, used for things such as beaconing and APRS. While they are a real and useful thing, the term appears in a couple of distractors (wrong answers).
Other comments
Off the exam, I understand that using DDS, Direct Digital Synthesis, it is possible to generate some of these modes directly, at RF.
Many radio have a "Digital" mode, in addition to the usual CW, USB, SSB, AM, FM, etc. You should consult the manual regarding its use.
Digital modes typically start 70 kHz above the lower edge of each HF band, at least in the pre-WARC bands. You can scroll down the HF bands here: ARRL Band-Plan, described thus: "A band plan refers to a voluntary division of a band to avoid interference between incompatible modes".
Fldigi
Off the exam, but useful for practical communications, including in emergency communications support work, the Fldigi (Fast light digital) is an open source suite of programs which use the soundcard as a modem. It offers 20 modes, with many variants.
Read more: Wikipedia: Fldigi
Relevant Questions
These are actual questions from the General exam pool.
G2E01
Which mode is normally used when sending an RTTY signal via AFSK with an SSB transmitter?
A. USB
B. DSB
C. CW
D. LSB
It is LSB, answer D.
G2E02
How can a PACTOR modem or controller be used to determine if the channel is in use by other PACTOR stations?
A. Unplug the data connector temporarily and see if the channel-busy indication is turned off
B. Put the modem or controller in a mode which allows monitoring communications without a connection
C. Transmit UI packets several times and wait to see if there is a response from another PACTOR station
D. Send the message: "Is this frequency in use?"
The system should have a monitor mode, which can tell you what traffic is being passed on a channel, answer B.
G2E03
What symptoms may result from other signals interfering with a PACTOR or WINMOR transmission?
A. Frequent retries or timeouts
B. Long pauses in message transmission
C. Failure to establish a connection between stations
D. All of these choices are correct
All these problems can be caused by interfering signals, answer D.
G2E04
What segment of the 20-meter band is most often used for digital transmissions?
A. 14.000 - 14.050 MHz
B. 14.070 - 14.100 MHz
C. 14.150 - 14.225 MHz
D. 14.275 - 14.350 MHz
14.070 to 14.100, answer B.
G2E05
What is the standard sideband used to generate a JT65, JT9, or FT8 digital signal when using AFSK in any amateur band?
A. LSB
B. USB
C. DSB
D. SSB
If the question is essentially "which" sideband, then the answer must be one of the first two, and in this case it is USB, answer B.
G2E06
What is the most common frequency shift for RTTY emissions in the amateur HF bands?
A. 85 Hz
B. 170 Hz
C. 425 Hz
D. 850 Hz
The most common on HF is 170 Hz, answer B.
G2E07
What segment of the 80-meter band is most commonly used for digital transmissions?
A. 3570 – 3600 kHz
B. 3500 – 3525 kHz
C. 3700 – 3750 kHz
D. 3775 – 3825 kHz
Digital modes are in the lower portion of the band, but not the very lowest, which is most likely Morse. Also, remember the Band edge plus 70 to 100 kHz concept above. Answer A.
G2E08
In what segment of the 20-meter band are most PSK31 operations commonly found?
A. At the bottom of the slow-scan TV segment, near 14.230 MHz
B. At the top of the SSB phone segment, near 14.325 MHz
C. In the middle of the CW segment, near 14.100 MHz
D. Below the RTTY segment, near 14.070 MHz
It is around 14.070 MHz, answer D.
G2E09
How do you join a contact between two stations using the PACTOR protocol?
A. Send broadcast packets containing your call sign while in MONITOR mode
B. Transmit a steady carrier until the PACTOR protocol times out and disconnects
C. Joining an existing contact is not possible, PACTOR connections are limited to two stations
D. Send a NAK response continuously so that the sending station has to pause
PACTOR uses a station to station connection, needed to allow retransmission requests, so others cannot join into an existing connection, answer C.
G2E10
Which of the following is a way to establish contact with a digital messaging system gateway station?
A. Send an email to the system control operator
B. Send QRL in Morse code
C. Respond when the station broadcasts its SSID
D. Transmit a connect message on the station’s published frequency
The systems software contains commands which allow you to send connect messages to stations, presumably a string of characters including their and your callsigns, answer D.
G2E11
Which of the following is characteristic of the FT8 mode of the WSJT-X family?
A. It is a keyboard-to-keyboard chat mode
B. Each transmission takes exactly 60 seconds
C. It is limited to use on VHF
D. Typical exchanges are limited to call signs, grid locators, and signal reports
Exchanges typically contain limited information, as indicated in answer D.
G2E12
Which of the following connectors would be a good choice for a serial data port?
A. PL-259
B. Type N
C. Type SMA
D. DE-9
This is the smaller of the two D-connectors used for serial data, the DE-9, answer D.
DE indicates the smallest of the shell sizes, and the 9 indicates 9 pins. The connector originally used for RS-232 is the much wider DB-25. A standard for the connector series is Mil Standard 24308.
G2E13
Which communication system sometimes uses the Internet to transfer messages?
A. Winlink
B. RTTY
C. ARES
D. Skywarn
Winlink is a system which allows Internet emails to be sent from Amateur and other radio links, answer A.
G2E14
What could be wrong if you cannot decode an RTTY or other FSK signal even though it is apparently tuned in properly?
A. The mark and space frequencies may be reversed
B. You may have selected the wrong baud rate
C. You may be listening on the wrong sideband
D. All of these choices are correct
Reversing frequencies or sidebands will have a similar negative effect; and the wrong baud rate will also prevent a digital signal being decoded, so all are correct, answer D.
G2E15
Which of the following is a requirement when using the FT8 digital mode?
A. A special hardware modem
B. Computer time accurate within approximately 1 second
C. Receiver attenuator set to -12 dB
D. A vertically polarized antenna
This family of modes are often is based on a system of stations taking turns to send, according to a transmit / receive cycle, with 15 seconds being used for FT8. Thus an accurately set clock is required, answer B.
This can be achieved by using an internet time server to set the time on the PC, or using a GPS based device.
You are over a quarter of the way through the question pool.
On to: HF Propagation 1 - Sunspots & Usable Frequencies
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, December 2022.
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