A ratio
Compares one power or voltage level with another. It has no absolute reference by itself.
How the DB Scale is Used in Receivers
RF Engineering Guide
Understanding dB, dBm and Receiver Limits
A practical introduction to the dB scale and how it applies to inter-station interference, transmitter products, receiver blocking, intermodulation and safe receiver input levels.
dBdBmdBm/HzdBcReceiver protection
Transmitter+62 dBm
Damage example+17 dBm
IMD example−35 dBm
Noise floor−135 dBm
Basics and terminology
Why this matters in a multi-radio station
Whenever two or more transceivers are used in close proximity, some level of coupling and interference is involved. In a well-designed station this may be barely noticeable. In a poor layout it can cause receiver overload, desensing, intermodulation, or even physical damage to receiver input components.
Important point
The problem is not only the wanted transmitted signal. Real transmitters also produce harmonics, phase noise and wideband noise. Real receivers also have limits: they can block, mix unwanted signals, become desensitised, or be damaged if excessive RF reaches the antenna input.
In simple terms, dB, dBm, dBm/Hz and dBc provide a common language for measuring very large differences in signal level. This is essential when planning filters, antenna spacing, coax separation and receiver protection.
TransmitterWanted carrier plus unwanted products
Band-pass filterRejects energy outside the selected band
Antenna systemSpacing and pattern provide isolation
Nearby receiverMay block, mix, desense or suffer damage
Understanding the dB scale
Why RF engineers use logarithmic units
Radio stations deal with enormous differences in power. A transmitter may produce hundreds or thousands of watts while a receiver is trying to hear signals close to its internal noise floor. These values are awkward to compare directly in watts or volts, so RF engineers normally use logarithmic units.
Absolute power
Power referenced to 1 milliwatt. Therefore, 1 mW equals 0 dBm.
Noise density
Describes the amount of noise power present in a 1 Hz bandwidth.
Relative to carrier
Harmonics, sidebands and noise around a carrier are often specified relative to carrier power.
+3 dBapproximately 2 × power
+6 dBapproximately 4 × power
+10 dB10 × power
?3 dBapproximately half power
Core ratio formulas
Power ratio in dB = 10 log??(P? ÷ P?)Voltage ratio in dB = 20 log??(V? ÷ V?) when both voltage measurements use the same impedance.
10 log??(1500 ÷ 0.001)10 log??(100 ÷ 0.001)10 log??(3 × 10?¹? ÷ 0.001)Licence reminder
The 1,500 W example comes from the original American reference and is retained as a mathematical example. Always operate within the power limits and conditions of your own amateur-radio licence.
Interactive calculator
dBm and power converter
Convert absolute power between dBm, watts and milliwatts in a 50 Ω RF system.
50 dBm = 100 W = 100,000 mW
dBm = 10 log₁₀(P watts ÷ 0.001)
Noise and bandwidth
Understanding dBm/Hz
For a narrow-band signal such as CW, power can be calculated directly from the power read on a wattmeter. Wideband noise is different. The amount of noise admitted by a receiver depends on both noise density and receiver bandwidth.
Noise-density example
If a receiver measures ?50 dBm of noise in a 500 Hz bandwidth:
Noise density = ?50 dBm ? 10 log??(500) Noise density = ?50 dBm ? 26.99 dB Noise density ? ?77 dBm/HzPractical meaning
The wider the receiver bandwidth, the more total noise power is admitted. Filter bandwidth therefore matters when assessing interference, sensitivity and receiver performance.
Interactive calculator
Noise and bandwidth
Calculate noise density from measured noise, or total noise from a known density.
Total noise → density
−50 dBm in 500 Hz = approximately −77.0 dBm/Hz
Density → total noise
−174 dBm/Hz across 2,400 Hz = approximately −140.2 dBm
Transmitter products
What else comes out of a transmitter?
Even a good transmitter produces unwanted products alongside the wanted signal. These products can interfere with nearby receivers, especially in contest stations, multi-radio club stations and special-event stations with several transmitters active at the same time.
Higher powerLower power
Wanted carrier
Phase noise
Wideband noise
Harmonics
Lower frequencyFrequencyHigher frequency
Harmonics
Harmonics occur at integer multiples of the transmitted frequency. A 7 MHz transmitter can produce harmonic energy near 14 MHz and 21 MHz, potentially affecting receivers on those bands.
The original reference gives a worst-case example of harmonics at ?40 dBc. From a 1,500 W carrier this corresponds to approximately 150 mW, or +21.8 dBm.
Phase noise
Oscillators and frequency synthesisers are not perfectly clean. Small random phase variations create noise sidebands around the transmitted carrier.
Phase noise is generally strongest close to the carrier and reduces with increasing frequency offset. This is a major issue when two stations operate close together on the same band.
Wideband noise
Solid-state transmitters use wideband gain stages. Noise generated in early stages can be amplified through the transmit chain and appear at the final output.
Low-pass filtering reduces harmonics and some out-of-band energy, but band-pass filtering and physical isolation remain important in a serious multi-radio station.
Receiver limits
How much signal is too much?
A receiver is designed to handle weak signals. A large unwanted signal at its antenna input can produce several different failure modes, ranging from increased noise and cross-products through to complete blocking or damage to input components.
Do not treat these figures as universal equipment ratings
The levels below are examples quoted from the source material. Actual limits depend on receiver design, preamplifier state, attenuator settings, frequency separation, protection components and manufacturer specifications. Consult the service manual or manufacturer data for the equipment being protected.
| Problem | Example level from source text | What happens |
|---|---|---|
| Receiver damage | Approximately +17 dBm maximum-safe-input example | Input resistors, switching diodes, protection devices or front-end components may be overstressed. |
| Receiver blocking | Approximately +5 dBm at 50 kHz offset and 0 dBm at 20 kHz offset | The receiver front end or signal path is overloaded and normal reception becomes unusable. |
| Intermodulation distortion | Approximately ?35 dBm | Two or more strong signals mix in a non-linear device and create false signals such as 2F? ? F? and 2F? ? F?. |
| Noise floor | Approximately ?135 dBm | The receiver's internal noise and selected bandwidth set the lower limit of what can be heard. |
Intermodulation example
With one strong signal at 7020 kHz and another at 7030 kHz, third-order products may appear at:
2F? ? F? = 7010 kHz2F? ? F? = 7040 kHz
Why third-order products matter
They often fall close to the original signals and can land inside the same amateur band, where ordinary low-pass filtering cannot remove them after they have been generated in the receiver.
Interactive calculator
Third-order intermodulation products
Enter two strong signal frequencies. The calculator shows the common 2F₁ − F₂ and 2F₂ − F₁ products.
2F₁ − F₂: 7010 kHz 2F₂ − F₁: 7040 kHz
Power-level chart
The dBm power ladder
The dBm scale makes it possible to compare transmitter output, harmonic energy, receiver-blocking levels, intermodulation thresholds and receiver noise floors on one common scale.
+70 dBm10 kW — above normal high-power amateur transmitter levels
+62 dBmApproximately 1,500 W transmitter example
+50 dBm100 W transmitter
+22 dBmApproximately 158 mW — example harmonic level at −40 dBc from 1,500 W
+17 dBmApproximately 50 mW — example maximum safe receiver-input figure from the source text
+5 dBmApproximately 3.16 mW — example receiver-blocking level at 50 kHz spacing
0 dBm1 mW reference level
−35 dBmApproximately 0.316 µW — example intermodulation threshold
−135 dBmApproximately 3.16 × 10⁻¹⁷ W — receiver noise-floor example
−150 dBmApproximately 1 × 10⁻¹⁸ W — below typical amateur receiver sensitivity
Example isolation calculation
To reduce a +61.76 dBm transmitter signal to the source text's +17 dBm receiver-input example:
61.76 dBm ? 17 dBm = 44.76 dB Approximately 45 dB isolation requiredWhy contest stations need more
Avoiding physical damage is not enough. Clean simultaneous operation normally requires much greater isolation to control blocking, intermodulation, transmitter phase noise and degradation of the receiver noise floor.
Station-planning calculator
Required isolation and margin
Compare the isolation required from transmitter output to a chosen receiver-input target with the isolation available in the station.
Required: 44.8 dB · Available: 50.0 dB · Margin: +5.2 dB
Engineering note: a positive damage margin does not guarantee clean simultaneous operation. Blocking, phase noise, transmitter noise and intermodulation usually demand substantially more isolation.
Summary
What to remember
Transmitters are not perfect
They produce harmonics, phase noise and wideband noise as well as the wanted carrier.
Receivers are not unlimited
Strong unwanted signals can cause blocking, intermodulation, desensing or damage.
dB makes the problem measurable
Using dB and dBm allows the required isolation between transmitters and receivers to be calculated.
Filtering and layout matter
Good band-pass filtering, coax separation, antenna spacing, bonding and station layout all reduce interference risk.
Reference and credit
Source material
This educational page is based on a free excerpt of Managing Interstation Interference by George Cutsogeorge, W2VJN.
Original reference: International Radio — INRAD.