Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers three power-rated dummy load designs (100W dry, 500W oil-cooled, 1 kW oil-cooled) and a precision 50Ω VNA calibration terminator. All designs target <1.1:1 SWR from 1.8 MHz through 450 MHz and <1.5:1 SWR through 1.3 GHz (VNA terminator only).

Chapter 2 — Theory of Operation

2-1 Resistor Network Fundamentals

Non-inductive resistors in parallel sets combine power ratings and maintain the 50Ω impedance. For N identical resistors each of value R in parallel: Z = R / N. Use:

• 100W load: 4 × 200Ω 25W resistors in parallel = 50Ω, 100W
• 500W load: 4 × 200Ω 125W in parallel = 50Ω, 500W
• 1 kW load: 8 × 400Ω 125W in parallel = 50Ω, 1000W

2-2 Parasitic Inductance and Frequency Limit

Each resistor lead has approximately 5–10 nH of lead inductance. At 150 MHz: X_L = 2π × 150e6 × 8e-9 = 7.5Ω, causing SWR to rise above 1.5:1. Minimizing lead length to ≤10 mm each side and using star wiring (all high ends bonded to a single center stud) reduces effective inductance to <2 nH, extending usable range to 450 MHz.

2-3 Oil Cooling

Transformer oil (mineral oil) provides thermal conductivity of 0.135 W/m·K, approximately 10× better than still air. Resistors submerged in oil can dissipate their full rated power continuously at 25°C ambient. Seal the container; oil expands approximately 7% from 20°C to 100°C.

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 100W Dry Load

1. Mount SO-239 / N connector on heatsink. Center conductor passes through to an M4 brass stud on the heatsink surface.
2. Cut resistor leads to ≤10 mm each side. Solder all high-side leads to center stud. Solder all low-side leads to ground bus on heatsink.
3. Orient resistors flat against heatsink surface for maximum thermal contact. Apply thermal compound between each resistor body and heatsink.
4. Verify DC resistance: 49–51Ω between connector center and shell.

4-2 500W / 1 kW Oil Load

1. Drill and mount N connector through lid of can. Extend center pin via threaded brass rod to center of can interior.
2. Wire resistors star-fashion around the center rod. Ground bus: bare copper wire ring soldered to can walls.
3. Test resistor assembly in air before sealing. Verify SWR <1.1:1 at 1.8 MHz through 50 MHz with NanoVNA.
4. Fill with mineral oil to within 25 mm of lid (room for thermal expansion). Seal lid with silicone RTV. Allow 24 hours to cure before transmitting.

Chapter 5 — Operating Procedures

1. Always confirm dummy load is connected before transmitting. A momentary key-down into open coax can damage the final transistor.
2. For CW/SSB testing: key down for ≤10 seconds, off for ≥30 seconds until thermal equilibrium is reached (approximately 15 minutes at full power).
3. For the oil loads: after extended use, check for oil seepage at the lid seal. If oil temperature exceeds 70°C (hot to the touch), reduce duty cycle or allow cooling.
4. Never use the 100W dry load above 100W continuous; use the oil loads for higher power or high duty cycle modes (FT8, WSPR, digital).

Chapter 6 — Calibration

1. Calibrate NanoVNA with SOLT at the measurement connector (not at the NanoVNA port).
2. Sweep 1.8 MHz to 450 MHz. Record SWR at 1.8, 3.5, 7, 14, 21, 28, 50, 144, 222, 432 MHz.
3. Acceptance criterion: SWR <1.1:1 at 1.8–50 MHz; SWR <1.3:1 at 50–150 MHz; SWR <1.5:1 at 150–450 MHz.
4. Record DC resistance: must be 49.0–51.0Ω.

Chapter 7 — Verification and Acceptance

1. Thermal test (100W load): apply 100W (from dummy transmitter or keyed transceiver) for 60 seconds. Heatsink must not exceed 80°C (use contact thermometer or thermal camera).
2. Thermal test (oil loads): apply rated power for 10 minutes. Oil must not bubble or emit smoke. If oil reaches 80°C, reduce power or duty cycle in future use.
3. Post-thermal SWR check: repeat SWR sweep immediately after thermal test. SWR must not have increased by more than 0.1 compared to cold measurement.
4. Log: date, resistors used, DC resistance, SWR at key bands, thermal test result, operator.

Appendix A — Power Derating Table

Appendix B — Resistor Selection

Non-inductive (wirewound non-inductive or thick-film) resistors only. Preferred types: Ohmite 270 series (wirewound NI), Vishay Dale RH series, Caddock MP series. Standard carbon film and metal film resistors are acceptable for the VNA calibration terminator (≤100 mW) but NOT for power loads (they are slightly inductive and not rated for RF).