Chapter 1 — Introduction and Scope
This manual covers the construction and operation of an RF noise bridge for antenna impedance measurement from 0.5 to 60 MHz. The bridge measures complex impedance Z = R + jX by balancing a calibrated reference arm against the unknown antenna. Null detection uses either an external communications receiver, a built-in audio amplifier, or an AD8307 logarithmic detector with digital readout. Accuracy: R ±5Ω (0–200Ω range), X ±10Ω (−200 to +200Ω range).
Chapter 2 — Theory of Operation
2-1 Bridge Circuit
A standard 4-arm Wheatstone bridge topology adapted for RF. Two fixed 51Ω arms establish the bridge reference. One variable arm (R_cal + jX_cal) is the calibrated reference. The unknown arm (Z_ant) is the antenna under test. The null detector sees zero voltage when the bridge is balanced:
Z_ant = R_cal + jX_cal (at null)
A 1:1:1 trifilar-wound transformer (T1) on a BN-43-202 core drives the two bridge legs from the noise source. Trifilar winding ensures <1° phase imbalance between the two drive ports across 0.5–60 MHz.
2-2 Noise Source
Two designs are provided:
- NGZ (Zener): 5.1V zener diode biased into avalanche breakdown produces wideband white noise. Output amplified by a MAR-6 MMIC (+20 dB, DC–2 GHz) to approximately −25 dBm into 50Ω.
- NGT (Transistor): Reverse-biased NPN transistor base-emitter junction noise; higher output level and more uniform spectral density than zener. Amplified by ERA-3SM MMIC.
2-3 Null Detection Modes
| Mode | Hardware | Sensitivity | Notes |
|---|---|---|---|
| A — External RX | Communications receiver/SDR | <−60 dBm | Best; frequency-selective |
| B — Audio amplifier | ERA-3SM MMIC + headphones | −50 dBm | Standalone; no RX needed |
| C — AD8307 | Log detector + CYD display | −70 dBm | Digital null indication |
Chapter 3 — Equipment and Materials
| Component | Value / Part | Quantity |
|---|---|---|
| Bridge transformer T1 | Trifilar, BN-43-202, 6 turns #26 AWG | 1 |
| Fixed bridge resistors R1, R2 | 51Ω 1% metal film, 1/4W | 2 |
| Variable resistance R_cal | 0–200Ω wirewound pot (non-inductive) | 1 |
| Variable reactance X_cal | 0–100 pF variable cap + 0–50µH roller inductor | 1 each |
| Noise source MMIC | MAR-6SM or ERA-3SM | 1 |
| Zener D1 | BZX55C5V1, 5.1V 500mW | 1 |
| RF bypass capacitors | 100 pF NP0 | 4 |
| RF choke RFC1 | 10µH SRF >50 MHz | 1 |
| Connectors | BNC female: noise out, DET, ANT | 3 |
| Power | 9V battery (80 mA typical) | 1 |
Chapter 4 — Construction
4-1 Transformer Winding
- Wind three identical windings of 6 turns #26 AWG simultaneously on a BN-43-202 binocular core. Keep the three windings in the same rotational direction (color-code the wire starts).
- Check winding: with an ohmmeter, verify all three windings are equal resistance (±0.1Ω) and have no shorts between windings.
- Test leakage inductance: connect one winding to NanoVNA port 1 with port 2 shorted to the other two windings. Leakage inductance should be <0.1µH at 10 MHz.
4-2 Calibration Dials
Mark the R_cal potentiometer dial in 10Ω steps from 0 to 200Ω using a resistance meter. Mark the X_cal capacitor dial in terms of −jX at 10 MHz using the formula X_C = 1/(2πfC). Mark the inductor in terms of +jX at 10 MHz using X_L = 2πfL. Actual X depends on operating frequency; provide a reactance chart (see Appendix A) for other frequencies.
Chapter 5 — Operating Procedures
5-1 Measuring Antenna Impedance
- Connect antenna to ANT BNC. Connect communications receiver (tuned to a clear frequency in the antenna's operating range) to DET BNC.
- Enable noise source (power switch ON). Receiver S-meter should show S3–S7 noise level. If no noise: check battery, MMIC bias.
- Set R_cal to 50Ω (center scale) and X_cal to zero (cap at minimum, inductor at minimum).
- Adjust R_cal for minimum S-meter reading. Note: the null may be sharp; sweep slowly.
- If null is imperfect at all R_cal values, the antenna has significant reactance. Adjust X_cal (capacitive for a high-SWR dipole that is too long; inductive for one that is too short) until the null is satisfactorily deep (≥10 dB below noise floor).
- Read R and X from the calibration dials. Reported impedance: Z_ant = R_cal + jX_cal at the measurement frequency.
Chapter 6 — Calibration
- Connect a precision 50Ω non-inductive resistor to ANT port. Set R_cal = 50Ω, X_cal = 0. Verify null at ≥20 dB below noise floor. If null is <20 dB: re-check bridge transformer balance.
- Connect a known 100Ω resistor. Set R_cal = 100Ω, X_cal = 0. Verify null is achieved at that setting.
- Connect a 47 pF NP0 capacitor. At 10 MHz: X_C = 1/(2π×10e6×47e-12) = −338Ω. Set R_cal = 0Ω (resistor is pure reactance), X_cal to −338Ω (capacitive). Verify null.
- Record calibration date and reference components used in log.
Chapter 7 — Verification and Acceptance
- Measure a known 50Ω dummy load: R must read 50 ± 3Ω, X must read 0 ± 10Ω.
- Measure a known antenna of known resonant frequency (verified by NanoVNA). At resonance, X should read 0 ± 15Ω; R should agree with NanoVNA feedpoint resistance within 10%.
- Log: date, reference impedances measured, null depth achieved, measured vs. expected values, operator.
Appendix A — Reactance vs. Frequency Chart
| Component | 1 MHz | 7 MHz | 14 MHz | 28 MHz |
|---|---|---|---|---|
| 47 pF cap | −3386Ω | −484Ω | −242Ω | −121Ω |
| 100 pF cap | −1592Ω | −227Ω | −114Ω | −57Ω |
| 1 µH inductor | +6.3Ω | +44Ω | +88Ω | +176Ω |
| 10 µH inductor | +63Ω | +440Ω | +879Ω | +1759Ω |
Appendix B — Worked Example
Measuring a 40m dipole at 7.100 MHz. Null achieved at R_cal = 72Ω, X_cal = +18Ω (inductive). This means the dipole is slightly too long (excess inductive reactance). To resonate it: shorten each element by:
ΔL = λ × (X / (2 × R)) × k (approximate) ΔL = 42.3m × (18 / 144) × 0.97 = 2.5 cm per side