RSN Study Guide — Learning Content

Companion study material for the Nigeria Amateur Radio Licence Guide and Syllabus.

This guide explains the technical and regulatory topics a candidate needs for the Novice, General and Advanced classes. It is organised to match the syllabus, so you can read the section for your target class and use the self-test questions at the end of each part.

How to use this guide

1. Read the Novice part first, even if you are aiming higher — General and Advanced build directly on it.
2. Work through the worked examples with a calculator. Most exam questions are arithmetic plus a definition.
3. Learn the formula reference at the back by heart.
4. Answer the self-test questions without looking, then check.
5. Use the glossary for any term you cannot define in one sentence.

Part 1 — Novice

1. Rules and licensing

1.1 What the Amateur Service is for

The amateur service exists for self-training, intercommunication and technical investigation. It is not a broadcasting service and not a commercial service. Amateur operators:

• experiment with radio technique
• train themselves in electronics and radio practice
• provide communication in emergencies when normal systems fail
• build international friendship

1.2 Who regulates it

1.3 Licence basics

• A licence is issued to a person (or a club), not to a piece of equipment.
• The licence states your callsign, permitted bands, modes, power and any special conditions.
• Licences are renewable and can be amended, suspended or revoked.
• You are responsible for everything transmitted from your station, including when someone else operates it under your supervision.
• Station categories include fixed, mobile, portable and temporary stations.

1.4 Callsigns

A callsign identifies the station and the country. Nigerian amateur callsigns begin with the national prefix 5N. The full callsign is made up of:

• a prefix identifying the country
• a numeral and letter(s) identifying the individual or club station
• a suffix that may indicate the licence class or region

Rules for using a callsign:

• Identify at the start and end of each contact, and at intervals during a long transmission.
• Never use a callsign that is not yours.
• When operating away from your licensed address, add the appropriate portable indicator.
• Club stations use the club callsign, not the operator's personal callsign.

1.5 Prohibited transmissions

You must not transmit:

• business or commercial traffic, or messages for payment
• advertising or promotional material
• music, entertainment or broadcasting programmes
• obscene, offensive or threatening material
• false or misleading distress signals
• encrypted or obscured messages, except where a specific rule permits limited control or security signalling
• communications on behalf of a third party where national rules forbid it
• signals that cause harmful interference to other services

1.6 Interference

Harmful interference is interference that seriously degrades, obstructs or repeatedly interrupts a radio service.

If your station causes interference:

1. Stop transmitting immediately.
2. Note the time, frequency, mode and power.
3. Check your equipment — antenna, feedline, filters, earthing.
4. Reduce power or change band if the problem continues.
5. Cooperate with the affected party and with the Ministry.

Never continue transmitting through a known interference problem.

2. Operating practice

2.1 The phonetic alphabet

Use the ITU spelling alphabet so callsigns survive noise and poor conditions.

Note the official spellings: Alfa (not Alpha) and Juliett (not Juliet).

2.2 Making a contact

A basic voice contact follows this shape:

1. Listen on the frequency first. Never transmit on a frequency already in use.
2. Call: "CQ CQ CQ, this is <your callsign>, <your callsign>, calling CQ and standing by."
3. Answer: "<their callsign>, this is <your callsign>, over."
4. Exchange: signal report, name, location, and any other information.
5. Close: "<their callsign>, this is <your callsign>, 73 and clear."

Use "over" to hand the transmission to the other station, "roger" to acknowledge, and "73" as a friendly sign-off.

2.3 Signal reports — the RST system

RST stands for Readability, Strength, Tone.

For voice, give two digits (for example 59). For CW, give three (for example 599). A report of 59 means perfectly readable and very strong.

2.4 Q-codes

Q-codes are three-letter shorthand. A few essential ones:

2.5 UTC and logging

• Amateur radio uses UTC (Coordinated Universal Time) so logs from different countries agree.
• A logbook records date, time in UTC, callsign, frequency or band, mode, and signal report.
• Keep your log accurate and available — it is evidence of compliant operation.

2.6 Simplex and repeater operation

Simplex means both stations transmit and receive on the same frequency. It is used for local contacts and is the simplest form of operation.

A repeater receives on one frequency and simultaneously retransmits on another, extending coverage.

Repeater etiquette:

• Listen before transmitting.
• Pause between transmissions so others can join or break in.
• Keep transmissions short.
• Identify with your callsign.
• Leave a gap before the repeater drops so it can reset.
• Do not use a repeater for long rag-chews when a simplex frequency is free.

2.7 Nets and calling frequencies

A net is an organised on-air meeting with a net control station. Check in when invited, give your callsign and location, and follow the net control's direction.

Calling frequencies are agreed frequencies where you may call CQ. Before calling, always ask "is this frequency in use?" and wait.

3. Electricity and electronics

3.1 Charge, current, voltage and resistance

Conventional current flows from positive to negative. Electron flow is the opposite direction — use conventional current for circuit calculations.

3.2 Ohm's law

Worked example. A 12 V supply drives 0.5 A through a resistor. What is the resistance?

3.3 Power

Worked example. A transceiver draws 20 A at 13.8 V. What power does it consume?

3.4 Series and parallel circuits

Resistors in series add:

Resistors in parallel:

For two resistors in parallel:

Worked example. Two 100 Ω resistors in parallel:

In a series circuit the current is the same everywhere and voltages add. In a parallel circuit the voltage is the same across each branch and currents add.

3.5 AC and DC

• DC (direct current) flows in one direction — batteries, power supplies.
• AC (alternating current) reverses direction periodically — mains, RF signals.
• Mains in Nigeria is nominally 230 V AC at 50 Hz.
• AC is described by its frequency and its amplitude. The peak value is √2 times the RMS value for a sine wave.

3.6 Conductors, insulators and semiconductors

3.7 Batteries, chargers and power supplies

• Lead-acid batteries are common for station backup. They are heavy but cheap per amp-hour.
• Lithium batteries are lighter and hold more energy but need careful charging and protection.
• Never short a battery, reverse its polarity, or charge it at the wrong voltage.
• A power supply converts mains AC to a stable DC output, usually 13.8 V for amateur equipment.
• Use fuses and circuit breakers sized just above the normal current draw.

3.8 Frequency, wavelength and period

• Frequency (f) is the number of cycles per second, measured in hertz (Hz).
• Wavelength (λ) is the distance the wave travels in one cycle, measured in metres.
• Period (T) is the time for one cycle: \(T = 1/f\).

where \(c\) is the speed of light, approximately \(3 \times 10^8\) m/s.

For amateur use, the convenient form is:

Worked example. What is the wavelength at 145 MHz?

This is why 145 MHz is called the 2 metre band.

4. Radio theory

4.1 Radio waves

A radio wave is an electromagnetic wave — a travelling combination of an electric field and a magnetic field at right angles to each other and to the direction of travel. Radio waves:

• travel at the speed of light in free space
• need no medium — they pass through a vacuum
• carry energy and can be modulated to carry information

4.2 The radio spectrum

Each band is named after its approximate wavelength, which is why HF is also called short wave.

4.3 Polarisation

Polarisation describes the orientation of the electric field.

• Vertical polarisation — electric field vertical. Typical of mobile and repeater antennas.
• Horizontal polarisation — electric field horizontal. Typical of HF dipoles and Yagis.

Matching polarisation between stations matters most on VHF and UHF. On HF, the ionosphere rotates polarisation so it matters much less.

4.4 Modulation

Modulation is the process of impressing information onto a carrier wave.

Bandwidth is the range of frequencies a signal occupies. Narrow modes fit more stations into a band.

4.5 Transmitters and receivers

A transmitter generates a carrier, modulates it, amplifies it and feeds it to the antenna.

A receiver picks up a weak signal, selects the wanted frequency, amplifies it and demodulates it back to audio.

A transceiver combines both in one unit.

Key controls and stages:

• Gain / RF gain — controls receiver amplification.
• Squelch — mutes the receiver when no signal is present.
• AGC (automatic gain control) — keeps audio level steady as signal strength varies.
• Noise blanker — reduces impulsive noise such as ignition interference.
• Microphone gain — sets audio drive to the transmitter.

4.6 RF power versus audio power

RF output power and audio power are different things. A transmitter rated at 100 W RF output produces only a few watts of audio in the receiver's speaker. Do not confuse the two when reading specifications.

4.7 Propagation basics

• Line of sight — VHF and UHF signals travel roughly in straight lines. The radio horizon is slightly beyond the visible horizon because of atmospheric refraction.
• Terrain and buildings — hills, buildings and trees absorb and block VHF/UHF signals.
• Antenna height — raising an antenna dramatically improves VHF/UHF range.
• HF skywave — HF signals refract off the ionosphere and can travel thousands of kilometres.
• Ground wave — follows the earth's surface, useful on LF and MF.

5. Antennas and feedlines

5.1 What an antenna does

An antenna converts electrical energy in a feedline into electromagnetic waves (transmitting) and the reverse (receiving). It is a reciprocal device: its transmitting and receiving characteristics are the same.

5.2 Common antenna types

5.3 Antenna length calculations

Half-wave dipole:

Quarter-wave vertical:

The constants are slightly less than the theoretical 150 and 75 because of the end effect and the velocity factor of the conductor.

Worked example. A half-wave dipole for 7.1 MHz:

Each leg is half of that: about 10.1 m.

5.4 Feedlines

Coaxial cable is the most common feedline. It has a centre conductor, a dielectric, a braided shield and an outer jacket.

Common connectors: PL-259/SO-239 (HF/VHF), BNC (test equipment and VHF), N-type (VHF/UHF, weatherproof).

Balanced line (twin-lead, ladder line) has lower loss than coax but must be kept away from metal and needs a balun at the transceiver.

5.5 SWR

Standing Wave Ratio (SWR) measures how well the antenna impedance matches the feedline.

A high SWR means power is reflected back to the transmitter, which can cause overheating and reduced radiated power. Causes include:

• antenna cut to the wrong length
• damaged or waterlogged coax
• loose or corroded connectors
• antenna too close to metal objects or the ground
• wrong feedline impedance

Reflected power is the portion of forward power returned by a mismatch. At 2:1 SWR roughly 11% of power is reflected; at 3:1 roughly 25%.

5.6 Antenna matching and baluns

• An antenna tuner (ATU) transforms the antenna impedance to match the transmitter. It does not fix a bad antenna — it only makes the transmitter happy.
• A balun connects a balanced load (dipole, ladder line) to an unbalanced feedline (coax).
• An unun connects unbalanced to unbalanced, often for end-fed wires.
• A choke suppresses common-mode current on the outside of the coax shield.

5.7 Ground systems

• Radials form an artificial ground plane for vertical antennas. More radials improve efficiency.
• Station earth provides a safety and RF reference. Bond equipment chassis together and to a single earth point.
• Lightning protection must be separate from, and better than, your station earth. Disconnect antennas when not in use.

6. Safety and interference

6.1 Electrical safety

• Treat all mains wiring as live until proven otherwise.
• Use RCD/GFCI protection on mains circuits.
• Fit fuses in the DC feed close to the battery or supply.
• Never work on equipment with the mains connected.
• Keep one hand in your pocket when probing live circuits — this reduces the risk of a current path across your chest.
• Discharge large capacitors before working on power supplies and amplifiers.

6.2 RF exposure

RF energy heats body tissue. The risk rises with frequency, power and proximity.

• Keep antennas away from occupied areas.
• Do not stand in front of a directional antenna when transmitting.
• Never touch an antenna or feedline while transmitting — RF burns are painful and slow to heal.
• Reduce power, increase distance, or reduce duty cycle to lower exposure.
• Mobile antennas must be mounted so that no person sits within the safe distance.

6.3 Antenna and mast safety

• Keep antennas clear of power lines. Assume any contact is fatal.
• Use proper ladders, harnesses and hard hats for mast work.
• Never climb alone.
• Check guy wires, anchors and mast fixings regularly.
• Lower antennas before storms.
• Ensure rooftop installations cannot be reached by the public.

6.4 Battery and power safety

• Vent lead-acid batteries — they release explosive hydrogen when charging.
• Do not short battery terminals; remove metal jewellery when working.
• Use the correct charger for the battery chemistry.
• Protect lithium packs with a BMS and never charge unattended.
• Keep a suitable fire extinguisher nearby.

6.5 Interference prevention

Interference has two directions:

You causing interference to others:

• Use a low-pass filter on the transmitter to suppress harmonics.
• Keep the antenna away from TV aerials and phone lines.
• Use good quality coax and well-made connectors.
• Ensure the station is properly earthed.
• Reduce power if problems persist.

Others interfering with you:

• Fit ferrite chokes on affected cables.
• Use a mains filter on the affected equipment.
• Improve your antenna's balance and earthing.
• Identify the source before accusing anyone.

7. Digital awareness

Rules for digital systems:

• Always identify with your callsign, even on a hotspot.
• Do not use a network to bypass licence conditions.
• Keep your hotspot and gateway secure — do not leave them open to abuse.
• Remember that a linked system may be heard worldwide.

Novice self-test

Work through these without looking. For multiple-choice practice with answers and explanations, see the Novice question pool.

1. What does RST stand for, and what does a report of 57 mean?
2. Calculate the wavelength for 50 MHz.
3. A 13.8 V supply feeds a 50 Ω load. What current flows?
4. What is the length of a half-wave dipole for 14.2 MHz?
5. What is the standard 2 m repeater offset?
6. Name three types of transmission that are prohibited.
7. What is the difference between CTCSS and DCS?
8. Why must you not touch an antenna while transmitting?
9. What does an SWR of 3:1 indicate?
10. What is the purpose of a balun?

Part 2 — General

8. Rules and operating procedures

8.1 Band plans

A band plan is a voluntary agreement that divides a band by mode and bandwidth so that different activities do not clash. Follow the IARU Region 1 band plan and any national variations.

Typical HF band plan structure:

• CW at the bottom of each band
• Narrow digital modes just above CW
• SSB voice in the middle and upper part
• Beacons at the very top of some bands
• Satellite segments in defined sub-bands

8.2 Allocations

• A primary allocation means the amateur service has equal rights with other primary services and is protected from interference.
• A secondary allocation means amateurs must not cause interference to primary users and must accept interference from them.
• On a secondary allocation, if a primary user appears, you must move or stop.

8.3 Operating procedures

8.4 Emergency communication

• Learn the emergency centres of activity in your band plan.
• These frequencies are not reserved, but during a declared emergency they must not be used for anything else.
• Use plain language, not codes, for emergency messages.
• A structured emergency message contains: precedence, callsign, message number, date/time, address, text, signature.
• Never impede an emergency net. If you are not trained, stay off the frequency.

9. HF propagation

9.1 The ionosphere

Solar radiation ionises the upper atmosphere, creating layers that refract HF signals back to earth.

9.2 Propagation terms

9.3 Solar activity

• The solar cycle lasts about 11 years, alternating high and low sunspot activity.
• The sunspot number and solar flux index (SFI) indicate ionisation levels. Higher values mean higher MUF and better HF conditions.
• Geomagnetic activity (measured by the K or A index) disturbs the ionosphere and degrades HF, especially at high latitudes.
• Solar flares can cause sudden ionospheric disturbances that black out HF for minutes to hours.

9.4 VHF and UHF propagation

Beyond line of sight, VHF/UHF signals travel by:

• Tropospheric ducting — temperature inversions trap signals and carry them hundreds of kilometres
• Sporadic E — intense E-layer ionisation reflects 6 m and sometimes 2 m signals
• Meteor scatter — brief reflections from meteor trails, used for short data bursts
• Auroral scatter — reflections from auroral ionisation; signals sound distorted
• Earth-Moon-Earth (EME) — signals bounce off the moon; very high path loss

10. Electronics and circuits

10.1 Capacitors and inductors

Reactance is frequency dependent:

• A capacitor's reactance falls as frequency rises — it passes AC and blocks DC.
• An inductor's reactance rises with frequency — it passes DC and blocks AC.

10.2 Impedance

Impedance (Z) combines resistance and reactance:

Impedance is a complex quantity with magnitude and phase. At resonance, \(X_L = X_C\) and the reactances cancel, leaving pure resistance.

10.3 Resonance

Worked example. A 100 µH inductor with a 100 pF capacitor:

• A series resonant circuit has minimum impedance at resonance.
• A parallel resonant circuit has maximum impedance at resonance.

10.4 Q factor

Q describes how selective a resonant circuit is:

High Q means a narrow bandwidth and sharp tuning. Low Q means a wide bandwidth and broader response.

10.5 Filters

10.6 Semiconductors

10.7 Power supplies

• A transformer steps mains voltage up or down.
• A rectifier converts AC to pulsating DC.
• A smoothing capacitor reduces ripple.
• A regulator holds the output voltage steady.
• Linear supplies are quiet but inefficient. Switching supplies are efficient but can generate RF noise.

10.8 Decibels

The decibel is a logarithmic ratio, used because signal levels vary over enormous ranges.

Absolute power is expressed in dBW (relative to 1 W) or dBm (relative to 1 mW).

Worked example. 100 W in dBm:

11. Transmitters and receivers

11.1 Transmitter stages

A typical transmitter chain:

1. Oscillator — generates the carrier at a stable frequency
2. Buffer — isolates the oscillator from later stages
3. Modulator — impresses information onto the signal
4. Driver — raises the signal to a level the final stage needs
5. Power amplifier — produces the output power
6. Low-pass filter — removes harmonics
7. Antenna matching — presents the correct load

11.2 Transmitter specifications

Duty cycle matters for both heating and RF exposure. CW and SSB have low duty cycles; FM and digital modes such as FT8 have high duty cycles.

11.3 Receiver architecture

The superheterodyne receiver is the standard design:

1. RF amplifier — amplifies the incoming signal
2. Mixer — combines the signal with the local oscillator
3. IF amplifier — amplifies at a fixed intermediate frequency, where filtering is easy
4. Demodulator — recovers the audio
5. Audio amplifier — drives the speaker

A double conversion receiver uses two IF stages to improve image rejection and selectivity.

11.4 SDR basics

A software-defined radio digitises the signal early and does filtering, demodulation and detection in software.

• ADC converts analogue to digital.
• DAC converts digital to analogue.
• DSP processes the samples.
• IQ signals carry both amplitude and phase information.

SDR advantages: flexible filtering, waterfall displays, multiple demodulators, easy recording. Disadvantages: needs a computer, can be affected by ADC overload and clock noise.

12. Antennas and transmission lines

12.1 Transmission line behaviour

Common-mode current causes feedline radiation, TVI and unpredictable SWR. Cure it with a choke or balun at the feedpoint.

12.2 Impedance matching

• A quarter-wave transformer matches impedances using a line of the correct impedance and length.
• An L network uses one inductor and one capacitor.
• A Pi network uses two capacitors and one inductor; common in amplifiers.
• A T network uses two inductors and one capacitor.
• A stub is a length of line used to cancel reactance.

12.3 Antenna characteristics

dBi is gain relative to an isotropic radiator. dBd is gain relative to a half-wave dipole. dBd = dBi − 2.15.

12.4 Antenna types

12.5 Ground systems

• Radials improve vertical antenna efficiency. Buried radials work well; elevated radials work better with fewer wires.
• A counterpoise provides the missing half of an end-fed antenna.
• Bonding all equipment to a single earth point prevents RF circulating in the shack.

13. Digital modes and satellite operation

13.1 HF digital modes

Time synchronisation is essential for FT8 and FT4. Use an internet time source or a GPS receiver. Being more than about one second off will stop you decoding.

13.2 Station interfaces

• A sound card interface connects the radio's audio in/out to the computer.
• CAT control lets the computer set frequency, mode and PTT over a serial or USB link.
• Set the audio level so the transmitter is not overdriven — ALC should barely move.
• Use VOX or a hardware PTT line for transmit switching.

13.3 Satellites

• A satellite has an uplink (you transmit) and a downlink (you receive).
• Doppler shift changes the apparent frequency as the satellite approaches and recedes. On 2 m the shift can be ±3 kHz; on 70 cm it can be ±10 kHz.
• Correct Doppler by tuning the downlink as the pass progresses.
• Full-duplex operation (hearing your own downlink) makes satellite work much easier.
• Respect the satellite's power and bandwidth limits — satellites are shared resources.

14. Safety and EMC

14.1 RF exposure assessment

Exposure depends on:

• frequency — the body absorbs most between 30 and 300 MHz
• power — average power, not peak
• distance — exposure falls with the square of distance
• duty cycle — average power over time
• antenna gain and pattern — a directional antenna concentrates energy

Reduce exposure by lowering power, increasing distance, raising the antenna, or reducing duty cycle.

14.2 EMC and interference

Electromagnetic compatibility means equipment works without interfering with, or being interfered with by, other equipment.

Sources of interference:

• transmitter harmonics and spurious emissions
• common-mode current on feedlines
• poor earthing and bonding
• cheap switch-mode power supplies
• plasma TVs, LED lighting and solar inverters

Cures:

• low-pass filter on the transmitter
• ferrite chokes on affected cables
• good station earthing
• balanced antennas and proper baluns
• physical separation between antennas and victim equipment

14.3 Station safety

• Earth and bond all equipment.
• Fit lightning arrestors and disconnect antennas when not in use.
• Keep a fire extinguisher suitable for electrical fires.
• Label fuses and breakers clearly.
• Never bypass a safety device.

General self-test

Work through these without looking. For multiple-choice practice with answers and explanations, see the General question pool.

1. What is the difference between MUF and LUF?
2. Which ionospheric layer causes daytime HF absorption?
3. Calculate the resonant frequency of a 25 µH inductor and a 100 pF capacitor.
4. Convert 400 W to dBm.
5. What is the velocity factor, and why does it matter?
6. Explain common-mode current and how to cure it.
7. What is the image frequency in a superheterodyne receiver?
8. Why does FT8 need accurate time?
9. What is Doppler shift and how do you correct for it?
10. Name four causes of a high SWR.

Part 3 — Advanced

15. Advanced RF theory

15.1 Complex impedance and the Smith chart

Impedance is written as \(Z = R + jX\), where R is resistance and X is reactance. A positive X is inductive; a negative X is capacitive.

The Smith chart is a graphical tool for solving transmission line and matching problems. It plots the reflection coefficient \(\Gamma\):

Key points on the chart:

• Centre — perfect match, \(\Gamma = 0\), SWR 1:1
• Left edge — short circuit, \(\Gamma = -1\)
• Right edge — open circuit, \(\Gamma = +1\)
• Outer circle — total reflection, \(\Gamma = 1\)

Moving along a transmission line rotates you around the chart. Adding series reactance moves you along a constant-resistance circle; adding shunt reactance moves you along a constant-conductance circle.

15.2 Return loss and reflection

15.3 Matching networks

Q of a matching network sets its bandwidth. Higher Q means a narrower match.

15.4 Noise and receiver performance

Cascaded noise figure (Friis): the first stage dominates. A low-noise preamplifier at the antenna improves the whole system; a lossy feedline before the preamplifier degrades it.

15.5 Oscillators and frequency synthesis

• Crystal oscillators are very stable but fixed in frequency.
• Phase-locked loops (PLL) multiply a reference to produce many frequencies.
• Direct digital synthesis (DDS) generates waveforms numerically; fast switching but can produce spurs.
• Frequency stability is expressed in parts per million (ppm). 1 ppm at 14 MHz is 14 Hz.

16. Advanced electronics

16.1 Amplifier classes

Linear amplifiers (Class A, AB, B) preserve the modulation envelope and are needed for SSB and AM. Non-linear amplifiers (Class C) are efficient but only usable for constant-envelope modes such as FM and CW.

16.2 Amplifier design issues

• Biasing sets the quiescent current and operating point.
• Negative feedback improves linearity and stability but reduces gain.
• Neutralisation cancels internal feedback in RF stages.
• Parasitic oscillation is unwanted self-oscillation; cure with ferrite beads, stopper resistors and layout care.
• Thermal design — heatsink sizing, airflow and derating matter more than peak ratings.
• Protection — SWR protection, over-current shutdown, over-temperature cutout.

16.3 Power supplies

For a station, a linear supply is quieter; a switching supply needs good filtering and screening.

16.4 EMC-conscious design

• Keep loop areas small.
• Use ground planes and short return paths.
• Filter every cable entering or leaving an enclosure.
• Separate noisy digital and sensitive analogue sections.
• Screen and bypass where needed.
• Design for the worst-case load and fault condition.

17. DSP, SDR and data systems

17.1 Sampling

The Nyquist theorem states that to reconstruct a signal you must sample at more than twice its highest frequency:

Sampling below this causes aliasing — high frequencies appear as false low frequencies. An anti-alias filter before the ADC prevents this.

17.2 Converter performance

Dynamic range in dB is roughly \(6.02 \times \text{bits}\).

17.3 Digital signal processing

• Digital filters — FIR and IIR; sharp, stable and repeatable.
• FFT — converts time-domain samples to a frequency spectrum; the basis of the waterfall display.
• IQ processing — separates amplitude and phase; enables image rejection and complex modulation.
• Decimation and interpolation — change the sample rate to match the required bandwidth.

17.4 Digital modulation

Bit rate is bits per second. Symbol rate is symbols per second (baud). With multi-level modulation, bit rate exceeds symbol rate. Bandwidth is set by symbol rate and filtering.

17.5 Error correction

• Forward error correction (FEC) adds redundancy so the receiver can correct errors without retransmission.
• Interleaving spreads burst errors so they can be corrected.
• Checksums and CRCs detect errors but do not correct them.

17.6 Remote and automated stations

A remote station must have:

• secure control — authentication, encryption where permitted
• positive identification — the station identifies correctly on air
• logging — a record of all transmissions
• fail-safe shutdown — the station stops transmitting if control is lost
• protection — against unauthorised access and interference

18. Advanced antennas and propagation

18.1 Arrays

• Phased arrays combine elements with controlled phase to steer the pattern electronically.
• Stacked Yagis increase gain and lower the take-off angle.
• Broadside arrays radiate perpendicular to the line of elements.
• End-fire arrays radiate along the line of elements.

18.2 Antenna modelling

Modelling software predicts pattern, gain and impedance. Inputs include element lengths, spacing, height above ground, ground characteristics and conductor diameter. Models are only as good as their inputs.

18.3 Propagation modes

18.4 Microwave

Above about 1 GHz, propagation is dominated by line of sight, and losses from rain, foliage and atmospheric absorption become significant. Antennas become physically small but require precise alignment. Feedline loss is severe, so the amplifier is often mounted at the antenna.

19. RF measurement and test equipment

Safety with instruments:

• Always attenuate before connecting a transmitter to a spectrum analyser.
• Never exceed the instrument's maximum input level.
• Use a dummy load for transmitter tests.
• Earth instruments properly.

20. High-power and site safety

20.1 RF exposure calculations

Power density from a directional antenna in the far field:

where S is power density in W/m², P is average power in W, G is numeric gain, and d is distance in metres.

Worked example. 400 W average into a 10 dBi antenna (G = 10) at 10 m:

Compare this with the applicable exposure limit for the frequency and adjust power or distance as needed.

20.2 High-power precautions

• Amplifier cooling must be adequate for continuous duty.
• Use SWR protection and over-temperature cutout.
• Mains distribution must handle the surge current.
• Bond all equipment and use a single earth point.
• Keep high-voltage areas enclosed and labelled.
• Never work on an amplifier with power connected — bleed the capacitors first.

20.3 Tower and high-site safety

• Use a proper harness, anchored above the working position.
• Never climb alone or in bad weather.
• Check for power lines before raising any mast.
• Use a gin pole or crane for heavy sections.
• Secure all tools and parts against falling.
• Have a rescue plan before you climb.

20.4 Station shutdown

A safe shutdown procedure:

1. Stop transmitting and disconnect the antenna.
2. Switch off the amplifier and let it cool.
3. Switch off the transceiver.
4. Switch off the power supply.
5. Disconnect the mains.
6. Disconnect antennas and fit lightning protection.

Advanced self-test

Work through these without looking. For multiple-choice practice with answers and explanations, see the Advanced question pool.

1. Calculate the reflection coefficient for a 100 Ω load on a 50 Ω line.
2. What return loss corresponds to an SWR of 2:1?
3. Explain why the first stage dominates the noise figure of a receiver chain.
4. What is the Nyquist limit, and what happens if you exceed it?
5. Compare Class A, AB and C amplifiers for efficiency and linearity.
6. What is the purpose of an anti-alias filter?
7. Calculate the power density 20 m from a 100 W transmitter into a 6 dBi antenna.
8. What is third-order intercept and why does it matter?
9. List the five requirements of a remote station.
10. Describe a safe station shutdown procedure.

Formula reference

Glossary

Recommended examination model

Important licence condition

This is a training and examination proposal. No operator should assume a privilege simply because it appears in this guide.

A Nigerian amateur may transmit only on the bands, frequencies, modes, bandwidths, station types and maximum power levels written on their current licence or authorisation from the Federal Ministry of Communications, Innovation and Digital Economy.