================================================================================ SCHEMATIC: WHEATSTONE BRIDGE CIRCUIT TM-NB-BRIDGE Rev A RF Noise Bridge, 0.5 - 60 MHz Measures: R (0-200 ohm), X (-200 to +200 ohm) ================================================================================ BRIDGE TOPOLOGY --------------- Standard 4-arm Wheatstone bridge adapted for RF impedance measurement. Two fixed arms (R1=R2=51 ohm) establish the bridge reference. One variable arm (R_cal + jX_cal) is the calibrated reference. One unknown arm (Z_ant) is the antenna under test. The null detector sees zero voltage when bridge is balanced. NOISE IN (from generator) | [T1 pri] (1:1:1 trifilar transformer, 50 ohm all ports) / \ T1 sec-A T1 sec-B (two 50-ohm bridge drive ports) | | R1 R2 (51R) (51R) | | +---[DET]----+ <-- NULL DETECTOR connected across bridge midpoints | | Z_CAL J_ANT (R_var+jX) (BNC to | antenna) | | GND GND FULL SCHEMATIC (all components): NOISE IN (J3, SMA) | C5 (100pF) <-- DC block | T1 primary (6 turns #26 AWG on BN-43-202) | GND (T1 center tap if used; for trifilar this is the common) T1 WINDING DETAIL (BN-43-202 binocular core): Wind all three windings simultaneously (trifilar): - Wire A: Noise In winding (IN+ to IN-) - Wire B: Calibration arm winding (CAL+ to CAL-) - Wire C: Antenna arm winding (ANT+ to ANT-) Number of turns: 6 trifilar (18 total wire passes through core) Wire: #26 AWG Teflon or enameled Label and verify with continuity meter before assembly. Interleave twist: twist all 3 wires together before winding. T1 EQUIVALENT CIRCUIT: Each secondary winding presents 50 ohm to its termination. Total impedance transformation: 1:1 (equal windings). Insertion loss < 0.5 dB, 1-60 MHz. Core material: Mix 43 (Mn-Zn ferrite) -- good 0.5-100 MHz. Alternatives: BN-61-202 (Mix 61, better above 30 MHz) BN-43-3312 (larger, higher power) RESISTANCE ARM (R_cal): ======================== T1 sec-A (arm terminal) | R4 (10 ohm, 1%, 1/4W fixed) <-- minimum R offset (prevents R=0) | VR1 (0-200 ohm, 10-turn cermet pot, Bourns 3590S-2-201) | (or 0-200 ohm multi-turn, Vishay T93YA-201) R5 (1 ohm, 1%, 1/4W) <-- anti-taper end-stop | +----+----+ | To X_cal arm and then to GND REACTANCE ARM (X_cal): ======================= CAPACITIVE REACTANCE (for capacitive antennas, XC positive in convention): R_cal junction | C_CAL (variable, 0-200 pF) <-- main reactance control | Polyvaricon air-variable: 12-230 pF GND Alternatives: Jackson Brothers, Millen OR (for wider range): Series switched capacitor banks: R_cal junction | S2 ----C10 (200pF) in series---- | S3 ----C11 (100pF) in series---- +-- to GND S4 ---- C12 (50pF) in series---- | S5 ---- C13 (25pF) in series---- | C14 (0-30pF trimmer in parallel) | <-- fine tune (S2-S5 = SPST micro switches or DIP switch) INDUCTIVE REACTANCE (for inductive antennas, XL negative in convention): R_cal junction | NOTE: Inductors are switched in to REPLACE the capacitor | for inductive loads. Cannot use both simultaneously. S6 ---- L10 (22uH) ---- | S7 ---- L11 (10uH) ---- +-- to GND S8 ---- L12 (4.7uH)--- | S9 ---- L13 (2.2uH)--- | (for fine tune: C_FINE trimmer 5-50pF in parallel with selected L) X_REACTANCE DIRECTION SWITCH: S_XL_XC (DPDT center-off toggle): Position CAP: connects variable capacitor to R_cal junction Position IND: connects inductor bank to R_cal junction Center (OFF): open circuit (R only, no reactance) DETECTOR OUTPUT: ================ T1 sec-B null point (this is the node between R2 and Z_ant) | C_DET (100pF, DC block) | R6 (51 ohm, 1%) -- optional 50-ohm input matching for external receiver | J4 (BNC) -- to receiver or built-in null detector ANTENNA PORT: ============= T1 sec-C lower node | J_ANT (SO-239 or BNC) -- antenna under test | Optional: BNC for external impedance standard (SHORT, OPEN, 50R for cal) CALIBRATION STANDARDS (build and label, store in kit): SHORT: BNC male with direct short across center-to-shell 50R: BNC male with 51R 1% across center-to-shell OPEN: BNC male cap installed, no resistor (open circuit) 100R: BNC male with 100R 1% across center-to-shell 200R: BNC male with 200R 1% across center-to-shell REACTANCE CALIBRATION (X scale derivation): ============================================ For CAPACITIVE reactance at frequency f: XC = 1 / (2 * pi * f * C_cal) TABLE: XC (ohm) vs C_cal at key frequencies C_cal 1.8 MHz 7 MHz 14 MHz 28 MHz 50 MHz ------ ------- ------ ------ ------ ------ 10pF 8842 2274 1137 568 318 20pF 4421 1137 568 284 159 50pF 1768 455 227 114 64 100pF 884 227 114 57 32 150pF 590 152 76 38 21 200pF 442 114 57 28 16 NOTE: At lower frequencies (1.8 MHz), even 200pF gives XC=442 ohm. For 160M operation: extend C range to 1000pF or add switchable C banks. For INDUCTIVE reactance at frequency f: XL = 2 * pi * f * L_cal TABLE: XL (ohm) vs L_cal at key frequencies L_cal 1.8 MHz 7 MHz 14 MHz 28 MHz 50 MHz ------ ------- ------ ------ ------ ------ 1 uH 11.3 44.0 88.0 176.0 314.0 2 uH 22.6 88.0 176.0 352.0 628.0 5 uH 56.5 220.0 440.0 --- --- 10 uH 113.0 440.0 --- --- --- 22 uH 249.0 --- --- --- --- BRIDGE BALANCE CONDITION: ========================= When null is achieved: Z_unknown = Z_calibration R_antenna = R_cal (reading of R pot) X_antenna = X_cal (reading from C or L bank) Sign convention used in this bridge: X positive = CAPACITIVE (XC) -- antenna has excess capacitance X negative = INDUCTIVE (XL) -- antenna is too long, has inductance NOTE: Some references use the opposite convention. The manual uses the convention most natural for above/below-resonance description: a SHORT antenna (too short = capacitive) has positive X. A LONG antenna (too long = inductive) has negative X. TRANSFORMER CONSTRUCTION DETAIL: ================================== Core: BN-43-202 binocular ferrite (Fair-Rite 2843000202) OR FT-37-43 toroid (use 2 stacked for adequate AL) OR FT-50-43 toroid (single core, adequate AL) Winding (trifilar method): 1. Cut 3 pieces of #26 AWG enameled wire, each 400mm long. Use three different colors if available (red, green, white). 2. Twist all three together: approximately 3 twists per 25mm. Use drill or pin vise to hold one end while twisting. 3. Wind 6 turns of the trifilar bundle through both holes of the binocular core (BN-43-202). Wind in the same direction. 4. The result is three windings of 6 turns each, all closely coupled. 5. Mark start and end of each winding with colored tape. 6. Verify with LCR meter: each winding ~2-3 uH (AL ≈ 50-80 nH/N^2 for Mix 43 at 1 MHz, N=6: L = 36 * AL ≈ 1.8-2.9 uH). 7. Verify coupling: short one winding, measure others -- should drop to < 50 nH if coupling is adequate. CRITICAL: The noise source winding and the two bridge arm windings must be phase-coherent. Verify: - Connect pin 1 of all three windings together (common start) - Short the load on winding B - Measure voltage on winding C: should be << 1% of driven voltage If not, reverse one winding's connections. COMPLETE PARTS LIST — BRIDGE CIRCUIT ===================================== Qty Ref Value/Part# Description --- ------ ---------------------- ---------------------------------------- 1 T1 BN-43-202 Binocular ferrite core (Fair-Rite) 18 -- #26 AWG enamel wire Trifilar winding (400mm each x 3) 2 R1,R2 51R 1% 1/4W Bridge arm resistors (match to <0.5%) 1 R3 51R 1% 1/4W Noise source series resistance 1 R4 10R 1% 1/4W R_cal offset (minimum R) 1 R5 1R 1% 1/4W R_cal end-stop 1 VR1 200R 10-turn cermet R calibration pot (Bourns 3590S-2-201) 1 C_CAL 12-230pF air variable X capacitive arm 1 C_FINE 3-40pF trimmer cermet X fine adjust (Murata TZB4) 4 C10-C13 25/50/100/200pF NP0 Switched capacitor banks 4 L10-L13 22/10/4.7/2.2uH Switched inductor bank 4 S2-S5 SPST micro switch Capacitor bank switches 4 S6-S9 SPST micro switch Inductor bank switches 1 S_XL DPDT center-off XL/XC direction switch 1 J1 SO-239 chassis Antenna port 2 J2,J3 BNC chassis Noise in, Detector out 5 -- BNC male plug Calibration standards (SHORT/50R/100R/200R/OPEN) 1 -- PCB or matrix board 80mm x 60mm minimum ================================================================================