================================================================================ PORTABLE RF POWER METER - SCHEMATIC DIAGRAM Wideband: 1.8 MHz - 1296 MHz (160M - 20cm Ham Bands) ================================================================================ MAIN CIRCUIT SCHEMATIC (ASCII) ================================================================================ ANTENNA INPUT (SO-239) | | +------+------+ | | R1 C1 100Ω 100pF | | +----------+-------------+----------+ | | | | R2 R3 R4 C2 50Ω 50Ω 50Ω 100pF | | | | | +----+----+ | | | | | | | | C3 FWD | | | 100pF DETECTOR | | | | | | | | +----+----+ | | | | | | | R5 | | | 1kΩ | | | | | | | +----+----+ | | | | | | | | C4 D1 | | | 100nF BAT15-03W | | | | | | | | +----+----+ | | | | | | | R6 | | | 10kΩ | | | | | | | +----+----+ | | | | | | | | C5 FWD_OUT | | | 10µF | | | | | | | | | GND TO ADC0 | | | | | +----------+-------------+----------+ | | C6 REV 100pF DETECTOR | | +-----+-----+ +----+----+ | | | | R7 R8 R9 D2 50Ω 50Ω 1kΩ BAT15-03W | | | | TRANSMITTER | C7 R10 OUTPUT | 100nF 10kΩ (SO-239) | | | | | C8 REV_OUT | | 10µF | GND GND| TO ADC1 GND POWER SUPPLY SECTION ================================================================================ 9V BATTERY (or USB 5V via LDO) | +--- S1 (Power Switch) | +---+--- C9 (100µF) --- GND | | | +--- U1 (LM7805 or LP2950-5.0) | | | | | | VIN GND | | | | | VOUT | | | | +--------+--- C10 (10µF) --- GND | | | +--- +5V RAIL | +--- LED1 (Power Indicator) --- R11 (1kΩ) --- GND MICROCONTROLLER SECTION (Arduino Nano or ATmega328P) ================================================================================ ATmega328P / Arduino Nano +-------------------------+ | | | ADC0 <--- FWD_OUT | Forward Power Reading | ADC1 <--- REV_OUT | Reverse Power Reading | ADC6 <--- Battery Mon | Battery Voltage Monitor | | | D2-D9 ---> LCD Data | 16x2 LCD Display | D10 ---> LCD RS | | D11 ---> LCD Enable | | | | D12 <--- CAL Button | Calibration Button | D13 <--- MODE Button | Mode Select Button | | | VCC <--- +5V | | GND <--- GND | +-------------------------+ LCD DISPLAY (16x2 Character LCD) ================================================================================ 16x2 LCD Module +------------------+ | VSS --- GND | | VDD --- +5V | | V0 --- Contrast (10kΩ pot to GND) | RS --- D10 | | RW --- GND | | E --- D11 | | D0-3 --- NC | (4-bit mode) | D4 --- D2 | | D5 --- D3 | | D6 --- D4 | | D7 --- D5 | | A --- +5V (via 220Ω) Backlight | K --- GND | +------------------+ BATTERY MONITOR ================================================================================ 9V Battery | +--- R12 (10kΩ) | | +--------+--- TO ADC6 | R13 (10kΩ) | GND Voltage divider for monitoring battery voltage USER INTERFACE ================================================================================ CAL Button: D12 --- Switch --- GND (with 10kΩ pullup) MODE Button: D13 --- Switch --- GND (with 10kΩ pullup) ================================================================================ COMPONENT DETAILS ================================================================================ DIRECTIONAL COUPLER: - Resistive bridge coupler using 50Ω resistors - Provides ~20dB coupling - Frequency response: DC to >2 GHz - Forward and reverse power sampling RF DETECTORS: - D1, D2: BAT15-03W Schottky diodes (or SMS7630) - Low barrier voltage for sensitivity - Fast response time - Wide frequency range POWER SUPPLY: - Input: 9V battery or USB 5V - Regulator: LM7805 or LP2950-5.0 (low dropout) - Output: Regulated +5V for logic and display MICROCONTROLLER: - ATmega328P (Arduino Nano compatible) - 10-bit ADC for power measurement - Multiple display modes and calibration CONNECTORS: - RF Input: SO-239 (UHF) female connector - RF Output: SO-239 (UHF) female connector - 50Ω impedance throughout signal path ================================================================================ CIRCUIT THEORY OF OPERATION ================================================================================ The RF power meter uses a resistive bridge directional coupler to sample both forward and reflected power. The coupler provides approximately 20dB of coupling, meaning 1% of the signal power is sampled. DIRECTIONAL COUPLER: The resistive bridge uses four 50Ω resistors (R2, R3, R4, R7) arranged in a bridge configuration. When properly balanced, forward power appears at the FWD port and reverse power at the REV port. R1 and C1 provide input matching and DC blocking. RF DETECTION: Schottky diodes (D1, D2) rectify the RF signal. The low forward voltage drop of Schottky diodes provides better sensitivity at low power levels. R5 and R9 provide DC return paths, while C4 and C7 filter the RF, leaving only the DC component proportional to RF power. OUTPUT FILTERING: R6/C5 and R10/C8 form low-pass filters with time constants of approximately 100ms, providing smoothing of the detected voltage while maintaining reasonable response time for SSB and CW operation. POWER MEASUREMENT: The microcontroller reads the filtered DC voltages through its ADC inputs. Using logarithmic curve fitting or lookup tables, the ADC values are converted to power readings in watts. The firmware implements calibration factors to compensate for frequency response variations. SWR CALCULATION: SWR is calculated from forward and reverse power: Reflection Coefficient (ρ) = sqrt(P_rev / P_fwd) SWR = (1 + ρ) / (1 - ρ) ================================================================================ PERFORMANCE SPECIFICATIONS ================================================================================ Frequency Range: 1.8 - 1296 MHz (all specified ham bands) Power Range: 0.1W - 100W (with calibration) Accuracy: ±10% (after calibration) Input Impedance: 50Ω Insertion Loss: <0.2 dB Power Supply: 9V battery or USB 5V Current Draw: ~50 mA (LCD backlight on) ~20 mA (LCD backlight off) Battery Life: ~10 hours (9V alkaline, backlight on) Display: 16x2 character LCD Update Rate: 2-5 Hz Connector Type: SO-239 (UHF) female ================================================================================ NOTES ================================================================================ 1. All RF traces should be 50Ω microstrip or use coaxial construction 2. Keep detector circuits compact to minimize stray capacitance 3. Use RF-quality capacitors (C0G/NP0) for all RF coupling capacitors 4. Ensure good grounding throughout the RF section 5. Calibration should be performed at multiple frequencies and power levels 6. Temperature compensation may be needed for high accuracy 7. Consider adding protection diodes at ADC inputs for static protection ================================================================================ END OF SCHEMATIC ================================================================================