Field Guide · hardware

Also known as: RF power meter, power meter, power sensor

An RF power meter measures the absolute power of a radio-frequency signal and reports it in dBm or watts.1 Where a spectrum analyzer shows how power is distributed across frequency, a power meter answers a simpler, more accurate question: how much total power is in this signal? — the measurement you need to set a transmitter’s output, verify an amplifier’s gain in decibels, or characterize a dummy load.

RF in Sensor head (detector) +37.0 dBm (≈ 5 W)
An RF power meter: a calibrated sensor head converts applied RF power to a DC signal, which the meter displays as an absolute reading in dBm (and watts).

How it works

A power meter is a sensor head plus a display unit. The sensor converts RF power to a proportional DC voltage; the meter applies calibration factors and shows the result. Three sensor technologies dominate:

  • Thermocouple / thermistor (thermal) sensors heat a load with the RF and measure the temperature rise. They respond to true average power regardless of waveform, are very accurate, and are the reference standard — but they are slower and less sensitive.
  • Diode detectors rectify the RF. In their square-law region they, too, read true power; driven harder they approach peak detection. Diode sensors are fast and sensitive (reading to −70 dBm or lower), which makes them the basis of most inexpensive and wideband meters, but they need correction to stay accurate on high- crest-factor modulated signals.

Modern wideband/peak power sensors digitize the detected envelope, so they report average, peak, and even a full power-versus-time trace for burst TDMA waveforms — important because a simple average reading understates the peak power of a bursty or high-PAPR signal.

In practice

  • Never exceed the sensor’s damage level. Sensors are low-power devices; measure a transmitter through a calibrated attenuator or a directional coupler, not by connecting the sensor straight to a PA.
  • Match the sensor to the waveform. Use a true-average (thermal) or corrected wideband sensor for modulated signals; a plain diode reading can err on high-PAPR waveforms.
  • Mind frequency range and cal factors. Each sensor is calibrated per frequency; applying the right cal factor is what turns a raw reading into an accurate dBm value.
  • Relative vs. absolute. For a quick gain check you need only a consistent relative reading; for compliance or link budgets you need traceable absolute accuracy.

Relevance to SDR

RF power meters live on the transmit side, so they matter less to receive-only SDR scanning than a spectrum analyzer or VNA do — an SDR reports level in relative dBFS, not calibrated dBm. Where a power meter does help the SDR enthusiast is on the bench: setting the output of a signal generator or the transmit level of a HackRF/LimeSDR, verifying amplifier gain, and confirming a dummy load is dissipating the expected power. GopherTrunk is a receiver and neither measures nor generates absolute power; a power meter is a general RF-bench aid, not part of its decode path.

Sources

  1. Power measurement — Wikipedia, on RF/microwave power measurement and thermal versus diode sensors. ↩

See also