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  BIRD-TYPE THRULINE DIRECTIONAL WATTMETER  —  TM-PWR-BIRD-001 Rev A
  Thruline body + interchangeable plug-in frequency/power elements
  Covers HF/VHF/UHF with separate slugs per band/range
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OPERATING PRINCIPLE
───────────────────────────────────────────────────────────────────────────────

  The Bird-type wattmeter uses a "thruline" body: a section of 50Ω coaxial
  line containing a transverse slot into which a plug-in "slug" element is
  inserted.  The slug contains a small directional coupler (loop or capacitive)
  that samples either the forward OR the reflected wave, depending on the slug
  orientation.  Rotating the slug 180° switches between FWD and REF readings.

  The thruline body maintains near-perfect 50Ω continuity whether or not a
  slug is inserted.  Without a slug, it is a transparent in-line connector.


BODY CROSS-SECTION (slug slot view)
───────────────────────────────────────────────────────────────────────────────

  RF IN (J1, SO-239)              RF OUT (J2, SO-239)
    │                                   │
    │←─────── CENTER CONDUCTOR ─────────│
    │                                   │
    │         ┌──────────────┐          │
    │         │  SLUG SLOT   │          │
    │─────────┤  (transverse │─────────-│
    │  OUTER  │   to RF axis)│  OUTER   │
    │  BODY   └──────┬───────┘  BODY    │
    │  (GND)         │          (GND)   │
    └────────────────┼──────────────────┘
                     │
               ┌─────┴──────┐
               │  SLUG      │
               │  ELEMENT   │
               │  (plugs in)│
               └─────┬──────┘
                     │
               METER JACK / BANANA


THRULINE BODY — ELECTRICAL SCHEMATIC
───────────────────────────────────────────────────────────────────────────────

  RF IN ─────────────────────────────────────────────── RF OUT
  (J1)   ←── Z0 = 50Ω mainline (center conductor) ──→  (J2)
  SO-239                                                SO-239
    │                   │SLOT│                            │
    │              ┌────┴────┴────┐                       │
    │              │   SLUG       │                       │
    │              │ ELEMENT      │                       │
    │              │              │                       │
    │              └──────┬───────┘                       │
    │                     │                               │
    │              METER OUTPUT                           │
    │              (J3, 2mm banana,                       │
    │               0–200mV FS)                           │
   GND                   GND                            GND


SLUG ELEMENT CROSS-SECTION  (forward power slug)
───────────────────────────────────────────────────────────────────────────────

  The slug is a cylindrical aluminum body, 1" diameter, 1" long (Bird Model 43
  compatible dimensions).  Inside the slug:

  RF CENTER ──────────────────────────────── (continues to body)
  CONDUCTOR     │                  │
                │    COUPLING      │
                │    LOOP (1T)     │
                │    or CARD       │
                │                  │
                └──────┬───────────┘
                       │
                    ┌──┴──┐
                    │ SLUG│
                    │SCHM │
                    │     │
                    └──┬──┘
                       │
                  METER OUTPUT


SLUG INTERNAL SCHEMATIC — FORWARD POWER DETECTION
───────────────────────────────────────────────────────────────────────────────

  The slug contains a directional coupler using a short coupling section.
  At HF: small ferrite bead or air-core loop
  At VHF: coupled microstrip card (PCB wafer inside slug)
  At UHF: capacitive coupling tab + microstrip

  COUPLING ELEMENT:

  Center conductor ──[LOOP L1, ~1T]──┬────────────────────
                                      │
                                 COUPLING
                                 WINDING (N2 turns)
                                      │
                              ┌───────┴────────┐
                              │                │
                           R_term             ►D1 SCHOTTKY
                           50Ω               1N5711 / HSMS
                              │                │
                              │           C_filter 10nF
                             GND               │
                                          R_load 10kΩ
                                               │
                                     METER OUTPUT (+)
                                               │
                                        (GND at meter jack)


  SLUG ORIENTATION:
    Slug inserted normal:  FWD coupling  → reads forward power
    Slug rotated 180°:     REF coupling  → reads reflected power
    SWR computed from two readings.


SLUG ELEMENT TYPES
───────────────────────────────────────────────────────────────────────────────

  Element  Freq Range    Power Range   Coupling   Core
  ───────  ────────────  ────────────  ─────────  ──────────────────
  HF-10W   1.8–30 MHz    1–10W         –26 dB     #43 bead, 3T/26T
  HF-100W  1.8–30 MHz    10–100W       –36 dB     #43 bead, 1T/20T
  HF-1KW   1.8–30 MHz    100–1000W     –46 dB     #43 bead, 1T/63T
  HF-3KW   1.8–30 MHz    1000–3000W    –50 dB     #43 bead, 1T/100T
  VHF-10W  50–148 MHz    1–10W         –26 dB     BN-43-202, 2T/20T
  VHF-100W 50–148 MHz    10–100W       –36 dB     BN-43-202, 1T/10T
  UHF-10W  420–1300 MHz  1–10W         –20 dB     Microstrip card
  UHF-100W 420–1300 MHz  10–100W       –30 dB     Microstrip card


METER CIRCUIT  (in body, driven by slug output)
───────────────────────────────────────────────────────────────────────────────

  SLUG     R_protect            METER
  OUTPUT ──[100Ω]──┬────────────┤M1 200µA FSD├──── GND
                   │
                  Z1 TVS 6.2V   (overload clamp)
                   │
                  GND

  Analog meter face provides two scales:
    — Watts scale  (calibrated for slug range, nonlinear due to V² law)
    — SWR scale    (% reflected overlay, labeled 1.0 – ∞)

  OPTIONAL DIGITAL INTERFACE:
    — ADS1115 samples slug output voltage (0–200 mV)
    — ESP32 computes power using calibration table
    — OLED or CYD display
    — BT/USB log output


METER BODY DIMENSIONS  (Bird Model 43 compatible)
───────────────────────────────────────────────────────────────────────────────

  Body:
    OD: 50mm  ×  155mm length
    Slug socket: 25.4mm (1") diameter cylinder, centered, transverse
    Connector: SO-239 both ends (or N-type for VHF/UHF version)
    Meter window: 70mm × 30mm on top face
    Retention spring for slug (radial O-ring groove)

  Slug:
    OD: 25.0mm  ×  25.4mm length
    Flat on one side (orientation key, asymmetric for FWD/REF)
    Banana jack: 2mm, bottom center, contacts body spring contact
    PCB inside: 22mm × 20mm wafer with coupler and detector

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  END OF BIRD-TYPE WATTMETER SCHEMATIC  — TM-PWR-BIRD-001 Rev A
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