================================================================================ TECHNICAL MANUAL RF FIELD STRENGTH METER — ANTENNA TESTING AND RF DETECTION TM-FSM-001 Rev A

FIELD STRENGTH METER SET, RF, PORTABLE PASSIVE / ACTIVE / DIGITAL (CYD) VARIANTS 1 MHz – 1300 MHz, S-METER, dBm, V/m, DATA LOGGING

NSN: XXXX-XX-XXX-XXXX (LOCAL FABRICATION)

THIS DOCUMENT CONTAINS TECHNICAL INFORMATION FOR THE CONSTRUCTION, OPERATION, CALIBRATION, AND MAINTENANCE OF THE RF FIELD STRENGTH METER SET, PORTABLE.

PRIOR TO PERFORMING ANY PROCEDURE, READ ALL APPLICABLE WARNINGS, CAUTIONS, AND NOTES. FAILURE TO COMPLY MAY RESULT IN INJURY OR EQUIPMENT DAMAGE.

================================================================================ RECORD OF CHANGES ================================================================================

================================================================================ TABLE OF CONTENTS ================================================================================

CHAPTER 1 GENERAL INFORMATION CHAPTER 2 THEORY OF OPERATION CHAPTER 3 EQUIPMENT DESCRIPTION CHAPTER 4 CONSTRUCTION CHAPTER 5 CALIBRATION PROCEDURES CHAPTER 6 OPERATING PROCEDURES CHAPTER 7 TROUBLESHOOTING CHAPTER 8 MAINTENANCE CHAPTER 9 PARTS LIST APPENDIX A ANTENNA FACTOR TABLES APPENDIX B S-METER REFERENCE TABLE APPENDIX C BAND FREQUENCY REFERENCE APPENDIX D FIELD STRENGTH CONVERSION TABLES APPENDIX E DATA LOGGING CSV FORMAT APPENDIX F GLOSSARY

================================================================================ CHAPTER 1 GENERAL INFORMATION ================================================================================

1-1. PURPOSE AND SCOPE

1-1.1 This manual provides complete technical data for the construction, operation, calibration, and field maintenance of the RF Field Strength Meter Set (hereinafter “FSM” or “the instrument”).

1-1.2 The FSM set consists of three instrument variants:

1. FSM-P (PASSIVE) — Battery-free germanium diode detector with analog moving-coil meter. Frequency range 100 kHz–500 MHz. No power required. Provides relative field strength indication.

2. FSM-A (ACTIVE) — ERA-3SM MMIC preamplifier with AD8307 logarithmic detector and analog meter output. Frequency range 1–500 MHz. Provides calibrated dBm readings. Includes audio tone output proportional to field strength.

3. FSM-D (DIGITAL) — ESP32-2432S028 (CYD) microcontroller with ILI9341 2.8” TFT display, AD8307 detector, SD card data logger with GPS coordinate tagging, and audio tone output. Frequency range 1 MHz–1300 MHz (with appropriate probe). Provides S-meter, dBm, V/m, dBμV/m display with Smith-chart-style direction finding capability. Firmware: field_strength_cyd.ino.

1-1.3 All three variants share a common probe set: - 50 cm telescoping whip antenna - λ/2 adjustable dipole with T-bar handle - 12.7 cm shielded single-turn loop (HF direction finding) - 3-element 2M/70cm mini-Yagi probe (VHF/UHF) - Calibrated terminated dipole (for traceable V/m measurements)

1-1.4 The FSM set is intended for: a. Verifying antenna radiation patterns and relative gain b. Locating sources of RF interference c. Measuring antenna field strength during transmitter testing d. Direction finding (locating hidden transmitters) e. Characterizing propagation paths in the field f. Compliance verification (non-traceable; for comparative use)

1-2. PRINCIPAL CHARACTERISTICS

1-3. SAFETY SUMMARY

WARNING

1. RF HAZARD — The FSM detects radiated RF fields. During measurement, the operator may be in the RF near field of a transmitting antenna. Observe all RF safety guidelines per ARRL RF Exposure guidelines or applicable regulations. Do not make measurements closer to a transmitting antenna than the minimum safe distance for the operating power.

2. NO TRANSMITTER CONNECTION — These instruments are passive or low-level detection devices. They are NOT designed to accept transmitter output power. Maximum safe input at any probe connector is +10 dBm (10 mW). Connecting a transmitter output to any probe connector WILL destroy the instrument.

3. BATTERY POLARITY — Reversed battery polarity will destroy MMIC devices (ERA-3SM, MAR-6SM) instantly. Verify polarity before every battery installation.

4. STATIC DISCHARGE — ERA-3SM and ESP32 are static-sensitive. Use ESD precautions during assembly and service.

CAUTION

1. The AD8307 input is damaged by input signals above +17 dBm. When measuring near a transmitter, always start with the maximum attenuator setting (−30 dB) and reduce only if needed.

2. Do not operate the FSM-D in rain without additional weatherproofing for the GPS and SD card connections. The CYD display is not sealed against moisture.

3. GPS fix requires clear sky view. Indoor or obstructed operation will prevent GPS position logging.

NOTE

1. Field strength measurements are inherently influenced by reflections, ground conductivity, antenna orientation, and nearby metallic objects. All measurements should be interpreted as relative comparisons unless a full calibrated reference measurement sequence has been performed.

2. The FSM-D antenna factor table in firmware is derived from theoretical free-space values. Actual antenna factor varies with frequency and probe construction. For precision measurements, perform user AF calibration per Chapter 5.

================================================================================ CHAPTER 2 THEORY OF OPERATION ================================================================================

2-1. FIELD STRENGTH AND POWER DENSITY

2-1.1 An electromagnetic wave propagating through free space carries power. The power density S (W/m²) at any point is related to the electric field intensity E (V/m) by:

S = E² / (120π) = E² / 377         [Equation 2-1]

where 120π ≈ 377 Ω is the impedance of free space.

2-1.2 Conversely, given a received power Pr at a load resistance RL, the equivalent open-circuit voltage Voc across an antenna is related to the field strength by the effective height he:

Voc = E × he                        [Equation 2-2]

For a half-wave dipole in free space: he = λ/π ≈ 0.318 λ [Equation 2-3]

Example at 14.175 MHz (λ = 21.17 m): he = 21.17 / π = 6.74 m If E = 1 V/m: Voc = 1 × 6.74 = 6.74 V (open circuit) Matched output into 50Ω: V = Voc/2 = 3.37 V, P = V²/50 = 227 mW = +23.6 dBm

This illustrates why a gain antenna at close range can overwhelm the FSM even at low transmitter power. Always use the attenuator.

2-1.3 FREE-SPACE PATH LOSS

For a transmitter of power Pt into an isotropic antenna: E (V/m) = √(30 × Pt × Gt) / d [Equation 2-4]

Where: Pt = transmitter power (watts) Gt = transmitter antenna gain (numeric: 1 = 0 dBi) d = distance from transmitter (meters)

Example: 100W into a dipole (Gt = 1.64) at 10 meters: E = √(30 × 100 × 1.64) / 10 = √4920 / 10 = 70.1 / 10 = 7.0 V/m

Example: Same at 100 meters: E = 7.0 / 10 = 0.70 V/m

2-1.4 FIELD STRENGTH UNITS

V/m: SI unit, field amplitude dBμV/m: 20 × log10(E_V/m × 10⁶) = 20 log10(E) + 120 dB (ref = 1 μV/m) dBV/m: 20 × log10(E) mV/m: E × 1000 μV/m: E × 10⁶

Common reference levels: S9 signal at antenna: E ≈ 50 μV into 50Ω → E_field depends on AF VHF FM broadcast 30 km: ~0.01–0.1 V/m (30–100 dBμV/m) AM broadcast 10 km: ~0.1–1 V/m (100–120 dBμV/m) 10W QRP at 100m, 40M: ~0.03 V/m (~90 dBμV/m) 100W at 10m from antenna: 7 V/m (~137 dBμV/m)

2-2. ANTENNA FACTOR

2-2.1 The antenna factor (AF) of a receive antenna is the ratio of the incident electric field to the voltage measured at the antenna terminals (into a standard 50Ω load):

AF = E / V_terminal                  [Equation 2-5]
AF (dB/m) = E_dBμVm − V_dBμV       [Equation 2-6]

2-2.2 For a λ/2 dipole (theoretical free space, 50Ω load): AF (dB/m) = 20 × log10(4π × f / c) = 20 log10(f_MHz) − 29.8 where c = 3×10⁸ m/s

At 14.175 MHz: AF = 20 log10(14.175) − 29.8 = 23.0 − 29.8 = −6.8 dB/m

NOTE: A negative AF means the antenna produces a larger voltage
than the field strength value (because the antenna is larger than 1 m
in effective height at low HF frequencies).

2-2.3 For a short monopole of physical length L (L << λ): AF (dB/m) ≈ 20 log10(λ / (2πL)) + 6 dB (rough approximation) A 50 cm whip at 7 MHz: AF ≈ 20 log10(42 / (2π × 0.5)) ≈ 20 log10(13.4) ≈ +22 dB/m

2-2.4 The FSM firmware stores pre-computed AF values for three probe types at each band. User calibration can apply a correction offset (±20 dB range) stored in EEPROM. See Chapter 5 for calibration procedure.

2-3. LOGARITHMIC DETECTOR OPERATION (AD8307)

2-3.1 The AD8307 is a successive-detection logarithmic amplifier covering −75 to +17 dBm (92 dB dynamic range) from 1 MHz to 500 MHz. Its output voltage is:

V_out = slope × (P_in_dBm − intercept)
      = 25 mV/dB × (P_in_dBm + 84 dBm)    [Equation 2-7]

At −75 dBm input: V_out = 0.025 × (−75 + 84) = 0.225 V At 0 dBm input: V_out = 0.025 × ( 0 + 84) = 2.100 V At +17 dBm input: V_out = 0.025 × ( 17 + 84) = 2.525 V

2-3.2 The AD8307 output is read by the ESP32 12-bit ADC (GPIO34). With the 3.3V ADC reference: P_dBm = (ADC_count × 3.3 / 4095) × 40 − 84

2-3.3 The ERA-3SM preamplifier (20 dB gain, P1dB = +12 dBm) extends the system minimum detectable signal to approximately: MDS = AD8307_min − ERA_gain = −75 − 20 = −95 dBm at probe input.

With a λ/2 dipole probe (AF ≈ −7 dB/m at 14 MHz), the minimum detectable field is: E_min_dBuVm = MDS_dBm + 107 + AF = −95 + 107 + (−7) = +5 dBμV/m E_min = 10^(5/20) × 10⁻⁶ = 1.78 μV/m ≈ 0.002 mV/m (excellent sensitivity)

2-4. S-METER CALIBRATION (IARU STANDARD)

2-4.1 The S-meter scale is defined by the International Amateur Radio Union (IARU) as follows for HF (below 30 MHz):

S1 = −121 dBm (input at 50Ω)
Each S-unit = 6 dB
S9 = −73 dBm = 50 μV at 50Ω

Above S9: reported as "S9 + X dB"
Example: −43 dBm = S9 + 30 dB

2-4.2 For VHF (above 30 MHz), IARU recommends: S9 = −93 dBm (stricter reference) Each S-unit still = 6 dB

2-4.3 FSM firmware uses the HF standard (S9 = −73 dBm) for all bands. A VHF correction of +20 dB can be applied in the settings.

2-5. DIRECTION FINDING WITH A LOOP ANTENNA

2-5.1 A small shielded loop antenna has a figure-8 radiation pattern: maximum response broadside (perpendicular to loop plane), and sharp nulls on the loop axis. The null is typically 20–30 dB below maximum.

2-5.2 SENSE AMBIGUITY

The basic loop has a 180° ambiguity: the null direction points both toward and away from the transmitter. To resolve this ambiguity, one of the following methods is used: a. CARDIOID PATTERN: Combine loop with a vertical whip (add an “adcock” sense antenna). The combined pattern breaks the 180° symmetry, giving a single null direction. b. ASYMMETRIC LOOP TECHNIQUE: Observe which side of the null the signal recovers more rapidly while moving toward the source. c. POSITION AMBIGUITY RESOLUTION: Take two bearings from different positions; the intersection identifies the transmitter location.

2-5.3 FSM-D DIRECTION FINDER (Screen 2)

The CYD direction finder screen displays: - Current RF level in dBm (tracks in real time) - Peak level bearing (bearing where maximum was recorded) - Minimum level (null depth indicator) - Bearing readout updated by operator pressing BEAR+ / BEAR− buttons

Operator rotates the loop probe, manually incrementing the bearing display with the touch buttons as the loop is rotated. The firmware records the bearing at peak level. This provides a bearing to the strongest source at the measurement frequency.

================================================================================ CHAPTER 3 EQUIPMENT DESCRIPTION ================================================================================

3-1. FSM-P (PASSIVE) DESCRIPTION

3-1.1 The FSM-P consists of: a. 1N34A germanium point-contact diode (envelope detector) b. 10-position rotary band switch with parallel LC tuned circuits providing ~6 dB selectivity per octave at each band c. 100 kΩ logarithmic-taper sensitivity potentiometer d. 100 μA FSD moving-coil panel meter (60×40 mm face) e. ASA enclosure (120×65×35 mm) with meter face window

3-1.2 The FSM-P requires no external power. The detector diode rectifies the received RF, producing a DC voltage proportional to the RF envelope. The meter deflects in proportion to the detected signal strength.

3-1.3 FREQUENCY RESPONSE

The FSM-P is inherently broadband. Without a tuned circuit, the detector responds to all signals across its frequency range. The band switch tuned circuit provides some selectivity, but a strong out-of-band signal may still deflect the meter. This is acceptable for relative measurements. For selective measurements, the FSM-A or FSM-D is preferred.

3-1.4 MINIMUM DETECTABLE SIGNAL

With a 50 cm whip probe and 100 μA FSD meter at 1/4 deflection: At 7 MHz (40M): approximately 1 mV/m (5 mV at meter for ≥5% deflection) At 14 MHz (20M): approximately 0.5 mV/m At 144 MHz (2M): approximately 0.2 mV/m

These values assume the sensitivity pot is at maximum and the band filter is set to the correct band.

3-2. FSM-A (ACTIVE) DESCRIPTION

3-2.1 The FSM-A adds an ERA-3SM MMIC preamplifier and AD8307 logarithmic detector to the passive detection chain. Key features:

1. Input protection: BAT54S Schottky clamp diodes limit to ±3.3V
2. 4-position Pi attenuator (0 / −10 / −20 / −30 dB, all 50Ω)
3. ERA-3SM preamp: 20 dB gain, 0.1–3000 MHz, P1dB = +12 dBm
4. AD8307: −75 to +17 dBm, 25 mV/dB slope
5. LM358 op-amp meter driver with zero/span potentiometers
6. NE555 audio VCO: tone frequency proportional to field strength

3-2.2 ATTENUATOR

The 4-position rotary switch selects: Position 0 (DIRECT): Full ERA-3SM gain. Use for weak fields. Position 1 (−10 dB): Reduced gain. Use for moderate fields. Position 2 (−20 dB): Moderate attenuation. Use near transmitters. Position 3 (−30 dB): Maximum attenuation. Use very close to antennas or for high-power transmitter near-field work.

CAUTION: Always start at −30 dB when first connecting to an unknown signal environment. Progressively reduce attenuation until the meter reads in the upper half of its scale.

3-2.3 AUDIO TONE

The 555 VCO frequency is controlled by the AD8307 output voltage. The tone frequency increases with increasing field strength. Weak field (−75 dBm): ~200 Hz low hum Medium field (−40 dBm): ~800 Hz medium tone Strong field (0 dBm): ~2500 Hz high pitch Overload (>+10 dBm): ~3500 Hz continuous high tone

The audio output is a useful indicator when the meter is not visible (e.g., climbing a tower, adjusting an antenna).

3-3. FSM-D (DIGITAL / CYD) DESCRIPTION

3-3.1 The FSM-D uses the CYD board (ESP32-2432S028) as its controller and display. All RF circuitry is identical to FSM-A (ERA-3SM + AD8307) but the meter and audio VCO are replaced by the CYD display and ESP32 PWM audio.

3-3.2 DISPLAY SCREENS

Six operating screens are provided, selectable by touch:

SCREEN 0 — S-METER MAIN (default power-on screen) Horizontal S-meter bargraph (S1–S9+30 dB scale) Large dBm readout (2× character size) S-unit readout (e.g., “S6” or “S9+15dB”) Field strength: V/m and dBμV/m Probe terminal voltage: μV or mV GPS coordinates (if fix acquired) Touch buttons: BND− | BND+ | HOLD | LOG | DIR | SET

SCREEN 1 — CALIBRATED READOUT Extra-large dBm (3× characters) V/m and dBμV/m Antenna factor readout and user correction S-unit reading

SCREEN 2 — DIRECTION FINDER Compass rose graphic Current/peak bearing pointers Peak and null dBm levels Touch buttons: RESET | BACK | BEAR+ | BEAR−

SCREEN 3 — DATA LOGGER GPS status (fix/acquiring, coordinates, satellites, altitude) Log file name, entry count Recent log entries (last 5 lines of CSV) Touch buttons: AUTO (toggle) | LOG NOW | CLEAR | BACK

SCREEN 4 — BAND SELECT 15-button grid: 160M through 20cm Highlighted selected band

SCREEN 5 — SETTINGS Attenuator (0/10/20/30 dB, toggles via ATT+ / ATT− buttons) Audio mode (Proportional / Pulse / Off) Probe type (WHIP / DIPOLE / LOOP — affects AF used) AF user correction offset (±20 dB in 1 dB steps) Log interval (Manual / 5s / 10s / 30s / 60s) Backlight brightness ADC offset and scale (calibration constants) SAVE: writes all settings to EEPROM

3-3.3 DATA LOGGING

Log entries are written to an SD card in CSV format. One file per day, named FSM_YYYYMMDD.CSV. If no GPS fix, file is FSM_LOG.CSV.

Each entry records: UTC time, date, GPS coordinates, altitude, satellite count, band name, frequency, attenuator setting, dBm level, probe terminal voltage (μV), field strength (V/m), S-units, notes.

The log file is compatible with Excel, LibreOffice Calc, or Python pandas for post-processing. A sample Python analysis script is provided in Appendix E.

3-3.4 GPS INTEGRATION

The GPS module (u-blox NEO-6M or compatible) connects via UART2 (GPIO16 RX, GPIO17 TX) at 9600 baud. NMEA 0183 sentences $GPRMC and $GPGGA are parsed for position, time, and altitude.

After cold power-on, GPS fix acquisition requires 90–180 seconds. The green status LED (GPIO27) blinks once per second while acquiring and twice per second when a fix is obtained.

3-3.5 AUDIO OUTPUT

GPIO25 PWM tone output feeds an RC low-pass filter and LM386 audio amplifier driving an 8Ω speaker. Three modes:

PROPORTIONAL: Continuous tone, frequency proportional to dBm level. PULSE: Geiger-counter-style click rate proportional to field strength. OFF: Audio disabled.

Mode is selected in Settings screen and saved to EEPROM.

================================================================================ CHAPTER 4 CONSTRUCTION ================================================================================

4-1. FSM-P CONSTRUCTION

4-1.1 PARTS REQUIRED

See schematics/sch_passive_fsm.txt for complete schematic.

4-1.2 PCB LAYOUT NOTES

The detector circuit is simple enough for a 50×40 mm piece of single-sided FR-4 or even Manhattan-style construction on copper clad board. Key layout rules:

1. Keep detector diode D1 and load R1 directly at the input connector (J1) with leads < 10 mm. Long leads add inductance that degrades HF detection sensitivity.

2. Place C1 (100 pF RF bypass) directly across R1 leads to provide a low-impedance RF return path.

3. The meter M1 and C2 (integrating cap) are in the DC output path; lead length is not critical.

4. Route the band switch S1 with short connections from each position to its corresponding LC tuned circuit. Keep LC circuits near the switch, not near the detector.

4-1.3 DETECTOR DIODE SELECTION

1N34A germanium point-contact diodes vary in quality. Before installation, check the forward voltage at 1 mA with a DMM: Good 1N34A: 150–250 mV forward voltage Silicon diode: 600–700 mV (do NOT use; too high for weak signals) BAT54 Schottky: 200–300 mV (acceptable substitute; better HF) OA47/OA90: 180–240 mV (acceptable European equivalents)

The lower the forward voltage, the better the weak-signal sensitivity.

4-1.4 TOROID INDUCTOR WINDING — BAND FILTERS

Wind toroid inductors on T-50-6 (green, for HF) or T-37-6 (yellow/green, smaller) cores:

Measure inductance with LCR meter after winding; adjust turns ±1 to reach target value within ±10%.

4-1.5 ENCLOSURE ASSEMBLY

Step 1: 3D print enclosure_passive_fsm.scad in PETG or ASA. Print body and lid separately. Deburr holes.

Step 2: Install M3 heat-set inserts (×4) at lid bolt holes.

Step 3: Mount BNC connector J1 on front face (12.5 mm hole). Torque: 0.6 N·m max.

Step 4: Mount rotary band switch S1 through lid hole (10.5 mm). Tighten hex nut, install knob.

Step 5: Mount sensitivity pot VR1 through lid hole (7.5 mm). Install knob.

Step 6: Mount panel meter M1 through meter aperture (62×42 mm). Secure with mounting screws from inside (M3, 4 corners if meter has tapped mounting holes) or with adhesive foam tape. Install acrylic glass window (1.5 mm, 64×44 mm, cut to fit).

Step 7: Install PCB on 4 mounting bosses with M2.5 screws. Connect BNC center pin to D1/C1 node. Connect band switch to PCB with 26 AWG hookup wire.

Step 8: Install lid; tighten 4× M3 screws.

4-2. FSM-A CONSTRUCTION

4-2.1 PARTS REQUIRED

See schematics/sch_active_fsm.txt for complete schematic.

4-2.2 CRITICAL ASSEMBLY NOTES — ERA-3SM

WARNING: ERA-3SM is an ESD-sensitive SOT-89 MMIC device. It is destroyed instantly by improper bias or reversed voltage. Before soldering, verify bias resistor R1 value (75Ω at 9V supply). Do not apply power without R1 installed.

The ERA-3SM SOT-89 pinout: Pin 1 (tab, bottom): RF INPUT Pin 2: GND Pin 3: GND Pin 4: Vcc / RF OUTPUT (same pin; bias through R1)

Solder with temperature-controlled iron (330°C max). Use minimum solder; the exposed collector/drain pad on pin 4 is the RF output.

4-2.3 AD8307 INSTALLATION

The AD8307AN (DIP-8) is preferred for hand assembly. Use a DIP-8 socket to allow replacement without thermal damage.

Post-installation verification: 1. Apply 5V to AD8307 VS pin. Measure: 4.8–5.2V. Pass. 2. Apply known signal (e.g., −30 dBm at 10 MHz from signal generator). VOUT should read approximately: V = 0.025 × (−30 + 84) = 1.35V ± 0.15V. 3. If VOUT is out of range, check input matching resistor R3 (51Ω) and INLO bypass capacitor.

4-2.4 ATTENUATOR PAD CONSTRUCTION

Pi attenuator resistors must be SMD (0402 or 0603) thin-film types rated ±1% for accurate calibration. Wirewound or carbon composition resistors are unsuitable.

Use E96 series values (nearest standard): −10 dB: 95.3Ω shunt, 71.5Ω series −20 dB: 61.9Ω shunt, 249Ω series −30 dB: 53.6Ω shunt, 787Ω series

Mount on a small piece of PCB with a 50Ω microstrip layout, or in a small metal box (brass or aluminum) for shielding.

4-3. FSM-D CONSTRUCTION

4-3.1 PARTS REQUIRED (additions to FSM-A)

4-3.2 CYD FIRMWARE UPLOAD

Step 1: Install Arduino IDE 2.x. Install ESP32 board support package v3.x via Boards Manager. Step 2: Install libraries: TFT_eSPI (Bodmer), XPT2046_Touchscreen. Step 3: Configure TFT_eSPI/User_Setup.h: #define ILI9341_DRIVER #define TFT_CS 15, TFT_DC 2, TFT_RST 12 #define TFT_MOSI 13, TFT_SCLK 14, TOUCH_CS 33 #define SPI_FREQUENCY 40000000 Step 4: Copy field_strength_cyd.ino and fsm_datalogger.h to same project folder. Step 5: Select board “ESP32 Dev Module”, partition “Default 4MB”. Step 6: Upload. Verify boot message on serial monitor at 115200 baud. Step 7: Verify all 6 screens accessible; verify touch response. Step 8: Insert microSD card (FAT32 formatted). Verify SD:OK on display. Step 9: Connect GPS module. Wait for fix (90–180s outdoors). Verify GPS coordinates appear on main screen.

4-3.3 WIRE ROUTING

All ADC input wires (GPIO34, 35, 36) should be shielded or routed away from the TFT SPI lines (GPIO13, 14) to prevent clock pickup on the ADC inputs. Use twisted pairs or coax for the RF signal path from the attenuator/ERA-3SM to the AD8307.

================================================================================ CHAPTER 5 CALIBRATION PROCEDURES ================================================================================

5-1. CALIBRATION OVERVIEW

5-1.1 Three levels of calibration are defined:

LEVEL 1 — FUNCTIONAL CHECK Verifies that the instrument powers on, deflects with RF present, and responds correctly to attenuator changes. No test equipment required beyond a working amateur transceiver.

LEVEL 2 — dBm CALIBRATION (FSM-A and FSM-D) Establishes the dBm reading accuracy to ±2 dB. Requires a signal source of known output level (e.g., a 50Ω QRP transceiver measured with the TM-DL-001 power meter and directional coupler at a fixed attenuated output).

LEVEL 3 — FIELD STRENGTH CALIBRATION (FSM-D only) Establishes the V/m and antenna factor accuracy. Requires a calibrated reference antenna and known transmit power (from the TM-DL-001 or TM-NB-001 power meter).

5-2. LEVEL 1 — FUNCTIONAL CHECK (ALL VARIANTS)

Step 1: With no probe connected, verify meter at zero (FSM-P, FSM-A) or dBm display shows < −85 dBm (FSM-D: noise floor).

Step 2: Connect a 50 cm whip to the input. Bring within 1 meter of a working VHF FM broadcast radio. Verify meter deflects (FSM-P), meter reads 1/4 or more FSD (FSM-A), or dBm display reads > −75 dBm (FSM-D).

Step 3: Increase attenuator setting one step. Verify reading drops.

Step 4: Test audio output (FSM-A / FSM-D): verify tone frequency increases as field strength increases.

Step 5: Test HOLD function (FSM-D): press HOLD, verify reading freezes. Press again to release.

ACCEPT: All steps show expected response. REJECT: Any step shows no response → troubleshoot per Chapter 7.

5-3. LEVEL 2 — dBm CALIBRATION (FSM-A and FSM-D)

5-3.1 EQUIPMENT REQUIRED

1. RF signal source with known output power ±0.5 dB. Options:

   • QRP transmitter at low output with calibrated power meter (TM-DL-001 directional coupler): recommended
   • NanoVNA in transmit mode (requires calibration)
   • Commercial signal generator (HP8640B, etc.)

2. BNC 50Ω precision attenuator (20 dB, if not built in) to reduce transmitter output to FSM safe input range.

3. BNC-to-SMA adapter (as needed for FSM-D SMA input).

5-3.2 CALIBRATION PROCEDURE

Step 1: Set signal source to 14.175 MHz (20M band center), output level exactly −20 dBm (10 μW at 50Ω).

Step 2: Connect through FSM attenuator at −20 dB position. Effective input to ERA-3SM = −20 − 20 = −40 dBm. AD8307 input (after ERA-3SM) = −40 + 20 = −20 dBm.

Step 3: FSM-A: Adjust VR1 (meter calibration) until meter reads at the −20 dBm mark on the scale. If scale has no dBm markings, calculate expected deflection: VOUT = 0.025 × (−20 + 84) = 1.60V Meter current = 1.60V / (R7 + R_meter) mA Adjust R7 (15 kΩ) or VR1 to match.

Step 4: FSM-D: Navigate to Screen 5 (Settings). Check ADC offset and scale. On screen, the displayed dBm should read −40 dBm (the input to the ERA-3SM, after ERA-3SM gain and attenuation are factored in by the firmware).

NOTE: The firmware computes: P_at_ADC = ad8307_raw_dBm (what AD8307 sees) P_at_input = P_at_ADC − ERA_gain = P_at_ADC − 20 dB P_displayed = P_at_input − atten_dB setting

Step 5: If FSM-D reads other than −40 dBm: Calculate correction: adc_offset = adc_offset + (V_measured − V_expected) where V_expected = 0.025 × (−20 + 84) = 1.60V and V_measured = (ADC_count × 3.3 / 4095) Or use the simpler formula: New offset = Old offset + (displayed_dBm − expected_dBm) × 0.025

Step 6: Repeat with −10 dBm input (expected FSM-D display = −30 dBm). Accept: both readings within ±2 dB of expected. Reject: if linearity is off (both points off in same direction: adjust offset; if they diverge: adjust scale factor).

Step 7: Press SAVE on Settings screen to store to EEPROM.

5-4. LEVEL 3 — FIELD STRENGTH CALIBRATION (FSM-D)

5-4.1 OVERVIEW

This procedure establishes the antenna factor (AF) for each probe against a reference transmitter of known power and geometry.

5-4.2 REFERENCE SETUP

1. Set up a half-wave dipole at a known height (≥λ/4 above ground). Orient horizontally. Feed with a precision 50Ω coax feedline.

2. Connect a calibrated power meter (TM-DL-001 directional coupler

   • power meter) to the feedline. Note forward power P_fwd in watts.

3. Calculate expected field strength at distance d: E = √(30 × P_fwd × Gt) / d [Equation 5-1] For half-wave dipole: Gt = 1.64 (2.15 dBi) E_dBuVm = 20 log10(E) + 120

5-4.3 MEASUREMENT

Step 1: Position FSM-D at distance d = 10 meters (measured) from the dipole’s center, in the broadside direction. Align probe for maximum received signal.

Step 2: Note FSM-D displayed E_Vm reading.

Step 3: Calculate actual E_field from Equation 5-1.

Step 4: AF correction: Delta_AF = E_expected_dBuVm − E_displayed_dBuVm Enter this delta on the Settings screen as “AF User Correction”. Press SAVE.

Step 5: Repeat for 2–3 bands. Note that the correction may vary by up to ±6 dB across bands. For best accuracy, store per-band corrections in a table (modify firmware band table as needed, or apply manually per band).

Step 6: Verify: return to reference position, re-check displayed E_Vm vs. calculated E_expected. Accept if within ±3 dB.

5-4.4 GPS TIMESTAMP VERIFICATION

Step 1: Verify GPS fix is active (GPS: FIX shown on Logger screen). Step 2: Record a manual log entry while at a known position. Step 3: Compare logged coordinates to GPS coordinates from a phone or dedicated GPS receiver. Accept: within 10 meters horizontal accuracy.

================================================================================ CHAPTER 6 OPERATING PROCEDURES ================================================================================

6-1. PREPARATIONS FOR USE

6-1.1 PRE-OPERATION CHECKLIST (ALL VARIANTS)

[ ] Probe connected to input connector (correct probe for band) [ ] Battery installed and charged (FSM-A/D: verify voltage indicator) [ ] Attenuator set to maximum (−30 dB) for initial measurement [ ] Power switch ON [ ] Display/meter reads correctly (no alarm indication) [ ] Band selection correct for measurement frequency (FSM-D) [ ] GPS acquiring (FSM-D; allow 2–3 minutes outdoors)

6-1.2 PROBE SELECTION GUIDE

6-2. RELATIVE FIELD STRENGTH MEASUREMENTS

6-2.1 Relative measurements compare signal levels at different positions or antenna orientations. Absolute accuracy is not required.

Step 1: Select appropriate probe. Connect to input. Step 2: Set attenuator to −20 dB initially. Step 3: Position instrument at measurement point. Step 4: Note reading (FSM-P: meter position; FSM-A: meter dBm; FSM-D: dBm or S-units on main screen). Step 5: Move to second position or change antenna orientation. Step 6: Compare readings.

NOTE: When comparing readings, the attenuator setting must remain constant between measurements. A change in attenuator setting introduces a known offset (10/20/30 dB) which must be accounted for.

6-2.2 ANTENNA PATTERN MEASUREMENT

To measure a transmitting antenna’s horizontal pattern: a. Set transmitter to known power, constant output. b. Measure FSM reading at 8–16 azimuths around the antenna at a fixed radius (5–20 wavelengths from antenna). c. Record readings (dBm or meter position) vs. azimuth angle. d. Normalize to the maximum reading (0 dB). e. Plot polar or rectangular pattern.

NOTE: Ground reflections, nearby structures, and antenna mounting will all affect the measured pattern. This technique measures the installed radiation pattern, not the free-space pattern. Both are useful for practical antenna evaluation.

6-3. CALIBRATED FIELD STRENGTH MEASUREMENTS (FSM-D)

6-3.1 For V/m field strength measurements, the following procedure produces calibrated results ±3 dB (with Level 3 calibration performed):

Step 1: Select band matching measurement frequency (Screen 4). Step 2: Select correct probe type in Settings (WHIP / DIPOLE / LOOP). Step 3: Verify AF user correction is applied (Settings screen). Step 4: Set attenuator to produce a reading in the upper half of the AD8307 range (aim for −50 to −10 dBm on display). Step 5: Orient probe for maximum response (broadside for dipole, horizontal for whip). Step 6: Read E_Vm and E_dBuVm from Screen 1 (Calibrated). Step 7: Log the reading (Screen 3 LOG button or auto-log).

6-3.2 MULTIPLE POSITION SURVEY (DATA LOGGING)

Step 1: Ensure GPS fix is active. Step 2: Set auto-log interval (Settings: 10s recommended). Step 3: Walk survey route, holding FSM at consistent height (1–1.5 m). Step 4: On completion, remove SD card. Import CSV into mapping software (QGIS, Google Earth with GPS Visualizer, or Python with folium library). Step 5: Color-code measurement points by field strength level to create a coverage or interference map.

TIP: Use the same frequency, attenuator, and probe throughout a survey session. Note attenuator setting in the Notes field of any manual log entries.

6-4. DIRECTION FINDING PROCEDURE

6-4.1 HF DIRECTION FINDING WITH LOOP PROBE

EQUIPMENT: FSM-D with shielded loop probe Compass (standalone or phone compass)

Step 1: Tune external receiver (or set FSM band) to the transmitter frequency. Step 2: Navigate to Screen 2 (Direction Finder, press DIR on main screen). Step 3: Press RESET to clear previous bearing data. Step 4: Hold loop in vertical plane, horizontal grip. Step 5: Slowly rotate loop. Watch dBm display. Find the orientation of MINIMUM signal (null). Step 6: The null axis of the loop points toward (and away from) the transmitter. Step 7: Note compass bearing of the loop axis at null. This is the bearing line to the transmitter (either direction). Step 8: Move to a second position (≥100 m from first). Repeat steps 4–7. Step 9: Plot both bearing lines on a map. Their intersection gives the transmitter location (resolves 180° ambiguity).

NOTE: In urban environments with multiple reflections, nulls may be indistinct or multiple. Move to a more open location, or use 3 bearings to triangulate if possible.

6-4.2 VHF/UHF DIRECTION FINDING WITH YAGI PROBE

The 3-element Yagi provides a directional forward lobe (~10 dBd) and a well-defined forward direction (maximum signal = transmitter direction). No ambiguity as with the loop.

Step 1: Mount Yagi probe on FSM-D input (SMA adapter). Step 2: Navigate to Screen 2 (Direction Finder). Step 3: Sweep horizontally to find MAXIMUM signal (peak, not null). Step 4: Note bearing at peak.

NOTE: The Yagi has significant sidelobe response. If the pattern seems broad or shows multiple peaks, try increasing attenuator to ensure AD8307 is not in saturation. A saturated AD8307 does not distinguish between different signal levels (all read near maximum).

================================================================================ CHAPTER 7 TROUBLESHOOTING ================================================================================

7-1. NO INDICATION (ALL VARIANTS)

Check 1: Power switch on? Verify battery voltage with DMM. FSM-A/D: ≥7V required.

Check 2: Is there a signal to detect? Confirm a transmitter is active on the measurement frequency.

Check 3: Correct probe connected? BNC/SMA connector seated? A BNC with only partial engagement does not make RF contact.

Check 4: Attenuator at maximum (−30 dB)? Try 0 dB setting.

Check 5: FSM-P only: verify diode D1 is germanium type and installed with correct polarity (cathode to load resistor R1, not GND).

Check 6: FSM-A/D: verify ERA-3SM bias voltage. Measure voltage at pin 4 of ERA-3SM: should be 4.0–5.0V. If zero or 9V (supply rail): bias resistor R1 is open or shorted.

7-2. READING TOO HIGH OR SATURATED

Check 1: Increase attenuator setting. If reading drops as expected, the ERA-3SM or AD8307 was in compression. Use more attenuation.

Check 2: FSM-D: if display shows approximately same value regardless of attenuator setting, AD8307 may be clipped at its maximum output. Increase attenuation by 20 dB.

Check 3: Check input for strong nearby transmitter (broadcast station, commercial RF source). Use band filter or move to a shielded location for lower-signal work.

7-3. ERRATIC OR NOISY READING (FSM-D)

Check 1: ADC noise: ensure ADC input (GPIO34) has 0.1 μF bypass to GND directly at the ESP32 pin. EMI from TFT SPI can couple into adjacent GPIO inputs.

Check 2: Ground loop: all shields, GND connections, and enclosure ground must be connected at a single star point. Multiple ground paths create ground loops that appear as noise.

Check 3: GPS module: ensure GPS UART2 lines (GPIO16/17) are routed away from ADC inputs. NMEA data stream at 9600 baud can alias into the ADC sampling.

Check 4: SD card clock (GPIO18) at 4 MHz: disable SD logging temporarily (Settings: Log interval = Manual). If noise reduces, SD SPI clock is coupling into ADC. Add ferrite bead on GPIO18 trace.

7-4. GPS NOT ACQUIRING

Check 1: Allow full 3 minutes for cold start. NEO-6M cold start is typically 1–3 minutes with clear sky.

Check 2: Verify GPS module receives +3.3V (or +5V per module spec). Some GPS modules with onboard LDO accept 3.3–5V.

Check 3: Confirm UART2 is configured: Serial2.begin(9600, SERIAL_8N1, 16, 17). Verify no conflict with other GPIO users on 16/17.

Check 4: In firmware, verify GPS is polling: gps_poll() is called in main loop; check that Serial2.available() returns > 0.

Check 5: GPS antenna: module must have external active patch antenna with clear sky view. Most NEO-6M modules include an antenna; verify it is connected.

7-5. SD CARD NOT RECOGNIZED

Check 1: Card is FAT32 formatted, ≤32 GB. SDXC cards >32 GB require exFAT and different SD library; use ≤32 GB.

Check 2: SD module SPI connections: CS=5, SCK=18, MISO=19, MOSI=23. Verify no signal inversion or float on MISO (add pull-up if needed: 10 kΩ from MISO to 3.3V).

Check 3: Level translation: if SD module is 5V type, 5V on MOSI/SCK from module will damage ESP32 inputs. Use 3.3V-only SD module.

Check 4: Reduce SPI frequency in SD.begin() call from 4000000 to 1000000 (1 MHz) for troubleshooting.

7-6. AUDIO TONE NOT WORKING

Check 1: Audio mode set to OFF? Check Settings screen. Check 2: MUTE_PIN (GPIO26): verify HIGH (not muted). Check 3: ledcSetup() channel 0 configured correctly in setup(). Check 4: Check RC filter (R=1kΩ, C=0.01μF) output with DMM AC. Should show AC voltage varying with tone. Check 5: LM386 supply: verify 5V present at VS pin. Check 6: Speaker: verify continuity; replace if open.

================================================================================ CHAPTER 8 MAINTENANCE ================================================================================

8-1. SCHEDULED MAINTENANCE

8-2. BATTERY REPLACEMENT (FSM-A)

Replace when battery indicator drops below 7.5V (FSM-A DMM reading). Replace both batteries in FSM-D (2× PP3 series) simultaneously. Do not mix alkaline and carbon-zinc types.

8-3. FIRMWARE UPDATE (FSM-D)

Step 1: Connect CYD via USB to computer running Arduino IDE. Step 2: Open field_strength_cyd.ino and fsm_datalogger.h. Step 3: Verify TFT_eSPI User_Setup.h matches CYD pinout. Step 4: Upload. Verify boot on serial monitor at 115200 baud. Step 5: Re-enter calibration constants (EEPROM may be reset if EEPROM address map changed between versions).

================================================================================ CHAPTER 9 PARTS LIST ================================================================================

See individual schematic files for complete BOMs: schematics/sch_passive_fsm.txt (FSM-P parts) schematics/sch_active_fsm.txt (FSM-A parts) schematics/sch_digital_cyd_fsm.txt (FSM-D additions) schematics/sch_probes.txt (probe parts)

Supplier recommendations: Mouser Electronics: mouser.com (passive components, ICs) Digi-Key: digikey.com (all component types) Mini-Circuits: minicircuits.com (ERA-3SM, attenuator pads) Analog Devices: analog.com (AD8307) Amidon: amidoncorp.com (toroid cores) AliExpress: aliexpress.com (CYD board, GPS module) McMaster-Carr: mcmaster.com (hardware, enclosure materials) 1N34A sources: eBay, RF Parts, or Antique Radio Supply

================================================================================ APPENDIX A ANTENNA FACTOR TABLES ================================================================================

A-1. THEORETICAL ANTENNA FACTOR (dB/m) — HALF-WAVE DIPOLE (free space, 50Ω)

Formula: AF(dB/m) = 20 log10(4πf/c) = 20 log10(f_MHz) − 29.8

A-2. APPROXIMATE ANTENNA FACTOR — 50 cm TELESCOPING WHIP (estimated)

A-3. APPROXIMATE ANTENNA FACTOR — 10 cm SHIELDED LOOP (estimated, near-field)

These values are for near-field measurements only. The loop is an H-field probe; E/H ratio is not 377Ω in near field.

================================================================================ APPENDIX B S-METER REFERENCE TABLE ================================================================================

================================================================================ APPENDIX C BAND FREQUENCY REFERENCE ================================================================================

================================================================================ APPENDIX D FIELD STRENGTH CONVERSION TABLES ================================================================================

D-1. FIELD STRENGTH vs. dBm (at 50Ω, various probes)

NOTE: dBm values assume λ/2 dipole at 14.175 MHz (AF = −12.5 dB/m) and include free-space conditions. Actual measurements will vary.

D-2. POWER DENSITY CONVERSION

================================================================================ APPENDIX E DATA LOGGING CSV FORMAT AND ANALYSIS ================================================================================

E-1. LOG FILE FORMAT

Column headers: UTC_Time, Date, Lat_deg, Lon_deg, Alt_m, Sats, Band, Freq_MHz, Atten_dB, dBm, V_rms_uV, E_Vm, S_units, Notes

Example line: 14:23:45,2026-04-25,37.30234,-120.48122,52.3,8,40M,7.150,0,-45.2,3.89,0.107,4.7,auto

E-2. PYTHON ANALYSIS SCRIPT (basic)

import pandas as pd import folium

df = pd.read_csv(‘FSM_20260425.CSV’) df[‘E_dBuVm’] = 20 * np.log10(df[‘E_Vm’] * 1e6)

# Create coverage map m = folium.Map(location=[df[‘Lat_deg’].mean(), df[‘Lon_deg’].mean()], zoom_start=14) for _, row in df.iterrows(): color = ‘red’ if row[‘dBm’] > -60 else ‘orange’ if row[‘dBm’] > -80 else ‘green’ folium.CircleMarker( location=[row[‘Lat_deg’], row[‘Lon_deg’]], radius=5, color=color, fill=True, popup=f”{row[‘dBm’]:.1f} dBm, {row[‘E_Vm’]:.4f} V/m @ {row[‘UTC_Time’]}” ).add_to(m) m.save(‘coverage_map.html’) print(df[[‘Band’,‘dBm’,‘E_Vm’,‘S_units’]].describe())

================================================================================ APPENDIX F GLOSSARY ================================================================================

AF Antenna Factor. Ratio of incident E-field to antenna terminal voltage. AF (dB/m) = E_dBμV/m − V_dBμV at antenna terminals.

dBm Decibels relative to 1 milliwatt, at 50Ω system impedance.

dBμV/m Decibels relative to 1 microvolt per meter; field strength unit.

ERA-3SM Monolithic Microwave Integrated Circuit preamplifier, 20 dB gain, 0.1–3000 MHz, made by Mini-Circuits. SOT-89 package.

Field Strength The magnitude of the electric (or magnetic) field at a point in space, typically expressed in V/m or dBμV/m.

Friis Friis Transmission Equation. Relates received power to transmit power, antenna gains, distance, and wavelength.

FSM Field Strength Meter. Generic term for this instrument set.

IARU International Amateur Radio Union. Sets standards including the S-meter calibration reference levels.

MDS Minimum Detectable Signal. The weakest signal a receiver or detector can distinguish from the noise floor.

NMEA 0183 National Marine Electronics Association serial data standard. Used by GPS modules to output position and time data.

S-meter Signal-strength meter. Calibrated to IARU standard: S9 = −73 dBm HF, S9 = −93 dBm VHF; 6 dB per S-unit.

V/m Volts per meter. The SI unit of electric field strength.

================================================================================ END OF TECHNICAL MANUAL TM-FSM-001 Rev A RF FIELD STRENGTH METER — ANTENNA TESTING AND RF DETECTION ================================================================================

DISTRIBUTION: Retain with instrument. For corrections, contact originating technical authority.

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