RF Power and Antenna Measurements
Measuring RF Power
The Directional Wattmeter
The most common power measurement tool in amateur radio. It measures forward power (going to the antenna) and reflected power (coming back) simultaneously.
- Net power to antenna = Forward − Reflected
- Example: Forward reads 100W, reflected reads 10W → antenna receives 90W
- You can also calculate SWR from forward and reflected readings
The Dummy Load
A non-radiating 50 Ω resistive load for testing transmitters without going on air. Essential for:
- Checking transmitter output power
- Adjusting drive levels
- Testing without causing interference
Good practice: Always test into a dummy load first, then switch to the antenna. This protects others from your test signals and lets you check everything before going on air.
dBm and dBW — Power in Decibels
Instead of saying "0.001 watts" or "400 watts," we use decibels referenced to a standard power level:
| dBm (ref: 1 mW) | dBW (ref: 1 W) | Actual Power |
|---|---|---|
| 0 dBm | −30 dBW | 1 mW |
| +30 dBm | 0 dBW | 1 W |
| +40 dBm | +10 dBW | 10 W |
| +50 dBm | +20 dBW | 100 W |
| +56 dBm | +26 dBW | 400 W (Advanced limit) |
The Antenna Analyser
One of the most useful instruments for any antenna experimenter. It sweeps across a frequency range and shows:
- SWR vs frequency: Shows you exactly where your antenna is resonant and how wide the usable bandwidth is
- Impedance (R + jX): Tells you if the antenna needs to be longer (capacitive, −jX) or shorter (inductive, +jX)
- Return loss: Another way to express how well the antenna matches 50 Ω
Modern antenna analysers like the NanoVNA have made this capability very affordable.
Frequency Counter
Directly measures the frequency of a signal. Key consideration: gate time determines resolution:
- 1 second gate → 1 Hz resolution
- 10 second gate → 0.1 Hz resolution
GPS-disciplined counters provide the highest accuracy timebase.