================================================================================ TERMINATING POWER METER WITH DUMMY LOAD — TM-PWR-TERM-001 Rev A True absorbing load + directional sample: 1W–3kW, DC–1300 MHz Used for transmitter testing, final amplifier tuning, TX characterization ================================================================================ PRINCIPLE OF OPERATION ─────────────────────────────────────────────────────────────────────────────── Unlike the thruline/inline meter (where RF continues to an antenna), a terminating power meter absorbs ALL transmitted power in a precision load. A small fraction is sampled by a coupling circuit, detected, and measured. No RF passes through to any output connector. Advantages: — Absolute power measurement with no reflections — No antenna or load mismatch affects reading — Safe for any output impedance transmitter — Used for output power calibration and spectral purity tests Disadvantages: — All power dissipated as heat (requires thermal management) — Cannot test antenna system match — Power capability limited by load thermal dissipation TERMINATING POWER METER BLOCK DIAGRAM ─────────────────────────────────────────────────────────────────────────────── RF INPUT J1 SO-239 │ │ ┌───────────┴───────────┐ │ │ RF COUPLER │ (–30 dB sample) │ │ │ DETECTOR RF LOAD D1/D2 50Ω Schottky (see below) │ │ ADC/METER │ DISPLAY │ GND RF LOAD DESIGNS ─────────────────────────────────────────────────────────────────────────────── LOAD TYPE 1: Non-inductive resistor array (≤ 100W) ─────────────────────────────────────────────────── RF IN ──┬──[R1 200Ω]─┐ ├──[R2 200Ω]─┤ ├──[R3 200Ω]─┤──── GND ├──[R4 200Ω]─┤ └──[R5 200Ω]─┘ 5 × 200Ω in parallel = 40Ω → use 5 × 200Ω 25W wirewound non-inductive Or 4 × 200Ω = 50Ω → use 4 × 200Ω 25W Better: 8 × 400Ω = 50Ω, each 25W → 200W total capacity Mount on aluminum heatsink or bolt to chassis. Non-inductive winding note: Standard wirewound resistors are INDUCTIVE. For RF use, specify non-inductive (bifilar wound) or use carbon composition. Above 30 MHz, use thick-film or metal-oxide resistors (lower inductance). Above 150 MHz, chip resistors (0805 or 1206) are preferred. LOAD TYPE 2: Oil-cooled resistor (100W–1kW) ───────────────────────────────────────────── Use 1 × 50Ω 100W non-inductive RFP resistor (e.g., RFP100-50) Immerse in mineral oil (transformer oil, heat-transfer fluid) or mount on aluminum chassis with thermal compound. Dissipation: 100W → 1 kg aluminum heatsink or oil bath. 1 kW requires forced air cooling or oil tank with pump. LOAD TYPE 3: Coaxial termination (UHF, 50W) ───────────────────────────────────────────── Uses coaxial resistor: Bird 8830, Weinschel 40-50, or homebuilt. Inner resistive element: thin-film 50Ω chip mounted inside N-type body. Rated to 6 GHz, 50W CW. For homebuilt: SMA or N-type bulkhead with 2 × 100Ω 0402 in parallel, inner series stub trimmed for VSWR < 1.2 to 1300 MHz. SAMPLING COUPLER CIRCUIT (for terminating meter) ─────────────────────────────────────────────────────────────────────────────── Since the load absorbs all power, only FWD power sampling is needed. A directional coupler or capacitive divider is used. METHOD A: Resistive voltage divider (DC–1 GHz, broadband) ────────────────────────────────────────────────────────── RF IN ──┬──────────────────────────────── [R_LOAD 50Ω]──GND │ [R_hi] (voltage divider, high side) │ ├──────────────────────────────── V_sample │ [R_lo] (divider low side, to GND) │ GND For –30 dB sample: R_hi = 1580Ω, R_lo = 51Ω (Thevenin equivalent) V_sample = V_rf × R_lo / (R_hi + R_lo) = V_rf × 51/1631 = V_rf × 0.031 = –30dB Advantage: flat from DC to 1+ GHz (resistive, no inductance issues) Disadvantage: R_hi + R_lo dissipates power (reduce R_hi to 500Ω for ~22 dB) METHOD B: Toroid coupler in-line to load (HF only, preferred) ────────────────────────────────────────────────────────────── RF IN ──[T1 1T/32T FT-114-43]──[R_LOAD 50Ω]──GND │ Detector circuit (same as inline meter) This gives true directional coupling but requires the toroid to handle full RF power. The toroid primary carries full transmitter current. Verify core saturation for the power level (see manual Chapter 4). METHOD C: Capacitive tap (VHF/UHF, broadband) ────────────────────────────────────────────── RF IN ──┬──[C_main 1–10 pF]──[R_LOAD 50Ω]──GND │ [C_tap 1 pF] (small capacitive divider) │ ├─────────────────── V_sample ──► Detector │ [R_term 50Ω] │ GND Divider ratio: C_tap / (C_main + C_tap) = 1/(10+1) = –20.8 dB Works well from 100 MHz to 2+ GHz. Adjust C_tap value for desired coupling level. TRUE RMS DETECTOR CIRCUIT ─────────────────────────────────────────────────────────────────────────────── For accurate measurement of SSB, AM, or complex modulation, true RMS detection is required. Two approaches: APPROACH 1: Log-domain RMS (AD8361 or LTC5596) ───────────────────────────────────────────────── RF INPUT ──[R_att]──► AD8361 VRMS PIN │ V_rms output (DC) │ ─────► ADS1115 A0 AD8361 (30 MHz – 2.5 GHz, –52 to 0 dBm range): V_out = 7.0 mV/µW × P_in (in square-law region) V_out = 250 mV at P = 0 dBm (1 mW into 50Ω) Dynamic range: 50 dB LTC5596 (100 MHz – 40 GHz): V_out = 35 mV/dB (logarithmic) Calibrate in dBm, convert to Watts APPROACH 2: Squaring + integration (firmware) ───────────────────────────────────────────── For signals < 100 MHz (within ESP32 ADC sampling rate): — Sample V_det at 50 kHz (fast ADC with external circuit) — Square each sample: V²[n] — Average: V²_rms = (1/N) Σ V²[n] — P_rms = V²_rms / (2 × 50) Practical: Use ADS1115 at 860 SPS for slowly-varying envelopes. For true high-frequency RMS, use log detector IC (AD8361 preferred). COMPLETE TERMINATING METER SCHEMATIC ─────────────────────────────────────────────────────────────────────────────── RF IN ──────┬───────────────────────────────── RF LOAD (50Ω, N×W) (J1 SO-239) │ │ │ COUPLING SECTION GND │ (Method A or B per band) │ │ │ V_SAMPLE │ │ │ ─►─[D1 1N5711]──┬───── V_DET_AVG │ (Schottky) │ │ R_term 50Ω C_hold 10nF │ │ │ │ GND GND │ │ ─►─[D2 BAT54]──┬───── V_DET_PEAK │ (peak det.) │ │ C_pk 1µF │ R_leak 1MΩ │ │ │ GND │ │ NTC1 ──────────────── TEMP SENSE │ └────── ADS1115 (A0=AVG, A1=PEAK, A2=TEMP) I2C → ESP32 or CYD THERMAL DESIGN (100W load) ─────────────────────────────────────────────────────────────────────────────── Power handling: 100W continuous Resistor choice: 2 × RFP100-50 (100W, 50Ω non-inductive, radial) or 4 × 200Ω 25W in parallel Heatsink required: θ_sa = (T_case_max – T_amb) / P at P=100W, T_case=85°C, T_amb=25°C: θ_sa = (85–25) / 100 = 0.6 °C/W Heatsink size: 200mm × 150mm × 2mm aluminum plate ≈ 0.5–0.8 °C/W Fan: 40mm 5V DC fan (add if θ_sa needed < 0.5 °C/W) Thermal sensor: DS18B20 1-Wire on heatsink, GPIO to ESP32 Firmware action: Thermal cutoff warning at T > 70°C Auto-shutdown indication at T > 85°C (does not disconnect RF — user must remove drive) WARNING: At 1kW, thermal mass is critical. Use forced-air cooling and add thermal runaway protection relay on RF input line. BILL OF MATERIALS (Terminating Meter, 100W version) ─────────────────────────────────────────────────────────────────────────────── Ref Value Description ───────── ─────────────────── ───────────────────────────────────────── J1 SO-239 RF input, silver-plated R_LOAD 2× RFP100-50 100W non-inductive 50Ω RF resistor T1 FT-114-43 Toroid coupler (HF version) D1 1N5711 Average detector, DO-35 D2 BAT54S Peak detector, SOT-23 C_hold 10nF NP0 Average hold capacitor C_pk 1µF/16V Peak hold capacitor R_leak 1MΩ Peak hold decay resistor NTC1 10kΩ/B3950 Thermistor, mounted on heatsink U1 ADS1115 16-bit I2C ADC U2 ESP32-WROOM Microcontroller (or CYD for display) ENCL 200×150×80mm Al Diecast or bent aluminum ================================================================================ END OF TERMINATING METER — TM-PWR-TERM-001 Rev A ================================================================================