RF Power Measurement: Power Meter vs Spectrum Analyzer vs VNA

Why there are three instruments

Power is one of the most basic RF quantities, yet three different instruments can report it. They do not answer the same question. A power meter answers "how much power arrives in total?" A spectrum analyzer answers "how much power is at each frequency?" A vector network analyzer (VNA) answers "how does this device change a signal?" Picking the wrong one gives a number that looks precise and means something else.

This page compares the three, shows what each does best and where each fails, and gives the dBm arithmetic needed to read the results.

The dBm arithmetic

dBm is power relative to 1 mW:

So 0 dBm is 1 mW, 30 dBm is 1 W, and −30 dBm is 1 microwatt. Two rules cover most bench work. Adding 3 dB roughly doubles the power. Adding 10 dB multiplies the power by 10. Subtracting works the same way in reverse: −10 dB divides the power by 10.

Gains and losses in dB are added to a level in dBm, and the result is again a level in dBm. Two levels in dBm are never added directly. To combine two signals, convert both to milliwatts, add them, and convert back. The dBm converter does this conversion.

Power meter: total power, one number

A power meter uses a sensor that absorbs the signal and converts it to a reading. It reports the total power over the bandwidth of the sensor. It does not care how that power is divided among frequencies.

This is its strength. Total power is exactly what you want when you check a transmitter output, set a drive level, or calibrate a signal source. The reading is a single number with a clear meaning.

It is also its weakness. If the signal contains the wanted carrier plus a harmonic or a spur, the meter adds them all together. It cannot tell you which part is which. A meter reading that is "too high" gives no hint about the cause.

Spectrum analyzer: power per frequency

A spectrum analyzer measures power within a resolution bandwidth (RBW) and sweeps that window across frequency. Because of this, it can separate signals at different frequencies. You see the carrier, the harmonics and the spurious signals as separate lines, each with its own level.

That makes it the right tool for spurious emissions, harmonic levels, and checking whether a second signal sits near the first. The same property creates a trap. A level read at one frequency is the power inside the RBW, not the total power of the signal. A modulated signal wider than the RBW spreads its power over many frequencies, and a single marker reading shows only part of it. To get total power, the analyzer must integrate across the signal bandwidth, or you should use a power meter.

VNA: ratios first

A VNA sends a known stimulus into a device and measures the waves that return and pass through. Its output is a set of ratios, the S-parameters. It tells you gain, loss and reflection, not the absolute power of an unknown signal.

For power work its role is therefore indirect. It characterizes the parts around the measurement: the loss of a cable, the loss of an attenuator, the reflection of a connector. Those numbers then correct a power reading taken with a meter or an analyzer. Use a VNA to find the loss between the device and the instrument, and use the other two to read the power. The related page on return loss and insertion loss explains the two ratios you will use most.

Mismatch affects every reading

A mismatch between the source and the sensor means that not all available power is absorbed. Part is reflected, so the reading is affected by the reflection. The reflection coefficient on a line of impedance Z₀ ending in a load Z_L is:

The fraction of power reflected is |Γ|², and the return loss is:

A perfect 50 ohm load gives Γ = 0, so nothing is reflected. A short or an open gives |Γ| = 1, so everything is reflected and nothing is absorbed.

The same applies to the input of a spectrum analyzer. A good input match, or a fixed attenuator in front of it, reduces the error from reflection and also protects the input.

Worked example: from volts to dBm to a reading

A source delivers 1 V rms into a 50 ohm load. The power is:

In dBm:

Check with the rules: 20 mW is 2 × 10 mW. Ten mW is 10 dBm, and doubling adds about 3 dB, so about 13 dBm. The two methods agree.

Now place a 10 dB attenuator between the source and the instrument. The level drops by 10 dB, from about 13 dBm to about 3 dBm. In power, 20 mW divided by 10 is 2 mW, and 10 log10(2) is about 3 dBm. The instrument reads 3 dBm. To report the source power, add the 10 dB back: 3 dBm + 10 dB = 13 dBm. Forgetting this correction is the most common error in the whole procedure.

Which instrument to pick

• Total output power of one signal: power meter.
• Level of the carrier, harmonics or spurs at separate frequencies: spectrum analyzer.
• Loss, gain and reflection of the parts in the path: VNA.
• Unknown signal with possible spurs: analyzer first to see what is there, then a meter for total power.

In plain words · for students and beginners

RF power measurement in plain words

An everyday picture

Think of a bathroom scale and a kitchen with many separate jars. A power meter is like putting a whole shopping bag on the scale. You get one weight for everything inside. A spectrum analyzer is like sorting the bag and weighing each item on its own. A VNA is different: it is like checking how much water a pipe loses on the way from the tap to the glass. It tells you about the pipe, not about the water you already have.

The picture breaks down in one place. Items in a bag do not reflect off the scale, but a radio signal does reflect when the sensor does not match the source. Part of the power bounces back and is never counted.

The same example in simple steps

Say a small source gives 1 volt (rms, a standard way to state an AC voltage) into a 50 ohm load.

1. Power is the voltage squared, divided by 50. One squared is 1, and 1 / 50 is 0.02 watts.
2. 0.02 watts is 20 milliwatts.
3. dBm is a scale where 1 milliwatt is 0 dBm. Twenty milliwatts is 20 times that, and the result is about 13 dBm.
4. Put a 10 dB attenuator (a part that reduces the signal) in front of the instrument. Take away 10 dB: the meter now reads about 3 dBm.
5. To get back to the source, add the 10 dB: 3 + 10 = 13 dBm.

Mistakes beginners make

• Adding dBm values. 10 dBm plus 10 dBm is not 20 dBm. It is about 13 dBm, because two equal powers double, and doubling adds 3 dB.
• Forgetting the cable and attenuator loss. The instrument shows what reaches it, not what the source sent.
• Reading one marker on an analyzer and calling it the total power of a wide signal.
• Ignoring a poor match. If the load does not match, some power reflects and the reading is off.
• Using a VNA to read the absolute power of an unknown signal. It is built for ratios.

Remember

• Every +10 dB is ten times the power, and every +3 dB is about double.
• A power meter gives the total, an analyzer gives power by frequency, a VNA gives ratios.
• Add back every loss between the device and the instrument, and check the match.

Related pages on RF View

• dBm to Watts, Volts and dBµV Converter
• Return Loss vs Insertion Loss: What Each Number Tells You