Amateur radio is full of numbers: watts, volts, ohms, decibels, megahertz, microvolts, SWR, SNR, SINAD, dBi, dBd, and many more. At first they look like a pile of technical jargon, but they are really just the language of radio. Once you understand the units, you can read radio specifications, choose antennas, estimate station performance, troubleshoot problems, and communicate more precisely.

This article covers the most important units and measurements every amateur radio operator should know.

1. Frequency: Hz, kHz, MHz, GHz

Frequency is one of the most basic concepts in radio. It describes how many cycles per second a signal makes.

The SI unit is the hertz, written as Hz.

Common radio units:

Amateur bands are usually described by either frequency or wavelength.

Examples:

Frequency tells you where you are transmitting or receiving. It also affects propagation, antenna size, bandwidth, and licensing rules.

2. Wavelength: m, cm, mm

Wavelength is the physical length of one complete radio wave cycle. The SI unit is the meter, written as m.

Radio operators often use wavelength names for bands:

The relationship between frequency and wavelength is:

wavelength in meters = 300 / frequency in MHz

Examples:

145 MHz: 300 / 145 = about 2.07 m
433 MHz: 300 / 433 = about 0.69 m

This matters because antenna size is usually related to wavelength. A half-wave dipole for 40 m is physically much larger than a half-wave antenna for 2 m.

3. Power: W, mW, kW

Power tells you how much energy your transmitter sends out. The SI unit is the watt, written as W.

Common units:

Examples:

More power can help, but it is not magic. Antenna efficiency, feedline loss, receiver noise, propagation, and operating skill often matter more than raw watts.

A useful rule:

Doubling power = +3 dB
Ten times power = +10 dB

So going from 5 W to 50 W is a 10 dB increase. Going from 50 W to 100 W is only 3 dB.

4. Voltage: V, mV, µV

Voltage is electrical pressure. The SI unit is the volt, written as V.

Common units:

In amateur radio, voltage appears in several places:

Many mobile and base radios are designed for about 13.8 V DC, because that is typical of a vehicle electrical system while charging.

Receiver sensitivity is often stated in microvolts. For example:

0.18 µV @ 12 dB SINAD

That means the receiver can detect a very weak signal and still produce usable audio.

5. Current: A, mA

Current is the flow of electric charge. The SI unit is the ampere, written as A.

Common units:

Examples:

Power, voltage, and current are related:

Power = Voltage × Current
W = V × A

Example:

13.8 V × 20 A = 276 W input power

The RF output may be 100 W, because the radio is not 100% efficient. Some energy becomes heat.

6. Resistance and Impedance: Ω

Resistance and impedance are measured in ohms, written as Ω.

Resistance applies mainly to DC circuits. Impedance applies to AC and RF circuits, where capacitance and inductance also matter.

Common amateur radio impedance values:

Most modern amateur radios expect a 50 Ω antenna system. If the antenna system is not close to 50 Ω, the radio may reduce power or require an antenna tuner.

7. SWR: Standing Wave Ratio

SWR means Standing Wave Ratio. It describes how well the antenna system matches the transmitter and feedline.

SWR is written as a ratio:

1.0:1
1.5:1
2.0:1
3.0:1

General guide:

SWR does not directly tell you whether an antenna is “good.” It tells you about impedance match. A dummy load can have excellent SWR but radiates almost nothing. A real antenna can have a decent SWR but still be inefficient if badly placed, lossy, or poorly built.

8. Decibel: dB

The decibel, written as dB, is one of the most important units in radio.

Strictly speaking, dB is not an SI unit. It is a logarithmic ratio. It compares one value to another.

In radio, dB is used for:

• antenna gain
• feedline loss
• amplifier gain
• filter attenuation
• signal strength
• path loss
• receiver performance

Useful dB rules:

Example:

If your coax has 3 dB loss, about half your power is lost in the feedline.

50 W transmitter power
3 dB feedline loss
about 25 W reaches the antenna

The decibel is powerful because gains and losses can be added.

Example:

Transmitter power: 50 W
Feedline loss: -2 dB
Antenna gain: +6 dBi
Net antenna-side gain effect: +4 dB

9. dBm and dBW

dBm and dBW are decibel units with fixed references.

Common values:

dBm is very common in receiver specs, spectrum analyzers, SDR software, link budgets, and weak-signal work.

Example:

-120 dBm = very weak signal
-90 dBm = stronger signal
-60 dBm = strong local signal

Less negative is stronger.

10. Antenna Gain: dBi and dBd

Antenna gain describes how an antenna concentrates radio energy in a particular direction compared with a reference antenna.

Two common units are dBi and dBd.

An isotropic radiator is a theoretical antenna that radiates equally in all directions. It is useful for math, but it does not physically exist.

A half-wave dipole is a real, common antenna.

Relationship:

dBi = dBd + 2.15
dBd = dBi - 2.15

Example:

Antenna gain = 5 dBd
Same gain = 7.15 dBi

Be careful when comparing antennas. A manufacturer using dBi may make the number look larger than one using dBd.

11. EIRP and ERP

EIRP means Effective Isotropic Radiated Power.

It is the apparent radiated power compared with an isotropic antenna.

ERP means Effective Radiated Power.

It is the apparent radiated power compared with a half-wave dipole.

These are important for regulations, repeater planning, satellite work, and link budgets.

Basic idea:

EIRP = transmitter power - feedline loss + antenna gain in dBi
ERP  = transmitter power - feedline loss + antenna gain in dBd

Example:

Transmitter: 50 W
Feedline loss: 3 dB
Antenna gain: 6 dBi

Power after feedline loss:

50 W - 3 dB = about 25 W

Antenna gain:

25 W + 6 dB = about 100 W EIRP

12. Bandwidth: Hz, kHz, MHz

Bandwidth is the amount of frequency space a signal occupies.

Common examples:

Bandwidth matters because radio spectrum is shared. A narrow signal uses less spectrum and may work better under weak conditions. A wide signal may carry better audio or more data but takes more space.

13. Signal Reports: RST and S-Units

Amateur radio commonly uses signal reports.

For CW and voice, the classic report is RST:

Example:

59
599
57

For voice, 59 means perfectly readable and strong. In contests, 59 is often exchanged quickly and may not be a precise measurement.

Many receivers also show S-units on an S-meter.

A common convention:

1 S-unit ≈ 6 dB
S9 on HF ≈ -73 dBm
S9 on VHF/UHF ≈ -93 dBm

This is a guideline. Many radio S-meters are not perfectly calibrated.

Above S9, reports are often given in dB:

S9 + 10 dB
S9 + 20 dB
S9 + 40 dB

14. SNR: Signal-to-Noise Ratio

SNR means Signal-to-Noise Ratio.

It compares the wanted signal with the noise level.

SNR = signal level compared with noise level

It is usually measured in dB.

Examples:

SNR is especially important for:

• HF weak-signal work
• digital modes such as FT8, JS8Call, packet, APRS, Winlink
• satellite communication
• EME
• SDR receiver displays
• link budget calculations

Digital modes often work at lower SNR than voice because computers can decode weak structured signals.

15. SINAD

SINAD means:

Signal + Noise + Distortion
divided by
Noise + Distortion

It is used mainly in receiver sensitivity testing, especially for FM voice receivers.

A common specification:

0.18 µV for 12 dB SINAD

This means that with a signal of 0.18 microvolts, the receiver produces audio with 12 dB SINAD, which is considered a usable audio benchmark.

When comparing receivers:

0.16 µV @ 12 dB SINAD

is generally more sensitive than:

0.25 µV @ 12 dB SINAD

because less signal is needed to reach the same audio quality.

16. Noise Figure: NF

Noise figure, written as NF, describes how much noise a receiver, preamp, or amplifier adds to a signal.

It is measured in dB.

Lower is better.

Examples:

Noise figure is especially important at VHF, UHF, microwave, satellite, and weak-signal operation. On lower HF bands, atmospheric noise may dominate, so receiver noise figure is often less critical.

17. Noise Floor: dBm

The noise floor is the background noise level in a receiver or measurement system.

It is often shown in dBm.

Example:

Noise floor: -120 dBm
Signal: -100 dBm
SNR: 20 dB

A lower noise floor allows weaker signals to be detected. However, real-world noise from power supplies, electronics, thunderstorms, solar activity, and urban environments can raise the effective noise floor.

18. Return Loss

Return loss is another way to describe antenna mismatch. It is measured in dB.

Higher return loss is better.

Approximate comparison:

Many antenna analyzers and VNAs show return loss as well as SWR.

19. Capacitance: F, µF, nF, pF

Capacitance is measured in farads, written as F.

In radio, practical values are usually much smaller:

Capacitors are used in:

• filters
• antenna tuners
• matching networks
• oscillators
• power supply filtering
• coupling and bypass circuits

Examples:

100 pF tuning capacitor
0.1 µF bypass capacitor
10 µF electrolytic capacitor

20. Inductance: H, mH, µH

Inductance is measured in henrys, written as H.

Common radio values:

Inductors are used in:

• antenna loading coils
• filters
• traps
• matching networks
• RF chokes
• oscillators

Examples:

2.2 µH coil
100 µH RF choke

Capacitors and inductors together form resonant circuits, which are central to radio tuning and filtering.

21. Battery Capacity: Ah and Wh

Battery capacity is commonly given in amp-hours, written as Ah.

Example:

12 V 20 Ah battery

This means the battery can theoretically supply:

20 A for 1 hour
or
1 A for 20 hours

In practice, usable capacity depends on battery chemistry, discharge rate, temperature, and cutoff voltage.

A better energy unit is watt-hour, written as Wh.

Wh = V × Ah

Example:

12.8 V × 20 Ah = 256 Wh

For emergency communication and portable radio, Wh is often more useful than Ah, because it includes voltage.

22. Temperature: °C and K

Temperature is commonly measured in degrees Celsius, written as °C.

Examples:

Radio operating temperature: -10 °C to +60 °C
Battery temperature limit: 0 °C to 45 °C while charging

The SI base unit is kelvin, written as K.

Kelvin is used in technical radio calculations involving thermal noise.

Important idea:

Higher temperature = more thermal noise

Most amateur operators use Celsius day to day, but kelvin appears in receiver noise and microwave engineering.

23. Time: s, ms, µs, UTC

Time is measured in seconds, written as s.

Common units:

Time matters for:

• digital modes
• packet timing
• propagation delay
• satellite passes
• logging
• contests
• emergency nets

Amateur radio commonly uses UTC, Coordinated Universal Time, for logging and international contacts.

Example log entry:

2026-06-29 1415 UTC
14.074 MHz
FT8
9M2PJU
-12 dB

Using UTC avoids confusion between time zones.

24. Data Rate: bps and Baud

Digital radio uses data rates.

They are related, but not always the same.

If each symbol carries one bit:

baud = bps

If each symbol carries multiple bits:

bps can be higher than baud

Examples:

25. Field Strength: V/m and dBµV/m

Field strength describes the strength of an electromagnetic field at a location.

Common units:

Field strength is used in:

• coverage prediction
• repeater studies
• EMC/RFI work
• broadcast engineering
• regulatory measurements

Most casual amateur operators do not use field strength daily, but it is important for interference investigations and serious station engineering.

26. Modulation Measurements

Several units and ratios describe modulation quality.

Deviation

FM deviation is measured in Hz or kHz.

Example:

±5 kHz deviation
±2.5 kHz deviation

Too much deviation causes splatter or distortion. Too little deviation makes audio weak.

Modulation Percentage

AM modulation is often expressed as a percentage.

100% modulation

Overmodulation causes distortion and unwanted emissions.

THD

THD means Total Harmonic Distortion.

It is usually given as a percentage or in dB.

Lower THD means cleaner audio or cleaner signal reproduction.

27. Digital Quality Units: BER, MER, Eb/N0

Digital communication introduces more quality measurements.

For most amateur operators, BER is the easiest to understand.

Example:

BER = 1 × 10^-5

This means about 1 bit error in 100,000 bits.

Lower BER is better.

These measurements are common in digital voice, satellites, microwave links, and data systems.

28. Common Prefixes

SI prefixes are essential in radio.

Examples:

100 pF
2.2 µH
13.8 V
500 mA
7 MHz
2.4 GHz

Be careful with capital letters:

m = milli
M = mega

So:

mW = milliwatt
MW = megawatt

Those are very different.

29. Quick Reference Table

30. Practical Examples

Example 1: Feedline Loss

You transmit 50 W into a coax cable with 3 dB loss.

3 dB loss = half power

So only about:

25 W

reaches the antenna.

Example 2: Antenna Gain

Your antenna has 6 dBi gain.

6 dB gain = about 4 times power in the favored direction

If 25 W reaches the antenna:

25 W × 4 = about 100 W EIRP

Example 3: Receiver Signal

Your SDR shows:

Noise floor: -120 dBm
Signal: -100 dBm

Then:

SNR = 20 dB

That is usually a very usable signal.

Example 4: Battery Runtime

Your radio draws:

1 A on receive
20 A on transmit

With a 20 Ah battery, runtime depends heavily on duty cycle.

If you transmit often, the battery drains much faster. For emergency work, always calculate using realistic transmit and receive time.

Conclusion

The most important amateur radio units are not just numbers on a specification sheet. They describe how your station actually works.

If you understand Hz, W, V, A, Ω, dB, dBm, dBi, dBd, SWR, SNR, SINAD, µV, Ah, and Wh, you can make better decisions about radios, antennas, feedlines, batteries, and operating technique.

A good amateur radio operator does not need to memorize every formula. But knowing what the units mean gives you a practical engineering sense: how much power is really reaching the antenna, how weak a signal your receiver can hear, how much loss your coax has, how long your battery will last, and why a better antenna often beats a bigger amplifier.

The post Amateur Radio Units Explained: dB, dBm, Watts, SWR, SNR, SINAD, dBi and More appeared on Hamradio.my - Amateur Radio, Tech Insights and Product Reviews by 9M2PJU.