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Amateur Radio Info & Exams - Modes 1 - Transmitter and Receivers

Modulation is using an "intelligence" signal to modify an RF signal. The intelligence can be voice, data, or image.

Transmitters

This is a device, or part or a device, which generates a radio signal for transmission. For broadcast systems this can be divided, with the first module termed the "exciter", which handles the modulation and puts out maybe 2 watts. This feeds the "PA", or power amplifier.

FSK

FSK is Frequency Shift Keying, where one frequency is the "mark", or 1 in a digital data stream; and another is the "space" or 0 in that stream. It involves directly altering the transmit frequency. This can be replicated using audio tones into an SSB transceiver. A variation is AFSK, used with FM transceivers, including for packet and APRS.

PACTOR

PACTOR is a data communications mode, for transferring messages and files via HF radio. As with various modes which work by connecting a modem to an SSB radio, the bandwidth of PACTOR version 3 is 2300 kHz, so that it fits within a voice frequency channel.

PACTOR is discussed on the following page.

SSTV

Slow-scan TV, or SSTV, an image transfer mode, likewise has a bandwidth, of around 2500 Hz. If used, this must be in the upper end of each HF band, beside voice. There are also fax, or facsimile modes, similar to those used for transmitting weather maps to ships.

Generating FM

To generate a VHF FM signal, often phase modulation is used at a much lower frequency, such as a twelfth or sixteenth of the desired frequency, and this is then multiplied up to this frequency. To transmit on 146.050 using a crystal controlled radios, we need a crystal on 12.170833333 MHz or 9.128125 MHz, depending on the multiplication factor. The section responsible is the multiplier. For UHF a larger factor is used.

More modern radios use true frequency modulation.

In some documents, especially from government agencies, "angle modulation" is used as a collective term for phase and frequency modulation.

AM and SSB

There are a few ways to produce SSB, with a balanced modulator and filter being one option, creating a DSB signal, and reducing it to SSB. The important point for the exam is this it is pretty much the narrowest common voice modulation method.

You can see AM envelopes at Wikipedia: Amplitude modulation

Note that VSB is silly in the contexts below. It is / was used for analogue TV broadcasting.

Link Budget and Link Margin

Sometimes the examiner randomly decides to throw in a new concept. These particular topics are normally associated with things like building microwave links, or moon-bounce.

Say we need to build a link between two repeaters, and we decide we need -80 dBm at the link receiver socket for a good signal. We work backwards, say taking into account 2 db of loss in coax, 9 dB of gain in the antenna, 123.5 dB path loss over 80 km at 448 MHz, 4 dB of antenna gain, and 5 dB of cable loss. This means we need 37.5 dBm at the transmitter socket, which is 5.6 watts.

Maybe as hams we can tolerate some degradation of signal quality, but if we are building a system to be used by non-ham volunteers (the link might be at 452 MHz in this case, and the loss 123.6 dB) we need a "Link Margin" so fading caused by atmospheric variations, bush fire smoke, rain storms, etc, and high atmospheric noise or lightning does not impede communications. We might decide this should be 15 dB.

There are a few ways we can "buy" this increased signal. We can up the transmitter power to 50 watts, gaining about 9 dB. We can purchase better antannas and coax, especially at the transmitter end. We can maybe add a masthead pre-amplifer at the receiever end. Thus under good conditions we have at least -65 dBm at the receiever antenna connector, and a high quality signal a very large percentage of the time.

For EME the biggest challenge is massive path losses of 250 to 310 dB. These increase with frequency, but higher gain antennas, moving to dishes, are possible with higher frequencies. For VHF and UHF multiple "stacked" Yagi antennas are often used; maximum legal power is used if possible, along with low loss coax and pre-amplifiers at the receiver antenna feedpoint. Advanced receivers, using digital signal processing, and specific WSJT-X or similar modes are used.

In designing a professional microwave communication system we can take, transmitter power, waveguide loss, dish size / gain, path loss, dish size / gain, waveguide loss, and reciever sensitivity into account. We need an often large margin for things such as rain induced signal fade. Using dBm and dB is the easiest way to do this.

Dishes can have very high gain, and 1 watt into a dish can generate a 1 kW ERP (+60 dBm) in a narrow beam:
30 dBm - 3 dB + 30 dB - 130 dB + 30 dB - 3 dB = -46 dBm
This would give a very good "Link Margin", given the receiver may work fine below -100 dBm.

Beyond the exam: While probably not a big deal at UHF, for comemrcial microwave systems we need to avoid having water at the point at which the signal would bounce from the ground, as this varying cancellation in the signal. On the old direct Admastown Heights to Gan Gan link it was even possible to hear an ocean wave-like variation in noise on the SSB intercom link between sites! Assuming it is still in use, the link now goes via a tower at Raymond Terrace. On a course I did the exercise was to find a suitable alternative relay point, including adjusting dish heights to avoid a bounce point in a lake or Port Stephens. I used a mountain at Tahlee. This generally should not prevent a ham contact.

There is no reason amateur or non-amateur microwave frequencies can't be used to link repeaters. The towers discussed above appear to be over-engineered, however this is necessary to prevent them twisting so the dishes move off-axis, causing signal loss, and intermittent link failure.

The graph used to design and characterise links can be seen here: Circuit design: Link Budget. Wikipedia pages: Link budget and Link_margin

Receivers

Mixers

A typical mixer takes two frequencies, and produces two outputs, these being the sum and the difference between the two. Where the Local Oscillator or carrier is not passed to the output, terms such as balanced or double-balanced may be used. To create DSB Full Carrier, aka traditional AM, we need the carrier to be included in the output signal.

Thus if we wanted to listen to 15 MHz (a time and frequency standard station), with a radio with a 10.7 MHz IF, we would need a Local Oscillator (LO) of 4.3 MHz. It is however possible that a strong signal on 6.4 MHz would also be heard, this being the "image". It is also possible to go up deliberately, say to receive a 7.1 MHz signal. Thus the LO would be set to 3.6 MHz. (This may allow the unintended reception of a strong signal on 14.3 MHz.)

Off the exam: Note that from HF, "double conversion" may be used, converting to 10.7 MHZ, or similar, then to 455 kHz, as used in medium wave receivers, even of the pocket size. There are also big dollar sets, with triple conversion, the first IF being around 70 MHz. Scanners, and other radios with broad coverage, were required to block spectrum used by AMPS analogue (FM) cellular 'phones. However, tuning to the "wrong-side" of the IF allowed strong signals to be heard. Despite digital modulation and encryption, this segment must still be blocked.

Often transformers are part of the IF signal path.

IF Filters

The output of the IF mixer is filtered before final conversion to audio frequency, and amplification to a speaker or headphones.

The width of the IF filter must be compatible with the bandwidth of the signal of interest.

For SSB the width would be 2.2 or 2.3 kHz (2200 or 2300 Hz), or thereabouts, and generally a sharp roll-off is good.

For CW 300 Hz or 500 Hz are typical options.

AM uses around 6 kHz.

For 9 or 10.7 MHz IF frequencies several crystals are used, along with shunt capacitors.

Several stages of inductors and capacitors are another option.

For 455 kHz filters a ceramic filter, containing piezoelectric ceramic elements, is typically used.

The best filters were mechanical filters made by Collins, a famous receiver manufacturer to the military, etc. Collins have ceased their production, so supplies held by companies such as InRad (International Radio), who fit them to the small carrier boards, are dwindling. Inrad sells sold / sells filters which are narrower and sharper that the factory filters for SSB, for CW, and for AM. See: InRad Filters. Yaesu also market(ed?) the same filters on their own carrier boards, prefixed "YSF-".

Using an SSB filter to receive CW means signals from several other stations may be in the receiver bandwidth, and even if we then have DSP, the ability of DSP to assist in the reception of a weak signal in the presence of significant noise, and/or a strong nearby signal is limited. A physical narrow filter, designed for CW reception is a valuable addition to a rig if you enjoy CW.

While the exam mentions matching the bandwidth to that of the communication, if you listen to broadcast AM on a ham receiver, then its narrowness may reduce the audio to communications style bandwidth. This may however be useful if you are listening to a "DX" station with a weak signal, as this also filters out noise in the upper audio frequencies.

Serious DXers, or DXpedition stations may even use a 1.8 kHz filter.

A friend had a good quality receiver, a Bearcat DX1000, which included wide 6 kHz and 12 kHz filters, for use when listening to good quality AM signals, in addition to a narrower 2.7 kHz one for SSB and CW.

Audio DSP filters the audio after the filtering above, and demodulation. Settings may be in menus, or via front-panel buttons and knobs. The FT-847 has a handy control using concentric knobs, the outer for the high pass (removing low frequency noise, buzzes, and similar interference), the inner for the low pass one, removing high end interference and noise. Two quick adjustments may this help provide usable audio. The amount of noise in-band noise reduction is also adjustable on many systems, but high setting may cause metallic, distorted audio.

The alternative is to digitise signals at the IF stage, and do all filtering and conversion to audio digitally.

If buying IF filters, characteristics to note are the width (-3 dB), the frequency of operation (be it 455 kHz or 9-odd MHz, for example), and the matching impedance. A -60 dB bandwidth may also be specified.

General comments: For the FT-857D the three small black TOYO ceramic filters soldered to the PCB can degrade, and may be replaced with similar looking muRata products, noting that this requires careful soldering. Several Hams have documented their work replacing them. One is DH1TW, another IZ2ZGG.

To add muRata and similar filters in metal cans as optional filters, these can be secured to the radios PCB with good double-sided tape, and short wires soldered between their pins, and the pins intended for the optional boards. Filter mounting pins may need to be shorted, to tell the radio the filter is there, and modes selected in menus.

Intermodulation

Any circuit which contains semiconductor junctions can be an accidental mixer. While it may be poor quality or faulty radio gear on a communications site, this includes a rusty bolt on a tower; or a wire resting on a galvanised water trough, near a transmitter site. Galvanising is zinc coating or plating.

While there are lots of examples of odd-order intermodulation products, twice one frequency, minus the other is a "classic" combination. You will note in the examples below that the product is close to transmitted frequencies. As far as I can determine an example of even-order product is that the sum of the two frequencies, mathematically f₁ + f₂. Clearly ~300 MHz is a lot further away.

Thus a 150 MHz and a 160 MHz transmitter can interact to cause interference at 140 MHz thus:
2f₁ - f₂ = 2 × 150 - 160 = 300 - 160 = 140 MHz.

Or if a combination of music and data appears on the VK calling frequency:
2 × 152.5 - 158.5 = 146.5 MHz.

Australian cities have ethnic "narrowcasters" at 151 and 152 MHz using communications bandwidth FM. The operators may then sell receivers to their community, no doubt at an inflated price, when a cheap scanner or HT will work. BBC World Service is also rebroadcast using it in Sydney.

This can be frustrating to find, and if one or both the signals are intermittent, these bursts will only happen periodically, and perhaps only during business hours. These bursts can be things such as a water tower reporting its level; or the need for more water to be pumped, or the pump stopped. While it is likely that the two lower powered transmitters which are interacting are within a few hundred metres, the interferring signal may travel many kilometres, especially to other high points.

In some cases there are more than two signals involved. It is also possible that both transmitters are in perfectly clean, and that the mixing occurs externally; or that one contains some fault which results in the generation of the interfering signal, in the presence of a second signal, or it is of poor design. Placing a Baofeng or similar in a high RF environment is asking for problems. Repeaters and the like tend to use Philips / Simoco, Tait, Motorola / Vertex-Standard, Icom, or other professional hardware.

Harmonics

Used in distractors, as well as a question, harmonics are multiples of the fundamental, and the naming is just a bit confusing. Say hit piano key A3 (A below middle C) firmly. This generates a tone at 220 Hz, quite deep or low. Most people behind a curtain could tell you that the instrument was a piano, not a harp, guitar, organ, or bassoon. This is because each has its own combination of harmonics. Different proportions of 440, 660, 880, and 1100 Hz, and so on, are present in the note produced by each instrument. The fundamental is also called the 1st harmonic; twice is the 2nd harmonic; 3 times, the third; 4 times the fourth, etc.

The other term, used especially in music is "overtone". The first overtone in twice the fundamental; the 2nd is 3 times, and 3rd is 4 times.

The attack and decay of the sound is also different; along with the difference between struck / plucked strings; and instruments blown mechanically, by mouth, or from a bag; or simple electronic oscillators; are also factors in the sound (timbre) of different instruments. If you use a 555, such as in the Funway 2 Monophonic Organ project, you'll have lots of odd harmonics in the audio, thanks to the square wave nature of the IC. Kawai organs from the 1970s or 1980s contained what were probably custom Single-In-Line IC oscillators, one for each of the white and black keys in an octave.

Note that each octave involves a doubling of frequency from one C to the next, or one A to the next.

Back to radio, harmonics can be useful, such as an antenna for 40 metres (7 MHz) works on 15 metres (21 MHz). If you have a 70 cm radio you need to check the reciever of, you can set it to 435 MHZ, and transmit on 145 MHz. Set a 23 cm radio to 1296 MHz, and you should hear a test transmission on 432 MHz. Note in each case the

Off the paper: In days of 23 channels, 27 MHz AM CBs needed a crystal pair for each channel, one for transmit, the other for receive. The TX one for Ch 10 might be marked 27.075, but it is an "overtone crystal". In a simple test oscillator you might find it oscillates at 9.025 MHz. The oscillator in the CB is designed to make it vibrate a the overtone. The second crystal operates at 455 kHz below the transmit one, this difference being the IF frequency.

This is different to generating harmonics using frequency doublers and triplers on an PM signal with very narrow deviation, to make an FM signal.

Between this system, and the current Phased Locked Loop (PLL) system, also used in the typical Ham or commercial FM (or DMR) transceiver, was a system which used an array of crystals. Adding a frequency from the first back to the second bank, then subtrancting a constant third one gives the desired transmit frequency. For example 23.340 + 14.950 - 11.275 = 27.015 MHz (Ch 5). Stepping up one channels selects a crystal 10 or 20 kH higher, and every four channels a crystal 50 kHz higher is used, restarting the secone bank at the first value. Ch 6 is 23.340 + 14.960 - 11.275 ; Ch 9 is 23.390 + 14.950 - 11.275. On recieve the third one is 11.730 MHz, generating a frequency 455 kHz lower, to take the IF into account. This gave all 23 channels, Tx and Rx, using 12 crystals.

There are other combinations, perhaps including values around 35 - 10 + 2 MHz.If you want to go down the rabbit home, tables for many units are here: CBCI Crystal Mixing Chart. Altering the third and fouth crystals meant that CBs could potentially be used on 10 metres by Hams, or unlawfully used for "Freebanding". PLLs and processors made out-of-band operation very difficult, at best.

Relevant Questions

As always, here are actual questions from the General exam pool.

G8A01
How is direct binary FSK modulation generated?
A. By keying an FM transmitter with a sub-audible tone
B. By changing an oscillator's frequency directly with a digital control signal
C. By using a transceiver's computer data interface protocol to change frequencies
D. By reconfiguring the CW keying input to act as a tone generator

True FSK alters the frequency of the transmitter's oscillator directly, in response to a digital signal, answer B.

The speed at which this must be done is too fast for a "CAT" interface, but this can be done using direct digital synthesis. It is worth noting that the function above is generally not available in the typical Amateur transceiver, but a function of dedicated radio teletype transmitter use by a military or government agencies, and are really a thing of the past. The Soviet Union used high powered LF / LW transmitters for teletype to communicate across its vast territory. They contained huge coils in the output stage, which were saturated with a massive DC current during the mark signal, to alter the frequency of the antenna tuning circuit.

G8A02
What is the name of the process that changes the phase angle of an RF signal to convey information?
A. Phase convolution
B. Phase modulation
C. Phase transformation
D. Phase inversion

This is phase modulation, answer B.

Many transceivers, especially older "PMR" or commercial / government FM transceivers actually use PM, followed by multiple stages of frequency multiplication to generate FM. Some Ham rigs are marketed with a claim of using "True FM".

G8A03
What is the name of the process that changes the instantaneous frequency of an RF wave to convey information?
A. Frequency convolution
B. Frequency transformation
C. Frequency conversion
D. Frequency modulation

A nicely written question, avoiding the concept of changing the frequency of the "carrier" which once it is off its centre frequency is no longer the carrier. This is frequency modulation, answer D.

G8A04
What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage?
A. Multiplex modulation
B. Phase modulation
C. Amplitude modulation
D. Pulse modulation

This is phase modulation, answer B.

G8A05
What type of modulation varies the instantaneous power level of the RF signal?
A. Power modulation
B. Phase modulation
C. Frequency modulation
D. Amplitude modulation

AM involves generating sidebands on either side of the carrier, and the power level of these varies with that of the modulating signal (voice), answer D.

G8A06
Which of the following is characteristic of QPSK31?
A. It is sideband sensitive
B. Its encoding provides error correction
C. Its bandwidth is approximately the same as BPSK31
D. All these choices are correct

These are all correct, answer D.

G8A07
Which of the following phone emissions uses the narrowest bandwidth?
A. Single sideband
B. Vestigial sideband
C. Phase modulation
D. Frequency modulation

SSB - single sideband is the narrowest analogue phone mode, answer A.

G8A08
Which of the following is an effect of overmodulation?
A. Insufficient audio
B. Insufficient bandwidth
C. Frequency drift
D. Excessive bandwidth

Overmodulation is all sorts of bad, and results in excessive modulation, answer D.

G8A09
What type of modulation is used by FT8?
A. 8-tone frequency shift keying
B. Vestigial sideband
C. Amplitude compressed AM
D. Direct sequence spread spectrum

The 8 in the name indicates 8-tone frequency shift keying, answer A.

G8A10
What is meant by the term "flat-topping," when referring to an amplitude-modulated phone signal?
A. Signal distortion caused by insufficient collector current
B. The transmitter's automatic level control (ALC) is properly adjusted
C. Signal distortion caused by excessive drive or speech levels
D. The transmitter's carrier is properly suppressed

In both AM or SSB transmissions, and audio amplifiers, if excessive level or "drive" is applied, the waveform closely approaches the supply rail voltage, and this results in a flat-top on the signal, visible on an oscilloscope, answer C.

This results in a distorted signal. Electric guitar players may be aware that some effects pedals involve deliberately over-driving small signal amplifiers to generate grungy sounds.

G8A11
What is the modulation envelope of an AM signal?
A. The waveform created by connecting the peak values of the modulated signal
B. The carrier frequency that contains the signal
C. Spurious signals that envelop nearby frequencies
D. The bandwidth of the modulated signal

The envelope of an AM signal is is a curve linking the peaks of the modulated signal. If we observe this on an oscilloscope, we see the audio waveform on the top of the screen, and this mirrored below, with a filling consisting of the RF in between, answer A.

G8A12
What is QPSK modulation?
A. Modulation using quasi-parallel to serial conversion to reduce bandwidth
B. Modulation using quadra-pole sideband keying to generate spread spectrum signals
C. Modulation using Fast Fourier Transforms to generate frequencies at the first, second, third, and fourth harmonics of the carrier frequency to improve noise immunity
D. Modulation in which digital data is transmitted using 0-, 90-, 180- and 270-degrees phase shift to represent pairs of bits

Quadrature Phase Shift Keying used signals phase shifted at any one time by one of the four angles in answer D, quad refering to four.

Have a stretch, a new section starts here.

G8B01
Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency (IF)?
A. Image frequency
B. Local oscillator
C. RF input
D. Beat frequency oscillator

Adjusting the local oscillator (LO) puts different frequencies into the IF passband, answer B.

G8B02
What is the term for interference from a signal at twice the IF frequency from the desired signal?
A. Quadrature response
B. Image response
?< C. Mixer interference
D. Intermediate interference

This is termed image response, answer B.

Sometimes it is handy, as the US requires scanners to block frequencies once used for analogue (AMPS) mobile phones, despite this being discontinued forever ago, but these frequencies can be received using the image.

G8B03
What is another term for the mixing of two RF signals?
A. Heterodyning
B. Synthesizing
C. Frequency inversion
D. Phase inversion

The mixing of signals is also called hetrodyning, answer A.

G8B04
What is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency?
A. Mixer
B. Reactance modulator
C. Balanced converter
D. Multiplier

This is a multiplier, answer D. Often there are several doubler stages and/or tripler stages.

G8B05
Which intermodulation products are closest to the original signal frequencies?
A. Second harmonics
B. Even-order
C. Odd-order
D. Intercept point

These are odd-order products are very close to the original frequencies, if not between them, answer C.

The second harmonics are twice the fundamental, so clearly wrong. The last answer is not the name of a product.

G8B06
What is the total bandwidth of an FM phone transmission having 5 kHz deviation and 3 kHz modulating frequency?
A. 3 kHz
B. 5 kHz
C. 8 kHz
D. 16 kHz

This adds to 8 kHz, but modulation is both down and up, so totals to 16 kHz, answer D.

The code for it is 16K0F3E for 16.0 kHz bandwidth, F for FM, 3 for a single analogue channel, and E for telephony (voice).

G8B07
What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 MHz FM phone transmitter?
A. 101.75 Hz
B. 416.7 Hz
C. 5 kHz
D. 60 kHz

The multiplication factor is 12, so the initial deviation must be 5000 / 12 = 416.6667 Hz, or 416.7 Hz answer B.

G8B08
Why is it important to know the duty cycle of the mode you are using when transmitting?
A. To aid in tuning your transmitter
B. Some modes have high duty cycles which could exceed the transmitter's average power rating.
C. To allow time for the other station to break in during a transmission
D. To prevent overmodulation

Modes which use AFSK (Audio Frequency Shift Keying) or PSK (Phase Shift Keying), such as RTTY often have a high duty cycle, and these can exceed the transmitter's average power rating, answer B.

Modes like FM on 10 metres also has a 100% duty cycle. This might be mitigated by reducing transmitter power for these modes. In some cases, after-market fans are available for rigs with passive heatsinking. (Duty cycle also affects electro-magnetic radiation exposure calculations.)

Something like SSB has a significantly lower duty cycle, which varies depending on the degree of processing.

G8B09
Why is it good to match receiver bandwidth to the bandwidth of the operating mode?
A. It is required by FCC rules
B. It minimizes power consumption in the receiver
C. It improves impedance matching of the antenna
D. It results in the best signal-to-noise ratio

Matching the receiver's bandwidth to the mode gives the best signal to noise ratio, answer D.

G8B10
What is the relationship between transmitted symbol rate and bandwidth?
A. Symbol rate and bandwidth are not related
B. Higher symbol rates require wider bandwidth
C. Lower symbol rates require wider bandwidth
D. Bandwidth is always half the symbol rate

The more data elements, or symbols sent, the wider the bandwidth needed, answer B.

Note that under good conditions especially, some modes supports more than one bit for each symbol, the bit rate thus exceeds the baud rate.

G8B11
What combination of a mixer's Local Oscillator (LO) and RF input frequencies is found in the output?
A. The ratio
B. The average
C. The sum and difference
D. The arithmetic product

This is the sum and the difference, answer C.

To make the maths easy, say we are listening to 28.455 MHz, our Local Oscillator will be at 28 MHz exactly, assuming a 455 kHz IF. However, were there an unlawful CB-style operation on 27.545 MHz nearby, using high power, you may hear this too, as an "image".

G8B12
What process combines two signals in a non-linear circuit to produce unwanted spurious outputs?
A. Intermodulation
B. Heterodyning
C. Detection
D. Rolloff

Intermodulation can cause interference, answer A.

G8B13
Which of the following is an odd-order intermodulation product of frequencies F1 and F2?
A. 5F1-3F2
B. 3F1-F2
C. 2F1-F2
D. All these choices are correct

This is 2F1-F2, answer C.

This is an "odd-order" intermod which places the product close to the original frequencies, or even between them. If you need to check, you need to avoid the subtractions giving an even number of Fs.

The question re image response above replaced one where a 13.800 MHz VFO (LO) is used with the intent to convert a 14.255 MHz signal to a 455 kHz intermediate frequency (IF) signal, but a 13.345 MHz signal is causing an image response. You will not a difference of 2 × 455 kHz, or 910 kHz between the (hoped for) amateur signal, and the undesired one being received. Note that this is not the result of the non-amateur user doing anything wrong. If this were a problem where you lived a filter termed a "pre-selector" may be needed, or you could try an antenna with the station end-on. Brisbane and Sunshine Coast hams might note that Telstra operates a "Brisbane Radio" transmitter on 13.342 MHz at Ningi on the mainland near near Bribie Island, to serve maritime users, using a kilowatt of SSB. Air Services Australia operates in nearby spectrum in the north of the country, ditto Defence nation-wide.

On to: Modes 2 - More on Digital

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, January 2025.

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