Chapter 1 — Introduction and Scope
This manual covers three categories of RF power meter in common amateur use: the Bird Model 43 slug-type (directional, interchangeable elements), the inline directional wattmeter (fixed range, inline measurement), and the terminating wattmeter (power absorbed by a built-in load). Operating frequency range: 1.8 MHz to 1.3 GHz depending on element / design. Power ranges: 1W through 3 kW.
Chapter 2 — Theory of Operation
2-1 Bird Model 43 Slug Type
The Bird 43 is an inline directional wattmeter using interchangeable plug-in elements (slugs). Each slug contains a toroid directional coupler wound for a specific frequency range and power range. The slug's flat face orientation determines forward vs. reflected reading: flat toward the meter face = forward power; flat away = reflected. Output is a DC voltage proportional to forward (or reflected) power, driving an analog panel meter.
Slug PCB dimensions: 25.0 mm OD × 25.4 mm length. Retention: O-ring at body midpoint. Contact: 2 mm banana pin. The flatted-D cross-section prevents 90° orientation errors.
2-2 Inline Directional Meter
Uses a toroid coupler (1-turn primary = center conductor; N-turn secondary) plus Schottky diode detectors (1N5711) for forward and reflected channels. An ADS1115 16-bit I2C ADC digitizes both channels; ESP32 computes forward power, reflected power, and SWR in firmware. CYD display shows real-time readings. Typical frequency range 1.8–600 MHz with appropriate coupler design.
2-3 Terminating Wattmeter
A terminating wattmeter absorbs all transmitted power in an internal 50Ω load (dry or oil-cooled) and measures the absorbed power via a coupler before the load. Because there is no transmitted power beyond the instrument, no antenna is needed during testing. Used for final amplifier output power verification and transmitter tuning.
Chapter 3 — Equipment and Materials
3-1 HF Plug-In Element (Bird 43 Compatible)
Slug body: 6061-T6 aluminum, black anodized, D-shaped cross section. PCB: 22×18 mm FR4. Key components per power/band range:
| Range | N2 turns | Core | Frequency |
|---|---|---|---|
| 100W / HF | 6 | Fair-Rite 5943000401 (#43, 9mm) | 1.8–30 MHz |
| 250W / HF | 10 | Same | 1.8–30 MHz |
| 1000W / HF | 32 | BN-43-202 | 1.8–30 MHz |
| 100W / VHF | 6 | Fair-Rite #67, 9mm | 50–175 MHz |
| 100W / UHF | Microstrip coupled | PCB | 420–1000 MHz |
3-2 Bird 43 Body
The Bird 43 body (if homebrew): SO-239 or N-type input/output connectors on a milled aluminum body with a 25.0 mm diameter slug cavity. Spring-pin contact engages the slug banana jack. Retention sleeve threads onto body.
Chapter 4 — Slug Construction
- Mill slug body to dimensions (OD 25.0 mm, length 25.4 mm). Machine the D-flat on one side. Drill and tap center hole for contact pin (2 mm banana jack, gold-plated).
- Fabricate PCB (22×18 mm). Drill 3 mm center hole for slug body contact. Route coupler, diode, and filter on PCB.
- Wind secondary winding on core per Table 3-1. Solder to PCB pads.
- Install O-ring in groove. Press PCB into slug body; solder banana pin.
- Mark power range and frequency on flat face (laser engraving or paint pen).
Chapter 5 — Operating Procedures
5-1 Bird 43 Use
- Insert correct slug for operating frequency and expected power level. Orient flat face for forward or reflected measurement.
- Connect IN to transmitter, OUT to antenna (or dummy load). Key transmitter at target power. Read panel meter. For reflected power: rotate slug 180° (flat face away from meter).
- Compute SWR from P_fwd and P_ref readings.
5-2 Inline Digital Meter
- Insert meter inline between transmitter and antenna (any orientation; no slug rotation needed).
- CYD displays P_fwd (W), P_ref (W), SWR simultaneously. Log readings to SD card if installed.
Chapter 6 — Calibration
- Connect calibrated power source to IN; calibrated 50Ω load to OUT.
- Apply 10W at 14.175 MHz. Panel meter (Bird 43) or CYD should read 10 ± 1W. Adjust calibration pot R_cal (Bird 43 body) if needed. For digital inline meter: update calibration coefficient in NVS.
- Verify at 50W and 100W. Response must be linear within ±5%.
- Repeat for each slug / frequency range in use.
Chapter 7 — Verification and Acceptance
- Power measurement within ±5% of reference (calibrated signal source) at all test frequencies.
- SWR reading within ±0.1 of calculated SWR for a known mismatch (e.g., 100Ω load gives SWR 2.0:1 into 50Ω system).
- Insertion loss with slug installed: <0.1 dB measured by comparing received signal with and without meter inline.
- Log: date, slug serial number (or homebrew tag), frequency, measured power vs. reference, SWR accuracy, operator.
Appendix A — Common Bird 43 Slug Equivalents
| Bird part | Power | Frequency | Homebrew N2 |
|---|---|---|---|
| 5H | 5W | 25–60 MHz | 6 turns, #67 core |
| 50H | 50W | 25–60 MHz | 10 turns, #67 core |
| 100D | 100W | 400–1000 MHz | Microstrip PCB element |
| 250B | 250W | 2–30 MHz | 10 turns, #43 core |
Appendix B — SWR from P_fwd / P_ref
|Γ| = sqrt(P_ref / P_fwd) SWR = (1 + |Γ|) / (1 − |Γ|) Example: P_fwd = 100W, P_ref = 5W |Γ| = sqrt(5/100) = sqrt(0.05) = 0.224 SWR = (1.224) / (0.776) = 1.58:1