Section 3: Power Measurement Reference
The tables you will actually photocopy.
The last section of any useful field book is the part you end up dog-earing. Quick lookup tables. Conversions between the units that engineers and standards bodies argue about. A product guide with enough information to narrow the options before you call sales. That is what Section 3 aims to be.
Chapter 10: Reference Tables
10.1 Amplitude Measurement Conversions
RF engineers spend most of their lives toggling between dBm, watts, mV, and VSWR. The following table covers the most common conversions for a 50-ohm system.
| dBm | mW | µW | Vrms (50 Ω) | Vpk (50 Ω) |
|---|---|---|---|---|
| +30 | 1000 | 1 × 10⁶ | 7.071 | 10.000 |
| +20 | 100 | 1 × 10⁵ | 2.236 | 3.162 |
| +10 | 10 | 1 × 10⁴ | 0.707 | 1.000 |
| 0 | 1 | 1000 | 0.224 | 0.316 |
| -10 | 0.1 | 100 | 0.0707 | 0.100 |
| -20 | 0.01 | 10 | 0.0224 | 0.0316 |
| -30 | 0.001 | 1 | 0.00707 | 0.0100 |
| -40 | 1 × 10⁻⁴ | 0.1 | 2.24 mV | 3.16 mV |
| -50 | 1 × 10⁻⁵ | 0.01 | 707 µV | 1.00 mV |
| -60 | 1 × 10⁻⁶ | 0.001 | 224 µV | 316 µV |
| -70 | 1 × 10⁻⁷ | 1 × 10⁻⁴ | 70.7 µV | 100 µV |
Quick mental shortcuts: - +3 dB ≈ double the power. +10 dB = ten times the power. - +6 dB ≈ double the voltage. +20 dB = ten times the voltage. - 0 dBm = 1 mW. Memorize this one. Everything hangs off it.
10.2 Return Loss, Reflection Coefficient, and VSWR
Three different ways to describe the same thing: how much of your signal is being reflected back at you.
| Return Loss (dB) | Reflection Coefficient (Γ) | VSWR | % Power Reflected |
|---|---|---|---|
| 40 | 0.010 | 1.020 | 0.01 |
| 30 | 0.032 | 1.065 | 0.10 |
| 25 | 0.056 | 1.119 | 0.32 |
| 20 | 0.100 | 1.222 | 1.00 |
| 15 | 0.178 | 1.433 | 3.16 |
| 14 | 0.200 | 1.500 | 3.98 |
| 10 | 0.316 | 1.925 | 10.0 |
| 7 | 0.447 | 2.615 | 20.0 |
| 6 | 0.501 | 3.010 | 25.1 |
| 3 | 0.708 | 5.848 | 50.1 |
| 0 | 1.000 | ∞ | 100 |
Rule of thumb: for laboratory measurements, look for VSWR better than 1.2 (about 20 dB return loss) at every connector in the path. Below that, mismatch becomes a dominant uncertainty contributor.
10.3 Wireless and Radar/Microwave Bands
| Band | Frequency Range | Primary Use |
|---|---|---|
| HF | 3 to 30 MHz | Shortwave, amateur |
| VHF | 30 to 300 MHz | FM broadcast, VHF TV, land mobile |
| UHF | 300 MHz to 3 GHz | UHF TV, cellular, GPS, Wi-Fi 2.4 GHz |
| L band | 1 to 2 GHz | GPS, DME, airborne radar |
| S band | 2 to 4 GHz | Weather radar, Wi-Fi 2.4, Bluetooth |
| C band | 4 to 8 GHz | Satellite downlinks, Wi-Fi 5 |
| X band | 8 to 12 GHz | Military radar, satcom |
| Ku band | 12 to 18 GHz | DBS television, VSAT |
| K band | 18 to 27 GHz | Automotive radar, police radar |
| Ka band | 27 to 40 GHz | Satellite uplinks, 5G mmWave |
| V band | 40 to 75 GHz | 5G mmWave, automotive radar |
| W band | 75 to 110 GHz | Automotive radar, imaging |
10.4 Sensor Cable Length Effects
A USB power sensor with a long cable behaves differently than the same sensor at the end of a short cable. Cable length affects triggering, signal integrity, and power delivery to the sensor.
USB 2.0 standard cables work well up to about 5 meters. Beyond that, hub-to-hub extension or active USB cables are needed. Active USB repeaters can push the reach to 15 or 20 meters without protocol issues.
Triggered measurements over long cables should use hardware triggering rather than software, to avoid USB round-trip latency.
For LAN-connected sensors (such as certain Berkeley Nucleonics variants with network interfaces), cable length is effectively unlimited up to standard Ethernet distances (100 m per segment).