RF & Communications — Complete Study Guide

A radio wave is electromagnetic energy — the same fundamental phenomenon as light, just at a much lower frequency. When alternating current flows in a conductor, it radiates an electromagnetic field that travels outward at the speed of light. Radio frequency (RF) is the slice of that spectrum useful for communication, roughly 3 kHz up to 300 GHz.

The spectrum is divided into named bands. You don't need to memorize every number, but you should recognize the order and what lives where:

The anatomy of a wave. Before going further it's worth naming the four properties every radio wave has, because the rest of RF is just manipulating them:

Modulation (Section 3) works by varying one of these — amplitude for AM, frequency for FM, phase for PM. So this little table is secretly the key to the whole guide.

How RF is made and radiated. Any time current changes, it produces a changing magnetic field, and a changing magnetic field produces a changing electric field — the two regenerate each other and the disturbance propagates outward as an electromagnetic wave. At low frequencies this radiation is negligible; as frequency climbs, a conductor of the right length becomes an efficient antenna, deliberately launching that energy into space. The electric and magnetic fields are perpendicular to each other and to the direction of travel — which is why "polarization" (Section 7) is defined by the electric field's orientation.

Why the band names matter in practice. The behavior shift across bands is gradual but real: at MF and below, waves follow the earth's curve (ground wave) and travel far at night. Through HF, the ionosphere becomes a mirror for long-distance "skip." From VHF up, waves punch through the ionosphere and travel essentially line-of-sight, trading range for abundant bandwidth — which is why high-data services (TV, Wi-Fi, cellular, GPS) live up high where there's room, while long-haul and low-data services sit down low.

Why it matters: The band sets the behavior. Knowing that HF bounces off the ionosphere (long range) while UHF is line-of-sight explains why a shortwave station reaches another continent but your Wi-Fi barely reaches the next room. Get comfortable converting between bands and wavelengths — it underlies antennas, propagation, and feedline choices.

Every wave has a frequency (how many cycles pass per second, in hertz) and a wavelength (the physical length of one cycle, in meters). They're locked together by the speed of light, so if you know one you can always find the other.

Velocity factor — waves slow down in cable. The speed of light applies in free space (and air, near enough). Inside a coax or wire, a wave travels slower — typically 0.66 to 0.95 of light speed — set by the cable's insulation. That ratio is the velocity factor (VF). It matters whenever a length of cable is part of a tuned circuit (a matching stub, a phasing line, a quarter-wave transformer):

Harmonics. A harmonic is an integer multiple of a frequency: the 2nd harmonic of 7 MHz is 14 MHz, the 3rd is 21 MHz. Harmonics matter two ways — antennas and circuits often work on harmonically related bands, and transmitters must suppress unwanted harmonics so they don't radiate interference. A wave's harmonics are always shorter in wavelength by the same integer factor.

Electrical vs physical length. An antenna's electrical length (what the wave "sees") is slightly longer than its physical length because of end effects — the wave fringes off the wire ends. That's why the practical half-wave dipole formula uses 468/f(MHz) in feet rather than the 492/f you'd get from pure free-space math (Section 7). The ~5% shortening is the end effect. Treat 468/f as an empirical starting point for thin bare wire at typical HF heights: the real shortening runs about 2–6% depending on conductor thickness, insulation and height, so cut a little long and trim.

Why it matters: Antennas are cut to a fraction of a wavelength (often a half or quarter). Get the wavelength wrong — or forget velocity factor in cable, or end effect on a wire — and every length that follows is off, detuning the system.

A bare radio wave at one frequency — the carrier — carries no information; it's just a steady tone. Modulation is how we ride information on top of it by varying one of its properties:

When you modulate a carrier, you don't get a single frequency anymore — you create sidebands, mirror-image copies of the carrier spaced above and below it by the modulating frequency. In AM, both sidebands carry the same information, and the carrier itself carries none.

That redundancy is wasteful, which is the whole insight behind SSB: throw away the carrier and one sideband, and you send the identical information in half the bandwidth using far less power. It's why long-distance voice on HF — and ham DX — is almost always SSB.

Modulation percentage (AM). How deeply you modulate matters. Modulation index for AM is the ratio of how far the amplitude swings to the carrier's level, usually given as a percentage:

At 100% the carrier amplitude swings from double down to zero. Push past 100% (overmodulation) and the envelope pinches off at zero for part of each cycle, generating spurious sidebands ("splatter") that interfere with adjacent channels and distort the audio. Undermodulation (say 30%) is clean but weak — you're not using the carrier efficiently. Broadcasters aim high but stay below 100%.

FM deviation and Carson's rule. FM doesn't have a modulation percentage; instead it has deviation — how far the carrier frequency swings from center. Wide deviation = louder/cleaner but more bandwidth. The occupied bandwidth of an FM signal is estimated by Carson's rule:

Why FM resists noise. Most natural and man-made noise shows up as amplitude spikes. Since FM carries its information in frequency, the receiver can clip off amplitude variations (with a limiter stage) and lose nothing — the noise is literally sliced away. AM has no such luxury, because amplitude is the signal.

Digital modulation, briefly. The same three knobs drive digital modes: ASK (amplitude-shift keying) varies amplitude, FSK (frequency-shift keying) shifts between tones, and PSK (phase-shift keying) flips phase. Combining amplitude and phase gives QAM, which packs many bits per symbol and underlies Wi-Fi, cable, and digital TV. They're not separate magic — just AM/FM/PM applied to discrete states instead of continuous audio.

Why it matters: Modulation type drives bandwidth, power efficiency, and noise behavior — and explains why broadcast FM sounds clean, why AM fades and hisses, why overmodulation gets operators in trouble, and why SSB is the workhorse of HF voice.

RF signals span an enormous range — from a transmitter's hundreds of watts down to a received signal of a millionth of a microwatt. Linear numbers become unwieldy, so RF lives in decibels: a logarithmic way to express ratios.

When the reference is fixed, dB becomes an absolute unit. The big one in RF is dBm — decibels relative to 1 milliwatt:

The real superpower: dB just add. Because decibels are logarithmic, the multiplications of a signal chain become simple additions. A gain is +dB, a loss is −dB, and you walk the path adding them up:

The reference-unit family. Append a letter to "dB" and you've fixed a reference, turning a ratio into an absolute level. The common ones:

Signal-to-noise and why faint signals still work. What lets a receiver pull in a vanishingly small signal isn't raw level — it's the signal-to-noise ratio (SNR), the dB difference between the wanted signal and the noise floor. A −120 dBm signal is perfectly usable if the noise floor is −130 dBm (10 dB SNR). This is why "sensitivity" is quoted as a level for a given SNR, and why low-noise front-end design (Section 6) matters more than brute gain.

Why it matters: Antenna gain, cable loss, amplifier gain, and receiver sensitivity are all quoted in dB. Adding and subtracting dB along a signal path (the "link budget") is everyday RF work — it tells you, before you build anything, whether the signal will make it.

A transmitter's job is to generate a clean carrier, put information on it, and boost it to enough power to reach the receiver. Every transmitter, simple or complex, is built from the same chain of stages:

The oscillator sets the frequency — often crystal-controlled for stability. The buffer isolates the oscillator so later stages don't pull it off frequency. The modulator adds the information, and the power amplifier (PA) raises the signal to its final output level.

RF power amplifiers often run in Class C — highly efficient but distorting — which is fine for a constant-amplitude signal like FM or CW because a tuned circuit reconstructs the clean waveform. SSB, digital modes and low-level-modulated AM, whose amplitude carries the information, need a linear chain (Class A, AB, or B push-pull) to avoid mangling the signal. The exception is high-level (plate or collector) modulated AM, the classic broadcast design: the final runs in Class C and the modulation is applied to it directly.

Setting and holding the frequency. An oscillator alone drifts with temperature and voltage. Three approaches keep a transmitter on frequency: a crystal oscillator (a quartz crystal's mechanical resonance is extremely stable — the piezoelectric effect), a VFO (variable-frequency oscillator, tunable but driftier), and the modern standard, a PLL frequency synthesizer, which locks a tunable oscillator to a crystal reference using a phase-locked loop — giving both crystal stability and the ability to step across a band.

Frequency multiplication. It's often easier to generate a stable signal at a lower frequency and then multiply it up. A multiplier stage deliberately distorts the signal to generate harmonics, then a tuned circuit selects the wanted multiple (×2, ×3, etc.). A 6 MHz crystal feeding a tripler yields a stable 18 MHz output. The trade-off: any frequency or phase change in the oscillator is multiplied too.

Keying and CW. The simplest "modulation" is just switching the carrier on and off — CW (continuous wave / Morse). It's still in use because it concentrates all power into a razor-thin bandwidth, punching through noise when voice can't. Keying must be shaped (soft on/off) to avoid "key clicks" — broadband splatter from switching too abruptly, the CW cousin of overmodulation.

Keeping the output clean. A transmitter must radiate only its intended signal. Two concerns: harmonics (multiples of the carrier) are suppressed by a low-pass filter at the output; and in tube/some high-power PAs, neutralization cancels internal feedback that would otherwise make the amplifier self-oscillate. A spurious or harmonic emission can interfere with services far from your own frequency — hence the legal requirement to filter them down.

Why it matters: Knowing the stage chain lets you troubleshoot a dead or drifting transmitter by signal-tracing stage to stage — "drifts when I key up" points at buffer/isolation, "interferes with the neighbor's TV" points at harmonic filtering, "no audio but carrier is there" points at the modulator.

A receiver has to pluck one faint signal out of the air from among thousands, amplify it enormously, and recover the information — all while rejecting everything nearby. The design that made this practical, and still dominates today, is the superheterodyne.

The key trick is heterodyning — mixing. The mixer combines the incoming signal with a tunable local oscillator (LO). When you mix two frequencies, you get their sum and difference. The receiver keeps the difference, which is always the same fixed intermediate frequency (IF) no matter what station you tune to.

Doing the amplification and filtering at a fixed IF is what gives a superhet its excellent selectivity (rejecting adjacent stations) and sensitivity (pulling in weak ones). After the IF stage, the detector recovers the audio and the audio amplifier drives the speaker.

Working the image frequency. The image sits two IFs away from your wanted signal (on the other side of the LO). With a 455 kHz IF and the LO above the signal, the image is 2 × 455 = 910 kHz away:

Sensitivity vs selectivity — the two receiver virtues. Sensitivity is the ability to hear weak signals (set by front-end gain and, crucially, low noise). Selectivity is the ability to separate one signal from its neighbors (set by the IF filter's bandwidth). They're different jobs: a sensitive-but-broad receiver hears everything at once; a selective-but-deaf one is quiet but clean. Good design needs both.

The detector depends on the mode. Recovering the information requires a detector matched to how the signal was modulated:

AGC — automatic gain control. Signals vary enormously as stations fade or you tune across strong and weak ones. AGC samples the output level and feeds it back to reduce gain on strong signals and raise it on weak ones, keeping the volume roughly constant and preventing strong-signal overload. It's why you can tune from a local blowtorch station to a distant whisper without lunging for the volume knob.

Why it matters: Nearly every radio you'll service — AM/FM, two-way, TV tuners — is a superhet. Understanding mix-to-IF, the image, and the per-mode detector tells you exactly where to look when a receiver is deaf, broad, or distorted.

An antenna converts the transmitter's guided RF energy into a radiated wave (and the reverse on receive). Its size is tied directly to wavelength — which is why Section 2 matters so much here.

Antennas have gain — they focus energy in some directions at the expense of others, like a flashlight reflector. Gain is measured in decibels relative to a reference:

Polarization is the orientation of the radiated electric field — vertical or horizontal. Transmit and receive antennas should match; a vertical and a horizontal antenna can lose much of the signal between them. The feedline (usually coax) carries energy between radio and antenna, and it has loss that rises with frequency — a cable that's fine at HF can waste half your power at UHF.

Radiation patterns — antennas aren't equal in all directions. An ideal isotropic antenna radiates equally everywhere (a sphere) — it's only a math reference. Real antennas have shape:

• A half-wave dipole radiates broadside to the wire (a doughnut pattern), with nulls off the ends. Its feedpoint impedance is about 73 Ω — conveniently close to 50 Ω coax.
• A quarter-wave vertical (against a ground plane) is omnidirectional in the horizontal plane — good for mobile/repeater use. Feedpoint ~36 Ω.
• A Yagi adds parasitic director and reflector elements to focus energy into a beam — high gain in one direction, like a flashlight.

Gain always comes from focusing, never from creating energy — a high-gain antenna is just robbing the directions you don't care about to strengthen the one you do.

Feedlines and their loss. The transmission line between radio and antenna matters more than beginners expect:

Loss rises with frequency and length — and it's doubly costly under high SWR (Section 8), because reflected power makes extra trips through the lossy line. This is why a marginal cable at HF becomes a power-wasting liability at VHF/UHF.

Baluns and matching. A balun (balanced-to-unbalanced) connects a balanced antenna like a dipole to unbalanced coax, preventing feedline radiation and current imbalance. An antenna tuner doesn't change the antenna — it transforms the impedance the transmitter sees so the PA is happy, even when the antenna isn't a perfect match.

Ground systems. A vertical antenna needs a ground plane or radials to work against — the missing "other half" of the antenna. Poor grounding wastes power as heat in the soil, and it can make the SWR look better, not worse: the ground-loss resistance adds to the vertical's ~36 Ω feedpoint and pulls it toward 50 Ω. A good SWR is not proof of a good ground system. (This is distinct from safety grounding for lightning and shock, which every installation also needs.)

ERP — putting gain and loss together. Effective radiated power combines transmitter power, feedline loss, and antenna gain (all in dB — see Section 4). A 50 W transmitter into 3 dB of cable loss feeding a 6 dBd antenna nets roughly 100 W ERP in the favored direction. Antenna gain can beat raw transmitter power — and feedline loss can quietly eat it.

Why it matters: The antenna and feedline are where signal is most easily won or lost. A few dB of feedline loss, a polarization mismatch, or a missing ground plane can dwarf the difference between a 5 W and a 50 W radio. This is also the bridge to the Amateur Radio antenna calculators — the physics here, the cut-to-length tools there.

For maximum power to flow from transmitter to antenna, their impedances must match — and match the feedline's characteristic impedance (commonly 50 Ω for RF, 75 Ω for video). Characteristic impedance comes from the cable's geometry (its inductance and capacitance), not a DC resistance you can read with an ohmmeter.

When impedances don't match, some of the forward power reflects back down the line. Forward and reflected waves combine into a stationary interference pattern — standing waves — measured as the Standing Wave Ratio (SWR).

High SWR means the transmitter sees a load that isn't the 50 Ω it was designed for. That off-nominal impedance pushes the final stage's voltage or current peaks and its dissipation past what it was built for — which is why a bad antenna or feedline can damage a radio. (In a lossless line the reflected power isn't simply "landing" in the final as heat; it re-reflects at the source.) Many transmitters fold back their power when they sense high SWR to protect themselves.

Reflection coefficient and return loss. SWR is one way to express a mismatch; there are two related measures you'll meet:

A perfect match means Γ = 0 (nothing reflected), SWR = 1:1, and infinite return loss. A total mismatch (open or short) means |Γ| = 1, SWR = ∞, and 0 dB return loss — everything bounces back. These are three views of the same thing: how much of your power the load refuses to accept.

How much power do you actually lose? Reflected power isn't all "lost" — in a lossless line it re-reflects and much eventually radiates — but it stresses the transmitter and exaggerates feedline loss. Rough reflected-power figures:

Fixing a mismatch. You don't always rebuild the antenna. A matching network — an L-network, pi-network, or antenna tuner — transforms the load impedance to what the source wants, using reactive components (inductors and capacitors) that add no loss in principle. A quarter-wave transformer (a precise length of line with a specific impedance) does the same trick at a single frequency. The tuner doesn't lower SWR at the antenna — it presents a happy match to the transmitter, which is what protects the PA.

Why it matters: SWR is the number every operator watches. It tells you, in one figure, whether your power is reaching the air or cooking your output stage — the most common bench and field check in all of RF. Knowing Γ, return loss, and matching networks turns "the SWR is high" into "here's why, and here's the fix."

Once a wave leaves the antenna, how it travels to the receiver depends mostly on its frequency. There are three main paths:

The ionosphere — charged layers high in the atmosphere — is the key to long-distance HF. It refracts HF sky waves back to earth, and its condition changes with the sun: time of day, season, and the 11-year solar cycle all shift which frequencies "open" for long-haul contacts. This is why a shortwave band alive at night may be dead at noon.

The ionospheric layers. The ionosphere isn't uniform — it has layers that behave differently and shift between day and night:

MUF and critical frequency. The ionosphere only refracts a wave back if the frequency is low enough for the current ionization. Two key numbers:

• Critical frequency — the highest frequency that reflects straight back when sent straight up. Above it, signals punch through to space.
• MUF (Maximum Usable Frequency) — the highest frequency that completes a given path (lower-angle signals reflect at higher frequencies than vertical ones). Operators work just below the MUF for best results; go above it and the signal escapes to space, below it and absorption increases.

A related idea is the skip zone — a dead ring between where the ground wave fades out and where the first sky-wave "hop" lands. Stations inside the skip zone can't hear a signal that's perfectly readable farther away.

Fading and multipath. Signals rarely arrive by a single clean path. When copies arrive via slightly different paths (different hops, or direct + reflected), they can add or cancel — producing fading. Rapid, frequency-selective fading distorts; slow fading just rises and falls (AGC handles that). Multipath is also why a moving car radio flutters under bridges and near buildings.

Above HF: it's not all line-of-sight. VHF/UHF are mostly line-of-sight, but a few modes extend them: tropospheric ducting (weather layers bending signals far beyond the horizon), sporadic-E (intense E-layer patches reflecting VHF), and knife-edge diffraction over ridges. Even "line-of-sight" reaches slightly past the visual horizon because the atmosphere bends RF a little — the radio horizon is about 15% farther than the optical one.

Why it matters: Propagation explains real behavior an operator sees daily — why AM stations travel farther at night, why a band is open to Europe at breakfast and Asia at dusk, why handheld VHF is local, why there's a dead zone at medium distance, and why operators check solar numbers before chasing DX.

Competency 19.4 asks you to differentiate the signals of AM and FM radio and TV and of land mobile radio, including CDMA, GSM, LTE and 5G. Section 3 explains modulation itself. This section compares how each system puts its signal on the air: how wide its channel is, how its users share the spectrum (the FDMA, TDMA and CDMA of Telecom, section 8, plus OFDMA) and how the carrier is modulated.

Four ways to share the spectrum. Narrowband FM and P25 Phase 1 cut the band into narrower channels (FDMA). GSM and P25 Phase 2 cut each channel into time slots (TDMA): Phase 2 fits two talk paths into 12.5 kHz, 6.25 kHz of spectrum each. IS-95 spreads every user across the same wide carrier and tells them apart by code (CDMA). LTE and 5G divide a wide channel into many narrow subcarriers and give each user a share of them (OFDMA). Broadcasting needs none of this: one transmitter sends one program to everyone.

Reading an emission designator. A license or a radio's data sheet names each emission with an FCC designator: the necessary bandwidth, then three symbols (47 CFR 2.201). In 11K0F3E, 11K0 is 11.0 kHz, F is frequency modulation, 3 is a single channel of analog information and E is telephony (voice). P25 Phase 1 voice, 8K10F1E, is 8.1 kHz, frequency modulation, a single channel of digital information, telephony. The old TV picture, 5M75C3F, is 5.75 MHz, C for vestigial sideband, carrying television.

Why it matters: The signal type tells you which test to make. Analog FM is checked for deviation and SINAD, P25 for modulation fidelity and bit error rate (Anritsu; VIAVI; Aeroflex), and section 12 shows the land mobile checks with a service monitor.

Competency 19.5 asks you to differentiate how communications radios and commercial broadcast receivers are used. Both are usually superheterodynes (section 6), but they are built for opposite jobs: a two-way radio carries short voice exchanges between licensed users, while a broadcast receiver plays one station's program for anyone who tunes in.

A communications radio's data sheet also rates how well it rejects signals other than the wanted one: adjacent-channel selectivity (67.5 to 70 dB on a 12.5 kHz channel for the APX 6000, measured per TIA/EIA-603), intermodulation (about 80 dB) and spurious rejection (Motorola).

SINAD and 12 dB SINAD sensitivity. SINAD is the ratio of signal plus noise plus distortion to noise plus distortion, measured on the receiver's audio output while the generator sends a 1 kHz test tone: SINAD = 20 log (rms of signal, noise and distortion ÷ rms of noise and distortion). A notch filter removes the 1 kHz tone to leave the noise and distortion (Keysight). A receiver's sensitivity is the lowest RF input that still gives the reference SINAD: 12 dB for a communications receiver, the point where noise and distortion are about 25% of the audio, the minimum deemed intelligible (Keysight).

Why it matters: “It can't hear” means different tests on the two kinds of set: a two-way radio is proved with 12 dB SINAD sensitivity, squelch and deviation on its programmed channels (section 12), and a broadcast receiver against its own, cleaner SINAD reference across its band.

Competency 19.8 asks you to explain radio circuit tuning and adjustments. There are two jobs: aligning a receiver's tuned circuits so they sit on frequency and track each other, and setting a transmitter's frequency, deviation and power with a service monitor. The stages being adjusted are in section 5 (transmitters) and section 6 (the superhet). Start with the service manual, and check before you adjust: some receivers need little alignment and some none (Silicon Chip), and Motorola's XTS 3000 manual gives its alignment as a fixed sequence, to be performed “in the sequence indicated”.

Receiver alignment: peaking the IF. Aligning a classic superhet needs an RF signal generator and an output meter (Silicon Chip). The IF stages come first. Inject a signal at the intermediate frequency, the industry-standard 455 kHz in AM sets, at the mixer (converter) input, and adjust each IF transformer's core or trimmer for maximum output, going round them several times. Keep the generator just strong enough to move the meter, and turn it down as the reading rises: otherwise the AGC (section 6) holds the output steady and hides the improvement. Use a non-metallic alignment tool, since a metal screwdriver detunes the coil and can cause a short or a shock (Silicon Chip).

Tracking. The antenna (RF) and oscillator tuned circuits share one tuning control, and each must tune exactly to its own frequency all across the dial; keeping them in step is tracking. The oscillator decides where a station falls on the dial: set its coil slug or padder capacitor at the low-frequency end, around 600 kHz, and its trimmer capacitor at the high end, around 1400 kHz, and repeat at both ends until nothing improves. Then peak the antenna trimmer and slug for maximum output, and an RF stage last (Silicon Chip). With a 455 kHz IF and the oscillator above the signal, the oscillator is then at 1055 kHz and 1855 kHz.

Synthesized radios. A PLL radio (section 5) has few slugs to turn. It is aligned from a computer with the maker's service software, which sets “softpots”: software potentiometers whose values set a D/A voltage inside the radio. Raising the reference-oscillator softpot, for example, raises the voltage on a varactor and so the frequency. For the XTS 3000 the order is reference oscillator; transmit power, at low, mid and high power on several frequencies across the band, measured at the antenna port; transmit deviation balance, which sets a 3 kHz tone's deviation within ±1.5% of an 80 Hz tone's so low and high audio deviate alike; transmit deviation limit; then the receiver's front-end filter, with a bit-error-rate test at the end (Motorola XTS 3000 service manual).

Transmitter checks with a service monitor. A service monitor, or communications analyzer, combines a signal generator with a receiver that measures carrier power, frequency error, deviation, SINAD, distortion, occupied bandwidth and squelch tones (Anritsu). Each reading has a limit:

Receiver checks. For 12 dB SINAD sensitivity, connect the monitor's generator to the radio's antenna port through any attenuation needed, lock out the radio's PTT, connect the monitor's audio input to the speaker output and set the volume for the rated audio output. Send the radio's receive frequency modulated with a 1 kHz tone at the deviation the manufacturer specifies, and raise the RF level from the minimum until the SINAD meter reads 12 dB; a CTCSS or DCS channel needs its tone added before it will unmute. 20 dB quieting is the older check: with an unmodulated carrier, the RF level that cuts the receiver's noise output to 10% of its no-signal level (Anritsu). A P25 receiver is rated instead by the RF level that gives a 5% bit error rate (Urgent Communications; Aeroflex).

Why it matters: An off-frequency transmitter interferes with users on the adjacent channels, and a misaligned front end costs sensitivity (Motorola). Alignment is how a technician puts a radio back on its specification, and the service monitor is how you prove it.

Job practice, not exam content. A typical bench check of a 12.5 kHz analog FM portable with a service monitor. The radio's own service manual, its limits and its alignment sequence, comes first.

1. Start from the manual. Find the radio's performance-check table and alignment sequence: rated power at each setting, the deviation limits for the channel bandwidth and the receiver sensitivity (Motorola).
2. Connect safely. Run the antenna port through a coupler or attenuator rated for the radio's power to the monitor's RF input, enter the loss as the power offset and set the monitor to the channel frequency (Anritsu).
3. Check the transmitter. Key the radio on a test channel and read power and frequency error. Convert the error to ppm and compare it with both the Part 90 limit and the maker's tighter target.
4. Check the deviation. Modulate with a 1 kHz tone at the manual's level, then speak loudly into the microphone (Motorola's check uses the word “four”) and read the peak: no more than ±2.5 kHz on a 12.5 kHz channel. On a tone-coded channel, confirm the CTCSS or DCS tone is there too.
5. Check the receiver. Lock out the PTT, feed the generator into the antenna port with a 1 kHz tone at the manual's deviation, set the rated audio output and raise the RF level until the SINAD reads 12 dB. Record the level in microvolts or dBm and compare it with the specification.
6. Adjust, then prove it. Adjust only what failed, in the manual's order with the service software (reference oscillator, power, deviation), then repeat steps 3 to 5 and record the before and after readings on the work order (Work Procedures).