UNCLASSIFIED
TM-CAL-014
50-OHM DUMMY LOAD POWER STANDARD
Non-Reactive Resistive Load for RF Power Measurement and Transmitter Testing
Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers construction and calibration of a 50 Ω non-reactive dummy load for measuring RF power. The load is used with a calibrated voltmeter or directional coupler to verify transmitter power output and calibrate wattmeters.
1-3. SAFETY
WARNINGRF power above 1 watt represents significant electric field intensity at close range. Do not contact the antenna output of a transmitter during operation. Always connect dummy load before applying RF drive.
CAUTIONResistor thermal rating: ensure dummy load power dissipation rating exceeds transmitter peak output by at least 2×. An oil-cooled load handles higher power but requires sealed construction.
CHAPTER 2 — THEORY OF OPERATION
2-1. RESISTIVE LOAD PRINCIPLE
Power delivered to a resistive load equals P = V2/R = I2R. For a 50 Ω load driven by an RF source matched to 50 Ω:
Power from RMS voltageP (W) = VRMS2 / 50
RMS voltage for given powerVRMS = √(P × 50)
| Power (W) | VRMS (V) | Vpeak (V) | IRMS (A) |
|---|---|---|---|
| 0.1 | 2.24 | 3.16 | 0.045 |
| 1.0 | 7.07 | 10.0 | 0.141 |
| 5.0 | 15.8 | 22.4 | 0.316 |
| 10.0 | 22.4 | 31.6 | 0.447 |
| 100.0 | 70.7 | 100.0 | 1.41 |
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. DRY LOAD (UP TO 25 W)
| Qty | Item | Specification |
|---|---|---|
| 10 | Non-inductive resistor | 500 Ω 5 W each, carbon film or metal oxide |
| 1 | BNC or PL-259 chassis connector | For RF connection |
| 1 | Copper-clad PCB or ground plane | For resistor mounting |
| 1 | Aluminum enclosure | With ventilation or heatsink |
3-2. CONSTRUCTION NOTES
Ten 500 Ω resistors in parallel = 50 Ω total. Each carries 1/10 of total power. For 25 W load, each resistor handles 2.5 W (rated 5 W). Mount resistors radially from center BNC connector with leads <10 mm. Keep all leads equal length to maintain SWR flatness to ≥100 MHz.
NOTECarbon composition resistors have good non-inductance but poor power stability. Metal oxide film is preferred: non-inductive and stable. Avoid wirewound at RF frequencies due to inductance.
CHAPTER 4 — ASSEMBLY PROCEDURES
- Drill BNC mounting hole in center of ground plane. Mount BNC chassis connector.
- Cut 10 resistor leads to equal length (15 mm each). Bend leads outward radially.
- Solder all resistor leads to BNC center pin simultaneously (one end) and to ground plane (other end). Use minimum solder.
- Verify 50 Ω with DMM at DC. Should read 50 Ω ±5%.
- Install in enclosure with ventilation holes (or heat-sink adhesive for high power).
- Apply label: '50 Ω 25 W DUMMY LOAD — NON-INDUCTIVE'.
CHAPTER 5 — CALIBRATION PROCEDURE
5-1. DC RESISTANCE VERIFICATION
- Measure DC resistance at BNC terminals with calibrated ohmmeter.
- Should read 50.0 ±2.5 Ω (5%).
- 4-wire measurement preferred for accuracy (TM-CAL-010).
5-2. RF POWER CALIBRATION
- Connect load to NanoVNA PORT 1. Measure |S11| from 1–100 MHz.
- SWR <1.5:1 (|S11| < −14 dB) across full range confirms non-reactive construction.
- For power measurement: connect calibrated Bird 43 wattmeter or TinySA tracking generator source in series with load.
- Measure VRMS at load terminals: Pcalc = V2/50.
- Compare Pcalc against wattmeter reading. Agreement <10% is adequate.
CHAPTER 6 — TUNING AND ADJUSTMENT
NOTEResistance can be adjusted within ±10% by replacing individual resistors. For exact 50.00 Ω: measure all 10 resistors before assembly, select from characterized lot, combine to achieve closest total.
CHAPTER 7 — VERIFICATION
- Apply 5 W from QRP transmitter to dummy load for 60 seconds. Verify resistors do not reach unsafe temperature (<100°C surface).
- Measure |S11| after thermal cycling to confirm mechanical stability.
- Log: DC resistance, |S11| at 10 and 50 MHz, power test level, date.
APPENDIX A — CALCULATIONS AND FORMULAS
Parallel resistanceRtotal = Reach / N for N equal resistors
10 × 500 ΩR = 500/10 = 50 Ω
SWR from Rload and Z0SWR = Rload/Z0 if Rload > Z0, else Z0/Rload
Maximum power per resistorPeach = Ptotal / N
APPENDIX B — EXAMPLE RESULTS
| Measurement | Expected | Measured | Pass/Fail |
|---|---|---|---|
| DC resistance | 50.0 Ω | 50.4 Ω | Pass |
| |S11| at 10 MHz | <−14 dB | −28 dB | Pass |
| |S11| at 50 MHz | <−14 dB | −22 dB | Pass |
| |S11| at 100 MHz | <−14 dB | −17 dB | Pass |
| Thermal (5 W, 60 s) | <100°C | 68°C | Pass |