UNCLASSIFIED
TM-CAL-017
FIELD STRENGTH REFERENCE
Calibrated RF Field for Receive Antenna and Field-Strength Meter Calibration
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Electronics — Not for commercial calibration use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers use of a calibrated transmitting antenna and power source to establish a reference electromagnetic field strength. The known field is used to calibrate receiving antennas, field strength meters, and EMC pre-compliance measurement setups.

1-3. SAFETY

WARNINGTransmitting RF energy above 1 W can cause interference to other radio services. Conduct tests on a licensed frequency (amateur bands if licensed) or at power levels below 10 mW. Comply with all local regulations.
CAUTIONMaintain minimum safe distance from transmitting antenna to avoid RF burns or biohazard. For 100 mW and below, >0.5 m is adequate. For QRP (5 W), maintain >3 m from antenna.

CHAPTER 2 — THEORY OF OPERATION

2-1. FAR-FIELD ELECTRIC FIELD STRENGTH

Far-field electric field strengthE (V/m) = √(30 × PW × G) / d

Where PW = transmitter power in watts, G = antenna gain (linear ratio, not dB), d = distance in meters from antenna.

Half-wave dipole gain (linear)Gdipole = 1.64 (2.15 dBi)
Isotropic antenna gainGisotropic = 1.0 (0 dBi)
Field Strength Reference Table (Dipole, G=1.64)
Power (W)Distance (m)E (V/m)E (dBμV/m)
0.117.01137
0.132.34127
1.037.40137
5.0316.5144
5.0104.96134

2-2. NEAR FIELD VS. FAR FIELD

The far-field approximation (above) applies only beyond the Rayleigh distance.

Far-field boundary (electrically short antenna)dfar = λ / (2π) ≈ 0.159 λ

At HF (3–30 MHz), λ = 10–100 m, so far-field begins at 1.6–16 m. Tests at <3 m at HF may be in the near-field where the 1/d law does not hold.

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. REFERENCE SETUP

Equipment
ItemSpecification
QRP transmitter or signal generatorCalibrated power output, e.g., 100 mW ±10%
Half-wave dipole (TX)See TM-CAL-016; length matched to test frequency
Calibrated dummy load + directional couplerBird 43 or TinySA with coupler for power monitoring
Calibrated receive antennaField-calibrated loop, calibrated dipole, or standard gain horn
Spectrum analyzer / receiverTinySA Ultra, for measuring received signal level
Steel tape measureFor setting exact TX–RX distance

CHAPTER 4 — ASSEMBLY PROCEDURES

  1. Set up TX dipole outdoors, clear of buildings, height ≥1 m above ground.
  2. Position receive antenna at measured distance d (e.g., 3.000 m ±10 mm).
  3. Both antennas must be co-polarized (both horizontal or both vertical).
  4. Align antenna boresights for maximum received signal.
  5. Connect transmitter to TX dipole through directional coupler for power monitoring.
  6. Connect receive antenna to TinySA or receiver.

CHAPTER 5 — CALIBRATION PROCEDURE

  1. Transmit CW at calibrated power PW (measured at TX antenna feedpoint).
  2. Record received signal level Srx (dBm or dBμV/m) at known distance.
  3. Calculate expected E: E = √(30 × P × G) / d.
  4. If receive antenna is a calibrated dipole: received power Prx = E2 × Grx × λ2 / (480 π2).
  5. Compare expected vs. measured received level. Difference is receive antenna (or receiver) calibration error.
  6. Iterate at multiple distances (3 m, 5 m, 10 m) to verify 1/d falloff.

CHAPTER 6 — TUNING AND ADJUSTMENT

NOTEField strength tests are sensitive to ground reflections (multipath). Conduct tests with antennas ≥1.5 m above ground for reduced ground bounce. Early morning or late evening provides reduced atmospheric refraction effects at VHF and above.

CHAPTER 7 — VERIFICATION

  1. Verify inverse-square-law (1/d): doubling distance should reduce E by 6 dB.
  2. Measure at d = 3 m and d = 6 m: received signal should decrease by 6.0 ±1.0 dB.
  3. Deviation >2 dB indicates near-field effects, ground reflections, or antenna orientation error.
  4. Log: frequency, TX power, TX antenna type, distance, measured E, expected E.

APPENDIX A — CALCULATIONS AND FORMULAS

Electric field (far field, from transmitted power)E (V/m) = √(30 × P (W) × G) / d (m)
dBμV/m from V/mE (dBμV/m) = 20 × log10(E × 106)
Effective aperture of half-wave dipoleAeff = G × λ2 / (4π) = 1.64 × λ2 / (4π)
Received power from E fieldPrx (W) = E2 × Aeff / (120π)

APPENDIX B — EXAMPLE RESULTS

Field Strength Measurements
FreqPTX (mW)d (m)E calc (V/m)E meas (V/m)Error (dB)
14.2 MHz1003.02.352.28−0.3 dB
14.2 MHz1006.01.171.14−0.2 dB
14.2 MHz10010.00.7040.690−0.2 dB
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