Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers three field strength meter (FSM) designs in ascending order of complexity: a passive germanium detector (no batteries), an active op-amp version with calibrated dB scale, and a digital CYD-based FSM with logging. All measure relative field strength; absolute calibration in V/m is possible with the active version using a known reference transmitter.

Chapter 2 — Theory of Operation

2-1 Passive Detector

An RF signal intercepted by the probe antenna develops a voltage across a germanium point-contact diode (1N34A). The diode rectifies the RF to DC; a 10µF capacitor integrates (smooths) the rectified DC; a 100µA FSD panel meter displays the result. Sensitivity is limited by the diode forward voltage (~0.2V for germanium vs. ~0.6V for silicon); minimum detectable field from a 50 cm whip at 7 MHz is approximately 0.5 mV/m.

2-2 Active Version

A common-base RF amplifier (BF199 or 2N3904) precedes the detector diode, providing 20–30 dB of gain before detection. An op-amp (LM386 or similar) drives a meter with logarithmic compression for a dB-proportional scale. Sensitivity improves to approximately 5 µV/m.

2-3 Digital CYD FSM

An AD8307 logarithmic amplifier (0.1–500 MHz, −74 to +17 dBm, 25 mV/dB) replaces the detector and meter. Its output drives an ESP32 ADC; the CYD display shows field strength in dBm (relative to 50Ω) and logs readings with GPS timestamp. Absolute calibration ties the ADC reading to a known field (calibrated reference transmitter at known distance).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 Passive FSM

1. Mount 1N34A diode cathode toward meter positive terminal. R_load = 100 kΩ; C_filter = 10µF 16V electrolytic. Meter series resistance sets FSD.
2. Connect probe antenna (50 cm rigid copper rod) via BNC to diode anode.
3. Mount in a small plastic enclosure. Keep RF lead from BNC to diode <20 mm to minimize stray capacitance.
4. Optional: add a rotary attenuator (100 kΩ potentiometer in the antenna lead) for sensitivity control near strong transmitters.

4-2 AD8307 Digital FSM

1. Mount AD8307 with 100 nF NP0 decoupling on each supply pin. INHI connects via 1 nF DC-blocking cap to SMA input; INLO to ground.
2. VOUT pin (25 mV/dB slope, intercept −84 dBm) connects to ESP32 ADC GPIO (12-bit, 3.3V range). Scale: 3300 mV / 25 mV/dB = 132 dB dynamic range from ADC alone, but ADC noise floor limits practical range to ∼80 dB.
3. Add 10 kΩ + 100 nF low-pass filter between VOUT and ESP32 ADC to reject RF on the DC output line.

Chapter 5 — Operating Procedures

5-1 Antenna Pattern Measurement

1. Set up a reference transmitter at fixed power and distance from the antenna under test. Distance must be ≥2 × far-field criterion: d_ff = 2D²/λ (D = antenna aperture).
2. Zero the FSM: rotate the test antenna to maximum signal; note the meter reading as the reference (0 dB or 100%).
3. Rotate antenna in 15° steps through 360°. Record reading at each step. Compute relative pattern in dB: ΔdB = 20 log10(V/V_ref).
4. Plot the resulting pattern. Front-to-back ratio, 3 dB beamwidth, and sidelobe levels are directly readable from the plot.

Chapter 6 — Calibration

6-1 Absolute Calibration (Active and Digital FSMs)

1. Place a calibrated signal generator (or TinySA in generator mode) at a measured distance r from the FSM probe.
2. Set generator output to a known level P (dBm) into a calibrated antenna with known gain G (dBi). Compute E-field at distance r:

E (V/m) = sqrt(30 × P_watts × G_linear) / r

3. Record FSM reading at this field level. This is the calibration reference point. Adjust offset constant in firmware so that the CYD display shows the calculated E-field value.
4. Repeat at −10, −20, −30 dB relative levels using known attenuators. Verify linearity within ±1 dB across the range.

Chapter 7 — Verification and Acceptance

1. Passive FSM: connect a 0 dBm signal (from TinySA or signal generator) at 7 MHz via 50Ω coax. Meter should deflect to a repeatable scale reading. Deflection should decrease by approximately half (6 dB) when a 6 dB attenuator is inserted.
2. Digital FSM: apply −40 dBm, −50 dBm, −60 dBm. Verify CYD readings within ±2 dBm of expected values. (AD8307 typical accuracy: ±1 dB from −74 to +10 dBm.)
3. Log: date, version (passive/active/digital), calibration signal source, calibration power level, measured offset, operator.

Appendix A — E-Field / Power Density Conversions

E (V/m) = sqrt(P_density_W/m2 × 120π)
P_density (W/m2) = P_tx (W) × G_linear / (4π × r2)
E (V/m) from dipole: E = sqrt(30 × P_tx × G) / r   (far field)

Appendix B — Worked Example

100 mW into a dipole (G = 2.15 dBi = 1.64 linear) at 10 m distance at 14 MHz:

E = sqrt(30 × 0.1 × 1.64) / 10 = sqrt(4.92) / 10 = 0.222 V/m = 222 mV/m

The AD8307 input sees approximately −37 dBm into 50Ω from a matched probe tuned to 14 MHz. The CYD should display approximately 0.22 V/m after calibration.