dBm, dBFS, V/m and µW/cm²: RF measurement units explained

RF readings come in a confusing mix of units: dBm and dBFS on a spectrum analyzer, dBµV on a test receiver, V/m or µW/cm² on an RF meter, mW/cm² in the FCC rules, dBi on an antenna datasheet. They measure related but different things. This guide explains each one, gives the formulas that connect them with worked examples, and covers the measurement details, resolution bandwidth, noise floor, peak versus average and calibration, that decide whether a number means what you think it means.

Decibels and logarithms

A decibel is a ratio on a logarithmic scale. For power:

dB = 10 × log₁₀(P₁ ÷ P₂)

The logarithm turns multiplication into addition. Radio signals span enormous ranges: a transmitter might put out tens of watts, and a receiver might pick up a millionth of a millionth of a milliwatt. In decibels that whole span fits into a couple of hundred units, and gains and losses along a signal path simply add and subtract.

Power ratios and amplitude ratios

Power is proportional to the square of voltage or field strength. So when the ratio is of voltages or field strengths, the formula uses 20 instead of 10:

dB = 20 × log₁₀(V₁ ÷ V₂)

Both formulas give the same number of decibels for the same physical change. Doubling the voltage quadruples the power, and both are +6 dB.

Negative decibels are the reciprocals: −10 dB is a tenth of the power, −3 dB about half.

Doing it in your head

You can convert most dB values with two facts: 10 dB is ×10 and 3 dB is ×2. Break the number into tens and threes.

• 23 dB = 20 + 3 = ×100 × 2 = ×200.
• 17 dB = 20 − 3 = ×100 ÷ 2 = ×50.
• −7 dB = −10 + 3 = ÷10 × 2 = ×0.2.
• 36 dB = 30 + 3 + 3 = ×1,000 × 4 = ×4,000.

These are approximations, because 3 dB is really ×1.995. The error is under 1 % per step, which is fine for reading a spectrum display.

Adding gains and losses

Gains and losses in decibels add. If a signal of −50 dBm passes through a 20 dB amplifier and then 4 dB of cable loss, it comes out at −50 + 20 − 4 = −34 dBm. This is the reason engineers work in decibels.

Adding two signals is different

You can't add the levels of two separate signals in dB. You have to convert to linear power, add, and convert back.

• Two equal signals of −60 dBm: 10⁻⁶ mW + 10⁻⁶ mW = 2×10⁻⁶ mW = −57.0 dBm. Two equal signals are 3 dB more than one, not twice the number.
• −60 dBm plus −70 dBm: 1.0×10⁻⁶ + 0.1×10⁻⁶ = 1.1×10⁻⁶ mW = −59.6 dBm.
• −60 dBm plus −80 dBm: −59.96 dBm. A signal 20 dB weaker barely changes the total.

This matters when you read a signal close to the noise floor: what you see is signal plus noise, so the display reads higher than the signal alone.

dBm and dBW: absolute power

A plain dB is only a ratio. Adding a letter fixes the reference, which turns it into an absolute unit.

• dBm is power relative to 1 milliwatt: P(dBm) = 10 × log₁₀(P ÷ 1 mW).
• dBW is power relative to 1 watt. Because 1 W = 1,000 mW = +30 dBm, dBW = dBm − 30.

Phones report LTE and 5G signal strength as RSRP in dBm. RSRP is the power of one reference-signal resource element, a very narrow slice of the channel, so it reads much lower than the total channel power. It's a dBm figure, but not one you can compare directly with a spectrum analyzer bin. The section on resolution bandwidth below explains why.

dBFS: decibels relative to full scale

A software-defined radio such as the HackRF One digitises the signal with an analog-to-digital converter (ADC). dBFS measures a level relative to the largest signal that converter can represent: 0 dBFS is full scale, and everything real is a negative number.

dBFS isn't an absolute unit. The same signal at the antenna can read −30 dBFS or −70 dBFS depending on how much gain sits between the antenna and the converter. The HackRF has an optional RF amplifier, an IF (LNA) gain adjustable from 0 to 40 dB in 8 dB steps, and a baseband (VGA) gain adjustable from 0 to 62 dB in 2 dB steps. Change any of them and every dBFS reading moves.

What 8 bits means

The HackRF's ADC has 8 bits, so each sample takes one of 256 values. The theoretical signal-to-quantisation-noise ratio for a full-scale sine wave is 6.02 × 8 + 1.76 ≈ 49.9 dB. A spectrum computed with an FFT spreads that quantisation noise across many frequency bins, so the noise in any one bin is lower than the total. Even so, the 8-bit converter limits how well very strong and very weak signals can be seen at the same time. If a strong signal pushes the converter to full scale, it clips, and the clipping creates false signals across the spectrum.

The practical rule

Set the gain so the strongest signals you care about sit comfortably below full scale, and the noise floor is still visible. Then leave the gain settings alone. Every comparison between dBFS readings is only valid at identical settings. Our HackRF One Android setup guide covers choosing them.

Voltage units: dBµV and dBmV

EMC test receivers and some broadcast and cable equipment express level as voltage at the input:

• dBµV is voltage relative to 1 microvolt: 20 × log₁₀(V ÷ 1 µV).
• dBmV is voltage relative to 1 millivolt, used in cable TV. dBmV = dBµV − 60.

Voltage and power are connected by the impedance: P = V² ÷ R. In a 50 Ω system, 1 mW corresponds to √(0.001 × 50) ≈ 0.2236 V, which is 20 × log₁₀(223,607) ≈ 107.0 dBµV. Hence the familiar rule:

dBµV = dBm + 107 (50 Ω)

In a 75 Ω system, used for TV and cable, the constant is 108.75, so 0 dBmV in 75 Ω is about −48.75 dBm.

Field strength: V/m, dBµV/m and A/m

Everything so far describes a signal inside a cable or receiver. Field strength describes the radio wave in the space around you, independent of any receiver.

• V/m, volts per metre, is the electric field strength. RF meters and exposure limits use it.
• dBµV/m is the same quantity in decibels relative to 1 µV/m: 20 × log₁₀(E ÷ 1 µV/m). Broadcast coverage and EMC emission limits use it. 1 V/m = 120 dBµV/m.
• A/m, amperes per metre, is the magnetic field strength. In the far field, H = E ÷ 377, so it carries no extra information there. Close to antennas and at low frequencies, it's measured separately.

The antenna factor

An antenna converts a field into a voltage at its connector. The antenna factor (AF), in dB/m, is the conversion between them:

E (dBµV/m) = V (dBµV) + AF (dB/m), plus any cable loss in dB

A calibrated measurement antenna comes with a table of AF against frequency. For a 50 Ω system, AF is related to the antenna's gain by:

AF (dB/m) = 20 × log₁₀(f in MHz) − G (dBi) − 29.77

The antenna factor rises with frequency, because a fixed-gain antenna collects less power from the same field as the wavelength shrinks. For an ideal 0 dBi antenna:

A real antenna's gain isn't a constant 0 dBi. It varies with frequency, direction and polarisation, which is exactly why calibrated antenna factors are measured rather than calculated.

Power density: W/m², mW/cm² and µW/cm²

Power density (S) is the RF power flowing through a unit area. It's the headline quantity in exposure limits above about 30 MHz. The FCC uses mW/cm², ICNIRP uses W/m², and many consumer RF meters show µW/cm².

• 1 mW/cm² = 10 W/m² = 1,000 µW/cm².
• 1 W/m² = 100 µW/cm² = 0.1 mW/cm².
• 1 µW/cm² = 0.01 W/m² = 10 mW/m².

To remember the factor of ten: a square metre is 10,000 cm² and a watt is 1,000 mW, so 1 W/m² = 1,000 mW ÷ 10,000 cm² = 0.1 mW/cm².

From field strength to power density

In the far field, the electric field and power density are related through the impedance of free space, about 377 Ω (more precisely 376.7 Ω):

S = E² ÷ 377 (S in W/m², E in V/m), and so E = √(377 × S)

• 1 V/m: S = 1 ÷ 377 = 0.00265 W/m² = 2.65 mW/m² = 0.265 µW/cm².
• 10 V/m: S = 100 ÷ 377 = 0.265 W/m² = 26.5 µW/cm².
• 61.4 V/m: S = 3,770 ÷ 377 = 10 W/m² = 1 mW/cm², the FCC general-population limit above 1.5 GHz.

In decibels: S (dBm/m²) = E (dBµV/m) − 115.8. Check: 1 V/m is 120 dBµV/m, and 120 − 115.8 = 4.2 dBm/m², which is 2.65 mW/m².

This relationship only holds in the far field. Close to an antenna the electric and magnetic fields have to be measured separately, as our exposure limits guide explains.

Antenna gain: dBi, dBd, EIRP and ERP

• dBi is gain relative to an ideal isotropic antenna, one that radiates equally in every direction. It doesn't exist physically but makes a clean reference.
• dBd is gain relative to a half-wave dipole, which has a gain of 2.15 dBi. So dBi = dBd + 2.15.
• EIRP (effective isotropic radiated power) is transmitter power plus antenna gain in dBi, minus feed losses: the power an isotropic antenna would need to produce the same field in the main beam.
• ERP (effective radiated power) is the same idea referenced to a dipole. EIRP = ERP + 2.15 dB.

Worked example: a broadcast licence lists 100 W ERP. That's 10 × log₁₀(100,000 mW) = 50 dBm ERP. Add 2.15 dB: 52.15 dBm EIRP, which is 10^5.215 mW ≈ 164 W EIRP. When you compare figures from different sources, check whether they're ERP or EIRP; mixing them is a 2.15 dB error.

A gain figure isn't extra power. A 15 dBi panel antenna doesn't create energy; it concentrates the transmitter's power into a narrower beam, so the field is stronger in the main beam and weaker elsewhere.

Effective area: how much power an antenna collects

An antenna placed in a field collects power from an effective area, or aperture:

A = G × λ² ÷ (4π)

Here G is the gain as a plain ratio (1 for 0 dBi, 2 for 3 dBi, and so on), λ is the wavelength in metres (λ = 3×10⁸ ÷ f in Hz), and A is in m². The received power is then P = S × A.

The area shrinks with the square of the wavelength. In the same power density, a 0 dBi antenna at 5.8 GHz collects about 1/3,400 of what it collects at 100 MHz. So a 5.8 GHz signal and a 100 MHz signal that read the same in dBm don't have the same power density. The higher-frequency one is far stronger in the air.

Worked examples: from received power to field and back

Received power to power density

Suppose a calibrated receiver with an isotropic (0 dBi) antenna reads −40 dBm on a 900 MHz cellular signal, measured across the whole channel, with no cable loss.

1. Power: −40 dBm = 10⁻⁴ mW = 10⁻⁷ W.
2. Wavelength: λ = 3×10⁸ ÷ 9×10⁸ = 0.333 m. Effective area: A = 0.333² ÷ (4π) ≈ 0.00883 m².
3. Power density: S = 10⁻⁷ ÷ 0.00883 ≈ 1.13×10⁻⁵ W/m² = 0.00113 µW/cm².
4. Field strength: E = √(377 × 1.13×10⁻⁵) ≈ 0.065 V/m.
5. For scale, the FCC general-population limit at 900 MHz is 0.6 mW/cm² (6 W/m²). This level is about 0.0002 % of it.

The same thing with the antenna factor

1. −40 dBm + 107 = 67 dBµV at the receiver.
2. AF of a 0 dBi antenna at 900 MHz: 20 × log₁₀(900) − 29.77 ≈ 29.3 dB/m.
3. E = 67 + 29.3 = 96.3 dBµV/m = 10^{(96.3 − 120)/20} V/m ≈ 0.065 V/m. Same answer.

Field strength to received power

Going the other way: what does a 0 dBi antenna at 900 MHz deliver from a 1 V/m field?

1. S = 1² ÷ 377 ≈ 0.00265 W/m².
2. P = S × A = 0.00265 × 0.00883 ≈ 2.34×10⁻⁵ W = 0.0234 mW.
3. In dBm: 10 × log₁₀(0.0234) ≈ −16.3 dBm. With the antenna factor: 120 − 29.3 − 107 = −16.3 dBm.

So even a field of 1 V/m, which is about 0.04 % of the FCC limit at that frequency in power-density terms (0.00265 ÷ 6), is a very strong signal at a receiver input.

Free-space path loss and the Friis equation

How much weaker does a signal get with distance? In free space, with nothing in the way, the Friis transmission equation gives the received power. In decibels:

P_r (dBm) = P_t (dBm) + G_t (dBi) + G_r (dBi) − FSPL (dB)

The free-space path loss, with distance in kilometres and frequency in MHz, is:

FSPL (dB) = 20 × log₁₀(d) + 20 × log₁₀(f) + 32.44

The constant comes from 20 × log₁₀(4π × 1,000 × 10⁶ ÷ c), which converts kilometres and megahertz into the plain formula 20 × log₁₀(4πd ÷ λ). The frequency term isn't a loss in the air. It's there because a fixed-gain receiving antenna has a smaller effective area at higher frequencies.

Worked examples

The last two rows show the two rules of thumb. Doubling the distance adds 6 dB of path loss. So does doubling the frequency, for antennas of fixed gain.

A link budget

A hypothetical 1.9 GHz transmitter puts 43 dBm (20 W) into a 15 dBi antenna through a feed with 1 dB of loss. A receiver with a 0 dBi antenna is 1 km away in the main beam.

1. EIRP: 43 + 15 − 1 = 57 dBm.
2. FSPL at 1 km, 1.9 GHz: 0 + 65.58 + 32.44 ≈ 98.0 dB.
3. Received power: 57 − 98.0 + 0 ≈ −41.0 dBm.

That's the free-space ideal. In a city, buildings, trees, walls and the ground usually make the real figure lower, often by tens of dB, and it changes from spot to spot. That variability is why Consentiva compares each block with its own history rather than with a formula.

Field strength from EIRP

In the far field, power density at distance d from a transmitter is S = EIRP ÷ (4πd²). Combining that with S = E² ÷ 377, and noting that 377 ÷ 4π ≈ 30, gives a handy formula:

E (V/m) = √(30 × EIRP) ÷ d (EIRP in W, d in m)

Worked example: 100 W EIRP at 1 km.

1. E = √(30 × 100) ÷ 1,000 = √3,000 ÷ 1,000 ≈ 0.0548 V/m.
2. In dBµV/m: 20 × log₁₀(54,772) ≈ 94.8 dBµV/m.
3. Check with power density: S = 100 ÷ (4π × 1,000²) ≈ 7.96×10⁻⁶ W/m², and 0.0548² ÷ 377 gives the same.

Resolution bandwidth and channel power

A spectrum analyzer doesn't measure the power of a signal directly. It splits the spectrum into slices, each the width of the resolution bandwidth (RBW), and shows the power in each slice. On an FFT-based instrument like a HackRF, each slice is an FFT bin. The bin spacing is the sample rate divided by the FFT size: at 20 Msps with a 1,024-point FFT, bins are 19.5 kHz apart.

Why a wide signal's reading depends on RBW

A narrow signal, such as an unmodulated carrier, fits inside one bin. It reads the same level whatever the RBW. A wide signal, such as a 5 MHz LTE channel, spreads its power over many bins, so each bin holds only a fraction. Make the bins narrower and each one reads lower.

Worked example: an LTE channel 5 MHz wide with a total power of −50 dBm, spread evenly.

• In 100 kHz bins: 50 bins, each with 1/50 of the power. Each bin reads −50 − 10 × log₁₀(50) = −50 − 17.0 = −67.0 dBm.
• In 1 MHz bins: 5 bins, each reading −50 − 10 × log₁₀(5) = −57.0 dBm.

The general rule: for a flat, wideband signal, the per-bin reading changes by 10 × log₁₀ of the RBW ratio. Ten times narrower RBW reads 10 dB lower.

Integrating across a channel

To get the total channel power, convert each bin across the channel to linear power, add them, and convert back.

1. 50 bins at −67.0 dBm each is 50 × 10^−6.7 mW = 50 × 2.0×10⁻⁷ mW ≈ 1.0×10⁻⁵ mW.
2. 10 × log₁₀(1.0×10⁻⁵) = −50 dBm, the channel power you started with.

Two refinements matter for accurate work. First, an FFT window spreads each bin's response a little wider than the bin spacing. A Hann window, for example, has an equivalent noise bandwidth of 1.5 bins, so summing bins for noise-like signals needs a correction for the window. Second, only sum the bins inside the channel; bins outside add noise.

Narrow and wide signals on the same display

Because RBW affects wide signals and noise but not narrow carriers, the picture changes when you change RBW. Take an unmodulated carrier at −80 dBm and a receiver noise floor set by a 10 dB noise figure:

• At 10 kHz RBW the noise floor is −174 + 40 + 10 = −124 dBm per bin. The carrier stands 44 dB above it.
• At 1 MHz RBW the noise floor is −174 + 60 + 10 = −104 dBm. The carrier still reads −80 dBm, but stands only 24 dB above the noise.

Neither display is wrong; they answer different questions. For comparing one walk with another, what matters is that the RBW is the same each time.

The noise floor

Every receiver adds noise. The starting point is thermal noise, which at room temperature (290 K) is −174 dBm per hertz of bandwidth (from 10 × log₁₀(kT × 1,000), where k is Boltzmann's constant). A real receiver adds its own noise on top, described by its noise figure (NF) in dB:

Noise floor (dBm) = −174 + 10 × log₁₀(bandwidth in Hz) + NF

The 10 dB noise figure here is just an example. A receiver's actual noise figure depends on its design, its frequency and its gain settings; turning up the front-end gain generally lowers the effective noise figure, until strong signals start to overload it.

A signal near the noise floor reads high, because you see signal plus noise. A signal exactly equal to the noise reads 3 dB above the floor. One 10 dB above the floor reads about 0.4 dB high.

Peak versus average

Many signals don't have a constant level. A reading can mean the peak, the average, or something in between, and the difference can be large.

• Pulsed signals. A radar sends short pulses and listens in between. Its average power is the peak power times the duty cycle. A hypothetical radar with 10 W peak (+40 dBm) and a duty cycle of 0.1 % has an average of 10 mW (+10 dBm): 30 dB lower, because 10 × log₁₀(0.001) = −30.
• Bursty signals. Wi-Fi, Bluetooth and many data links transmit only when there's something to send. A sweep that lands during a burst sees the full level; the average over a minute may be far lower.
• OFDM signals. LTE, 5G and Wi-Fi use OFDM, which has a high peak-to-average power ratio: the instantaneous power often rises well above the average, by 10 dB or more on occasional peaks.

Spectrum analyzers handle this with detectors and trace modes. A peak detector or a max-hold trace shows the highest value seen; an average or RMS detector shows the mean power. Max hold is good for finding intermittent signals; averaging is what power-based comparisons need. A reading should always say which it is.

Peak versus average also matters for exposure limits, which are time averages. A peak-hold reading of a bursty signal overstates its average, sometimes by a lot.

Why dBFS depends on gain, and why that's fine for comparisons

A dBFS reading is the signal's level after every gain stage, relative to the ADC's full scale. Write it as a sum of decibels:

Reading (dBFS) = power at the antenna connector (dBm) + receiver gain (dB) + a fixed constant for the converter

The receiver gain includes the RF amplifier, the LNA and VGA settings, and the receiver's own frequency response. If you change the LNA gain by one 8 dB step, every reading moves by about 8 dB. The signal in the air hasn't changed at all.

That's why relative comparisons work. Subtract two readings taken at identical settings and the gain and constant cancel:

Reading₂ − Reading₁ = Power₂ − Power₁

Worked example: a block's normal level on a channel is −70 dBFS, and tonight it reads −58 dBFS at the same settings. The signal is 12 dB stronger than normal, about 10^1.2 ≈ 16 times the power, and that's true whatever the calibration is. But if the LNA had been one step higher tonight, 8 dB of that 12 dB would be the settings, and the real change would be only 4 dB (about 2.5 times). A gain change between walks shows up as a fake rise or fall across every frequency at once, which is one way to recognise it.

The cancellation is only approximate if the two readings are at very different levels, because near overload the receiver compresses, and near the noise floor the noise adds to the reading. Comparisons are most reliable for signals well above the noise and well below full scale.

Calibrating a HackRF against a known source: the concept

Calibration turns dBFS into dBm by measuring the offset between them. In concept, at one frequency and one set of gain settings:

1. Get a known source. A signal generator with a specified output level, or a source you've measured with a calibrated instrument. Use an unmodulated carrier, so the RBW doesn't affect the reading.
2. Connect by cable, not over the air. Over the air, the antenna gain and the room's reflections would be part of the result. By cable, the only unknown is the receiver. Use an attenuator if needed, so the level is modest and well within the receiver's safe input range.
3. Account for the cable. Subtract the cable and attenuator loss at that frequency from the generator level to get the power at the receiver's input.
4. Use your survey settings. Same RF amplifier, LNA and VGA settings, same sample rate and FFT size.
5. Read the level in dBFS. Centre the carrier on a bin, or take the peak of its response, because a tone between two bins reads lower; for a Hann window, the worst case is about 1.4 dB low.
6. Work out the offset: offset (dB) = known input power (dBm) − reading (dBFS). Afterwards, dBm = dBFS + offset.
7. Check linearity. Change the generator by 10 dB. The reading should change by 10 dB. If it changes by less, the receiver is compressing and you need a lower level or less gain.
8. Repeat across frequency. The offset varies across 1 MHz–6 GHz, so measure it at the frequencies you care about.

Worked example (hypothetical numbers)

1. The generator is set to −50.0 dBm at 900 MHz. The cable loses 1.5 dB at that frequency, so the HackRF input sees −51.5 dBm.
2. The HackRF reads −38.2 dBFS on the carrier.
3. Offset = −51.5 − (−38.2) = −13.3 dB.
4. Linearity check: at −60.0 dBm from the generator, the reading should be −48.2 dBFS.
5. Later, at the same settings and frequency, a channel integrates to −52.0 dBFS. Its power at the input is −52.0 + (−13.3) = −65.3 dBm.

That calibrates the receiver only. The offset is valid for that unit, at those settings, near that frequency, and it can drift with temperature. To go from dBm at the connector to field strength in the air, you'd still need the antenna's calibrated antenna factor, the cable loss, and attention to direction and polarisation. Continuing the example with an assumed 0 dBi antenna at 900 MHz: −65.3 + 107 = 41.7 dBµV; plus AF 29.3 dB/m gives 71.0 dBµV/m ≈ 3.5 mV/m. With a real, uncalibrated antenna, that last figure could be off by many dB.

Consentiva lets you enter a calibration offset in its settings, after which readings are shown in dBm. It's the receiver-only calibration described above, so it applies to the settings and frequencies where you measured it.

Reading a survey with these units: four situations

The arithmetic above turns into quick judgements when you look at a sweep. All of these assume readings taken at identical gain settings and RBW, so the dB differences are real.

A signal 20 dB above the block's normal

20 dB is a power ratio of 100. Under free-space conditions, that's the change you'd get from moving ten times closer to the same transmitter, because each halving of distance adds 6 dB and 20 × log₁₀(10) = 20. In a city it could also be a new transmitter, a transmitter running at higher power, or simply a different line of sight. The number tells you how much stronger, not why.

A rise of exactly 3 dB on a channel

If a channel's level rises by 3 dB, the power has doubled. One common cause is a second signal of similar strength appearing on the same frequency, because two equal powers add to +3 dB. Another is a busier network transmitting more of the time. Look at the shape: a second carrier may show a slightly different centre frequency or bandwidth.

A wide hump versus a narrow spike

A wide signal and a narrow one can have the same total power but look very different. A 5 MHz channel at −50 dBm total appears as a flat hump about 17 dB lower per 100 kHz bin; an unmodulated carrier at −50 dBm appears as a spike at −50 dBm. Don't judge which is "stronger" by peak height alone. Integrate the hump to compare channel powers.

A faint signal just above the noise

If a signal reads 3 dB above the noise floor, it's roughly as strong as the noise itself, and the true signal is about 3 dB lower than the displayed total. A small change of the noise floor, from a gain change or a nearby wideband source, can make it appear or disappear. Treat changes in signals this close to the floor with caution. Our guide to surveying radio signals covers repeat walks and how to tell a real change from noise.

What Consentiva shows

Consentiva uses a HackRF One on an Android phone to sweep the spectrum as you walk: 30 MHz–2.7 GHz on the Free and Standard plans and up to 6 GHz on the Full plan. Each sweep is recorded with its GPS position.

• Default units: dBFS. Without calibration, levels are relative to the receiver's full scale.
• Optional dBm. With a calibration offset entered in the settings, levels are shown in dBm at the receiver input. They're still not field strength or power density.
• Relative comparisons. Consentiva learns a baseline for each block of about 150 m, separately for night, morning, afternoon and evening, and reports how many dB a signal is above that normal. A signal stronger than that baseline is flagged Unusual. Keep the gain settings the same between walks and these comparisons stay meaningful even though the absolute levels are uncalibrated.

Reports show these relative figures alongside the frequencies, places and times. See how to read a Consentiva report, the sample report, or the plans.

Quick-reference conversion tables

Formulas

Power

Field strength and power density (far field)

The last two rows are the FCC general-population limits at 30–300 MHz (27.5 V/m, 0.2 mW/cm²) and above 1.5 GHz (1.0 mW/cm²), for reference.

Common mistakes

• Using 10 log for a voltage ratio, or 20 log for a power ratio. A field that doubles is +6 dB, not +3 dB.
• Adding dBm values directly. Two −60 dBm signals make −57 dBm, not −120 dBm. Convert to milliwatts first.
• Treating dBFS as dBm. A dBFS reading has no absolute meaning until you've calibrated that receiver at those settings.
• Comparing dBFS readings taken at different gains. An 8 dB gain step looks exactly like an 8 dB stronger signal everywhere.
• Ignoring RBW. A wide signal's per-bin level depends on the bin width. Compare channel power, or keep the RBW identical.
• Comparing a phone's RSRP with a spectrum bin. RSRP measures one narrow resource element, not the whole channel.
• Mixing W/m² and mW/cm². They differ by a factor of ten.
• Mixing ERP and EIRP. They differ by 2.15 dB.
• Comparing dBm across frequencies as if they were the same field. At higher frequencies a 0 dBi antenna collects less, so the same dBm means a stronger field.
• Using far-field formulas next to an antenna. S = E² ÷ 377 and the inverse-square law only hold in the far field.
• Reading a peak as an average. Max hold shows the highest moment, not the mean power.
• Overloading the receiver. A converter at or near full scale creates false signals. If many new signals appear at once near a strong transmitter, reduce the gain and look again.

For which instrument suits which measurement, see RF meter vs spectrum analyzer. For the bands you'll see most often, see 5G frequency bands explained.

Glossary

• ADC. Analog-to-digital converter: turns the receiver's signal into numbers. Its full scale defines 0 dBFS.
• Antenna factor (AF). The dB/m figure that converts antenna output voltage into field strength.
• Channel power. The total power of a signal across its occupied bandwidth, found by integrating bins.
• dB. A logarithmic ratio: 10 log₁₀ for power, 20 log₁₀ for voltage or field.
• dBd. Antenna gain relative to a half-wave dipole.
• dBFS. Level relative to the ADC's full scale; depends on gain.
• dBi. Antenna gain relative to an ideal isotropic antenna.
• dBm / dBW. Power relative to 1 milliwatt / 1 watt.
• dBµV, dBmV. Voltage relative to 1 microvolt / 1 millivolt.
• dBµV/m. Electric field strength relative to 1 µV/m.
• Effective area (aperture). The area from which an antenna collects power: G × λ² ÷ (4π).
• EIRP / ERP. Radiated power in the main beam referenced to an isotropic antenna / a dipole.
• Far field. The region far enough from an antenna that power density falls with the square of distance.
• FFT bin. One frequency slice of a digitally computed spectrum; its width sets the RBW.
• FSPL. Free-space path loss: the loss between two isotropic antennas in empty space.
• Noise figure (NF). How much noise a receiver adds above thermal noise, in dB.
• Power density (S). Power per unit area: W/m², mW/cm², µW/cm².
• RBW. Resolution bandwidth: the width of each slice in which a spectrum analyzer measures power.
• RSRP. Reference Signal Received Power: a phone's LTE and 5G signal measurement for one resource element.
• Thermal noise. Noise from the random motion of electrons: −174 dBm/Hz at 290 K.

Common questions

What does dBm mean?

dBm is power relative to one milliwatt on a logarithmic scale: 0 dBm is 1 mW, -30 dBm is 1 microwatt, -60 dBm is 1 nanowatt and -90 dBm is 1 picowatt. Every 10 dB is a factor of ten and every 3 dB roughly doubles or halves the power.

What is the difference between dBm and dBFS?

dBm is an absolute power. dBFS is relative to the full-scale input of a receiver's analog-to-digital converter, so 0 dBFS is the largest signal it can digitise. dBFS readings can only be turned into dBm with a calibration for that receiver, gain setting and frequency.

How do I convert V/m to µW/cm²?

In the far field, power density equals the field strength squared divided by 377 ohms. 1 V/m is 2.65 mW/m², which is 0.265 µW/cm². 10 V/m is 26.5 µW/cm².

How do I convert dBm to dBµV?

In a 50 ohm system, add 107: -60 dBm is 47 dBµV. In a 75 ohm system the constant is about 108.75.

Why does a signal's level change when I change the resolution bandwidth?

A wide signal spreads its power across many bins, so narrower bins each hold less of it. For a flat wideband signal, ten times narrower RBW reads 10 dB lower per bin, while a narrow carrier reads the same.

What is the free-space path loss formula?

FSPL in dB equals 20 log10 of the distance in km, plus 20 log10 of the frequency in MHz, plus 32.44. At 1 km and 900 MHz it is about 91.5 dB.

How do I calibrate a HackRF to read dBm?

Feed it a carrier of known power by cable at your usual gain settings, note the dBFS reading, and take the offset as known dBm minus dBFS. The offset only applies to that unit, those settings and nearby frequencies.

What is the thermal noise floor?

Thermal noise at room temperature is -174 dBm per hertz. Add 10 log10 of the bandwidth in hertz and the receiver's noise figure: in 1 MHz with a 10 dB noise figure the floor is about -104 dBm.