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During the 1900s, digital modes required strong signals to provide a coherent signal into a modem based on physical filters. Modern digital signal processing (DSP) either in dedicated ICs, or using a PC sound-card and software processing allows suitably designed digital signals to be resolved at very low received levels.
RaspberryPi, OrangePi, maybe BananaPi, and related products can be used too. Or a mini PC. USB connected soundcard modules are available at little cost via online marketplaces, and can be used with these devices, or PCs, etc.
Once you set this up for one mode or package you can use it on many others, such as fldigi, useful for passing messages, including for emergency communications support. JS8Call is another example. Traditional modes like RTTY, AX-25 Packet, and decoding Morse are also possible. I believe DSP based noise reduction and filtering is also something you can try.
Moving to the voice and image segment, you can also send "slow scan TV", SSTV, one of the distractors, EasyPAL is an example. Hellschreiber is an interesting hybrid text mode. You can also receive weather fax from NOAA (at least before midday 20 January 2025, or perhaps a few weeks beyond); and send and receiver facsimile images. List can be found on DX Zone: Weather and FAX, and a wider Software page.
As of 20 February the charts are still on the NOAA website; but with Golfo de México bearing the name a certain orange Gringo Loco thinks it should have.
The Ham developed, and open source CODEC2 based digital voice system is also an option.
One family of modes uses WSJT-X software, standing for Weak Signal by Joe Taylor. Examples are JT9, JT65, the FT8 family, and WSPR, below.
The inventor, Joseph Hooton Taylor Jr. K1JT is an astrophysicist who received the Nobel Prize in Physics for discovering a "new type of pulsar". He previously held VK2BJX.
The important commonality with the included modes is the use of varying numbers of tones, such as 4 or 8.
The Princton pages have been deleted, with content and downloads moved to: https://wsjt.sourceforge.io/wsjtx.html
WSPR, pronounced "whisper", stands for "Weak Signal Propagation Reporter". Is a low powered beaconing system. It assists in understanding propagation. It uses Frequency Shift Keying, coded F1D. Messages contain a compressed version of callsign, Maidenhead locator, and power in dBm. Each element lasts 0.682687 seconds.
It has gained public attention as reflections from the hull or contrails of flight MH370 in an otherwise empty region of sky provide a possible location for its end point.
You can view recent "decodes" at WSPRnet Map
There are a few options to run this mode. You can use a PC and radio to both send and decode messages, and send reports to the site above. Another is to use a RaspberryPi or similar device to beacon only; it can toggle a data line at radio frequencies to generate a low powered signal, probably under 20 milliwatts.
Another popular mode is PSK31, a mode which uses phase shift keying, and 31.25 symbols per second (meaning each element lasts 32 milliseconds). To speed communications, instead of a fixed number of bits per character, this mode uses "Varicode", meaning common characters are send using a shorter sequence than longer ones. Lower case also have shorter varicodes than upper case ones, so SHOUTY upper-case messages take longer than ones using normal cases.
There are two modes of operation, BPSK31, standing for Binary PSK, with two positions, 0 and 180 degrees. The other is QPSK31, meaning quadrature, with 4 different angles of shift, permitting additional error correction.
Operators use it to type messages to each other, similar to RTTY.
Stations operating using these modes usually use only modest power, maybe 20 watts, but even lower power (QRP) works well.
Read more at: PSK31 page at University of the Basque Country and https://bpsk31.com/
Frequencies are listed on Wikipedia: PSK31, with links to original articles on the subject.
This popular mode allows the fairly automated exchange of callsigns and signal reports between Amateurs globally. The examiner focuses on its very low signal-to-noise ratio requirement, and this is a real benefit, as a Ham in a terraced house in an electrically noisy urban environment can still make long distance contacts on HF, with a modest antenna.
Stations take turns transmitting, each period being 15 seconds long. Your PC clock should be set via NTP or GPS.
FT4 has faster exchanges (7.5 seconds per turn) for contests.
While you can certainly monitor operations using this mode on HF as Technician or Novice, the lowest band you can use in on is 10 metres, with activity on 28.074 MHz. FT4 is on 28.18 MHz, and JT9 is on 28078-28079 kHz. JS8Call is also on 28.078 MHz. I understand that all Amateurs can use it on their VHF and UHF allocations. For Novices 222.065 MHz is within their 1.25 metre allocation (which is the the entire general access (upper) portion of the band). On 23 cm 1296.174 MHz isn't, so you'd have to arrange a contact below 1295 MHz.
These are packet based data modes, used on HF especially.
In Packet and PACTOR, an ACK is an acknowledgement reply, meaning the last packet was decoded OK, and the next packet should be sent. A NAK is a negative acknowledgement which means the packet most be resent.
Among oher things these form the basis systems such as Winlink, an email by radio system, although there is a shift from WINMOR to VARA.
The examiner now groups these as ARQ modes, standing for Automatic Repeat reQuest, partly due to changes on modes used over time, as above.
If a station connects to another, but the error rates are excessive, the link will be "dropped", or disconnected.
A concept not explored by the examiner is the idea of layers in data communiations, things like the physical layer with bits and modulation, to frames, to packets, and up to the application.
These can be implimented using physical modems, or in software, as above.
This is a 5 bit code, although it does use stop and start bits. It only supports English language upper-case characters, and via shift characters, numerals, and selected punctuation, plus a Bell (BEL) to gain the operator's attention. It is used in RTTY.
The 5-bit code was used by Émile Baudot in the 1870s, to develop a printing telegraph system. The baud is named for him.
In radio it is used with FSK (Frequency Shift Keying) modes, including RTTY. They name one frequency the mark, the other the space. These equate to 1s and 0s.
The question on this has been withdrawn, but the following remains interesting.
This band is centred on 2.4 GHz, which is also an Industrial, Scientific, and Medical band. The main feature of the ISM bands is that non-Amateur activities do not require a licence from the FCC, ACMA, etc. Being above 1 GHz, it is generally considered a microwave band.
This ISM band is also used by things such as Wi-FI, your microwave oven, BlueTooth, industrial heating; and for often analogue video links from video baby monitors, horse-box cameras, and trailer rear cameras. (Yes, your microwave can interfere with your Wi-Fi, especially if your fibre / VDSL / ADSL / etc modem is in the front room, the kitchen in the middle, and your PC in the back room).
The full US allocation is: 2.300 to 2.310 GHz, and from 2.390 to 2.450 GHz. The gap is spectrum used in North America for Sirius XM satellite delivered pay radio. It is worth noting that the lowest part of the Ham band is below the ISM band.
Discussed elsewhere, Wi-Fi equipment can be hacked to transfer large amounts of data within the ham segment of the ISM band (the latter extends to 2.5 GHZ in the US, or 2.4835 GHz Downunder).
The VK Ham allocation is 2300 - 2302 MHZ & 2400 - 2450 MHz.
The band has plenty of bandwidth for analogue video, including use for things like retrofitted back-up cameras on cars and trailers. A friend had one powered fom the reversing light line on his trailer, so it was usually blank, but things like the inside of horse trailers, or his own car viewed from the back of a semi-trailer would appear on it. Assuming operation in the Amateur allocation, you can make short range contacts, or use a dish or other gain antenna for field-day fun. Just point the camera at a card with your callsign on it periodically. Successful transmission and reading of such as images is typically what is needed for a formal contact. Quite a few cameras send mirrored video, to emulate a mirror.
The 5 cm band at 5.6 GHz overlaps another ISM band, also used for Wi-Fi, aka WLAN (Wireless Local Area Network). 5.6 GHz analogue video senders are the easiest way to get on this band, much cheaper and easier than building voice gear; just place the patch antenna at the feed-point position of an offset satellite dish. pointed at the other station.
For those with the big bucks, the Icom IC-905 is a great radio which covers this band, and several others, using a radio consisiting of a user interface which looks like a radio (the IC-705), and a mast-top box which has the actual radio bits in it. This is the first plug-and-play rig for these bands, and was apparently built in resonse to a use-it-or-lose-it edict from the Japanese regulator. The traditional method is to use a 2 metre or 70 cm SSB rig, along with a transverter. 2304.100 FT-290, FT-290R or FT-290R II; FT-790R or FT-790R II; FT-817ND; or FT-857D, or something similar from ICOM, etc
To use the SSB calling frequency, which is in the 2400 MHz segment, using a 2 metre radio, you can determine the Local Oscillator thus: 2403.1 - 147.1 = 2256 MHz. Using 70 cm gives a wider frequency range with a single LO: 2403.1 - 435.1 = 1968 MHz. For this MiniKits transverter, strictly for the 2300 MHz segment, different LOs are needed. This SG Lab unit includes a switchable LO. Discussion: VK3FS 13 cm page.
An example of a suitable antenna for the lower microwave BiQuad. Another is a parabolic dish.
Apparently Wi-Fi channel 1 can interfere with the 2403 MHz area. If using this band at home, you may want to see if you can set your router to use a higher channel, perhaps ditto your neighbours' systems. There are 'phone apps which show channels in use. If you each move to different channels you may improve your Wi-Fi performance!
Hams in Slovenia (S5) have been operating an extensive high speed packet network on these bands since the 1990s. See: High speed AX-25 network in Slovenia
Modes discussed here are for UHF and VHF. Most radios are capable of FM as well.
One question, and a new section required! There are multiple digital voice modes, some almost proprietry, used by governemnt users; some commercial grade, but with equipment with ham-friendly pricing, and some Amateur only.
DMR is an mostly open European system called Digital Mobile Radio, developed by European Telecommunications Standards Institute. You need a Tier II radio for most uses. It is likely the most popular system, with commericial grade (Vertex Standard, Motorola, etc), and various Chinese made ham grade products.
It uses talk-groups, which reflect the location (broad or narrow), language, or purpose of the communication, such as emergency support. Physically, the user transmits on a different frequency to they listen, as is the case with FM repeaters. In Australia the vast majority of amateur DMR activity is on 70 cm. In the US and UK 2 metres is also used.
The big attraction of this, and most other digital systems, is that the repeaters are connected to the Internet, allowing regional or global contacts. It is also possible to connect them via a hatspot, typically an MMDVM. There are simplex and duplex versions, and ones duplex ones. Note that these require either a RaspberryPi Zero W (the W indicating Wi-Fi), or a full sized RPi. Listings may show the price of the case with the prcie of the MMDVM board revealed once this is selected.
In Australia VKDMR is the primary system, and 505 is the talk-group (TG) for calling for contacts, using Timeslot 2. Using TS1, TGs 3801 to 3909 are for general use, previously state based. 3810 is for WICEN use. These groups are "user activated", meaning you generally must briefly key up to be able to listen in - don't worry, your datastream contains your identification. TG 1910 is for ARNSW supported repeaters; 1911 is Far North Queensland.
Australian TG 8409 (TS1) links to the UK's TG 4409, which links to Wires-X (below). I've spoken to people in Canada and Norway via it, and heard a South African station.
All TS1: TG 1 is global, TG 3 is the US. TG 13 is English World-Wide, and TG 80 is a UK group. TG 9 is the local repeater only. TG 8 is regional. TG 17 is for Nordic languages. 11 and 21 appear to be French language; World-Wide and Europe respectively.
The other setting is "Color Code", which is an access control code, maybe a little like CTCSS on FM repeaters, meaning repeateres of other organisations are not accidently accessed. In Australoa all Ham repeaters use a CC of 1. In the US a range of numbers are used, as can be the case with WICEN NSW non-ham systems.
BrandMeister is a separate, global network using DMR. To access it you need to connect to a BrandMeister access point or hotspot, which allows you to connect to repeaters in specific countries or cities; as an example, 425 is Israel. DMR-MARC is a Motorola-centric system.
Thanks to TDMA (Time Division Multiple Access), two conversations can take place on one repeater at the same time, as long as the timeslot (TS) is different. However, if someone on a reapeater is using say, 3802 (TS1), and I try to use 8409, the radio will emit a deep tone to warn the repeater is in use. The IPSC2 websites for the various areas can tell you who is using what, in real time.
Many set each repeater to have its own "zone", and this can be a good way to store a comprehensive list of TGs for each repeater. You can however make a zone with the most important local channels and TGs, both digital and analogue. If type approved it can include things like the local UHF CB channels (although this is often very potty-mouthed). Local WICEN net channels could also be added.
A friend has a Radioddity GD-77, with modified firmware which has a very nice feature: The up-down buttons select repeaters, and the left-right the Talk Group. Instructions in the firmware: Open GD-77.
Amateur users need to obtain a user ID from RadioID.net. This is a 7 digit number based around the internal cell site numbering system used by GSM. An Australia number might be 5051234, a US one 3211888 might be a US number.
D-Star was developed by the Japanese association, with ICOM as the initial manufacturer. It supports voice and data.
SystemFusion or "System Fusion" is Yaesu's product. It uses C4FM. Repeaters are FM compatible. Repeaters are linked by Wires-X, as development of the Wires linking system used with FM.
The sequentially numbered Project 25 / APCO 25 / P25, not to be confused with the ending of democracy and decency, is a system designed for US law enforcement. Motorola is the major player, with used products (especially ex Sydney Olympics) going into the Amateur and UHF CB spaces. Tait and Kenwood also sell P25 products, with new radios costing thousands of dollars. The year 2000 era Quantar repeaters support P25 and FM, although all in the conversation must use the same mode. Inter-mode bridges exist. Quantars are VERY power hungry, needing 24 volts at around 16 amps, just on standby!
TETRA is the European equivalent, used by LE and emergecy services. There is (very?) limited ham use. I think Brisbane in VK4 has an access point.
NXDN is an open system which uses a single stream with a 6.25 kHz spacing. A spin put on it is that it uses FDMA - Frequency Division Multiple Access, which is exactly what an FM handheld or AM CB uses - each group of users uses a different frequency, at any one time. It is used commercially, with some amateur use, perhaps mainly in the US. Hobart, in Lutruwita (aka Tasmania - VK7) has the only one in VK. As above, IDs are issued by RadioID.net. There are 272,618 DMR IDs vs 11,328 NXDN ID's. There are 10096 repeater IDs across both systems.
The short answer is "DMR". The long answer is "Digital Mobile Radio". The more serious answer is to ask what local hams using, what repeaters are available nearby, and what is used where you travel. That said, DMR coverage may not be as extensive as FM repeaters just yet, but it is getting close in many regions.
See these maps of DMR Repeaters and NXDN Repeaters.
DMR radios, whether Ham market Chinese units, or Motorola / Vertex Standard ones, support FM, sometimes in narrow and wide modes. UHF versions of the latter can be programmed with Australian UHF-CB channels, as they are not a "non-standard device", to use government double-speak; ditto the Ham-only ones, but only to listen, with transmitting only in dire emergencies. Marine 16 and 67 are handy in VHF versions.
A new ham-developed, open source mode is M17. Unlike the systems above which use proprietry AMBE or AMBE-2 chips, this uses the free, Australian developed Codec 2. The CSI CS7000-M17 runs this mode, although you apparently have to load the required software. There is also the more expensive Connect Systems CS7000-M17 PLUS handheld radio.
DroidStar is an Android app which allows you to connect the various systems above.
WICEN NSW Inc, and its parent body, the Volunteer Reascue Association (VRA), hold licences for non-amateur repeaters using DMR and P25. To use these the radio must be type-approved. Both the ~160 MHz and ~460 MHz bands are used on DMR, and 460-odd MHz on P25.
These are actual questions from the General exam pool.
G8C01, regarding the 2.4 GHz being in the proximity to WI-Fi, has been withdrawn.
G8C02
Which digital mode is used as a low-power beacon for assessing HF propagation?
A. WSPR
B. MFSK16
C. PSK31
D. SSB-SC
Sounding like whisper, this is WSPR, answer A.
G8C03
What part of a data packet contains the routing and handling information?
A. Directory
B. Preamble
C. Header
D. Trailer
This is the header, answer C.
G8C04
Which of the following describes Baudot code?
A. A 7-bit code with start, stop and parity bits
B. A code using error detection and correction
C. A 5-bit code with additional start and stop bits
D. A code using SELCAL and LISTEN
Baudot, used in TELEX and RTTY uses 5 bit codes, answer C.
G8C05
In an ARQ mode, what is meant by a NAK response to a transmitted packet?
A. Request retransmission of the packet
B. Packet was received without error
C. Receiving station connected and ready for transmissions
D. Entire file received correctly
This stands for Negative Acknowledgement, and means the racket was not received correctly, and thus needs to be retransmitted, answer A.
G8C06
What action results from a failure to exchange information due to excessive transmission attempts when using an ARQ mode?
A. The checksum overflows
B. The connection is dropped
C. Packets will be routed incorrectly
D. Encoding reverts to the default character set
The connection is dropped (disconnected), answer B.
G8C07
Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?
A. MSK144
B. FT8
C. AMTOR
D. MFSK32
FT8 can work down to 20 dB below the noise, or -20 dB, answer B.
G8C08
Which of the following statements is true about PSK31?
A. Upper case letters are sent with more power
B. Upper case letters use longer Varicode signals and thus slow down transmission
C. Error correction is used to ensure accurate message reception
D. Higher power is needed as compared to RTTY for similar error rates
Only a semi-literate RWNJ on Facebook would believe the first option, and given normal messages are mostly lower case, it makes sense that these use shorter varicodes, and upper case uses longer, answer B.
In most cases only modest power is used on PSK31.
G8C09
Which is true of mesh network microwave nodes?
A. Having more nodes increases signal strengths
B. If one node fails, a packet may still reach its target station via an alternate node
C. Links between two nodes in a network may have different frequencies and bandwidths
D. More nodes reduce overall microwave out of band interference
Just as if one street is blocked in a city grid, you can dodge sideways, assuming it isn't peak hour, when one node fails, a packet can still reach its destination via an alternate node, answer B.
G8C10
How does forward error correction (FEC) allow the receiver to correct errors in received data packets?
A. By controlling transmitter output power for optimum signal strength
B. By using the Varicode character set
C. By transmitting redundant information with the data
D. By using a parity bit with each character
This is done by sending "redundant information", meaning that content is repeated to some extent, answer C.
G8C11
How are the two separate frequencies of a Frequency Shift Keyed (FSK) signal identified?
A. Dot and dash
B. On and off
C. High and low
D. Mark and space
This is Mark and Space, answer D.
G8C12
Which type of code is used for sending characters in a PSK31 signal?
A. Varicode
B. Viterbi
C. Volumetric
D. Binary
Like in Morse Code, the most common characters have the shortest length, and this variable-length code is called Varicode, answer A.
G8C13
What is indicated on a waterfall display by one or more vertical lines on either side of a digital signal?
A. Long path propagation
B. Backscatter propagation
C. Insufficient modulation
D. Overmodulation
Overmodulation can cause spurious signals either side of the transmitted signal, answer D.
G8C14
Which of the following describes a waterfall display?
A. Frequency is horizontal, signal strength is vertical, time is intensity
B. Frequency is vertical, signal strength is intensity, time is horizontal
C. Frequency is horizontal, signal strength is intensity, time is vertical
D. Frequency is vertical, signal strength is horizontal, time is intensity
Frequency increases across the screen, while brightness indicates the signal strength. Over time the displayed information scrolls down the screen, as water flows over a waterfall, answer C.
Intensity may be replaced by a false colour representation. Some digital modes appear as two parallel lines, while slow Morse may be visible as short and long elements. Ionospheric sounding transmissions and some interference from poor quality power supplies may be seen as diagonal lines wandering across the waterfall. To see and example click the green image on this page.
There is even software which can generate a signal which will place text and symbols directly into the waterfall display.
G8C15
What does an FT8 signal report of +3 mean?
A. The signal is 3 times the noise level of an equivalent SSB signal
B. The signal is S3 (weak signals)
C. The signal-to-noise ratio is equivalent to +3dB in a 2.5 kHz bandwidth
D. The signal is 3 dB over S9
Reporting singal strength is an important part of an FT8 exchange. +3 indicates a signal-to-noise ratio of +3 dB, answer C.
For FT8 this is a good signal, but is much less than needed for traditional modes.
G8C16
Which of the following provide digital voice modes?
A. WSPR, MFSK16, and EasyPAL
B. FT8, FT4, and FST4
C. Winlink, PACTOR II, and PACTOR III
D. DMR, D-STAR, and SystemFusion
Just find a digital voice mode you recognise (D is a clue), and you get the banana. DMR (Digital Mobile Radio) is a mostly open European-designed system, D-Star a Ham specific Japanese system, and SystemFusion Yaesu's product. Answer D.
The wrong answers all are text based data modes, with one image mode, so likely easy to discount these distractors.
If you have been watching the question numbers, you may notice that we only have 2 sections to go. I'm covering this in 4 pages.
Andrew Viterbi is a Jewish / Italian / American electrical engineer, professor, and co-founder of Qualcomm. His work included the recovery of data from noisy signals, including that used in CDMA, an important telecommunications system. These are / were vital for rural and regional communications.
On to: Feedlines and SWR
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, February 2025.
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