RF Exposure Calculator
Magnetic Loop RF Exposure Calculator
FCC Approved Methods for Station Evaluation
Source: FCC, OET Bulletin 65 Sup B (1997) p 15
Commission's limits on human exposure to radiofrequency (RF) electromagnetic fields. Amateurs can select from a number of technically valid methods that can be useful in performing the required station evaluations. In general, it will be appropriate to use one of the following methods:
Estimated compliance distances using tables developed from field-strength equations
Estimated compliance distances using tables derived from antenna modeling
Estimated compliance distances using antenna modeling (NEC, MININEC, etc.)
Estimated compliance distances using field-strength equations
Estimated compliance distances using software developed from field-strength equations
Estimated compliance distances using calibrated field-strength measurements
FCC Limits for maximum Permissible Exposure (MPE)
Source: FCC, OET Bulletin 65 (1997) p 67
Table 1 Limits for Maximum Permissible Exposure (MPE)
Assumptions and mathematical determination
The process for estimating compliance distances using field-strength equations to find the minimum safe distance is a twenty plus step process as loop current must be determined to estimated field strength;
Determine loop current from User inputs, used for H Field determination
Determine EIRP from User inputs, used for E Field determination
Determine MPE from Table 1 Limits for Maximum Permissible Exposure, FCC, OET Bulletin 65 (1997) p 67, calculated from the frequency input by the user.
Rearrange the magnetic field = µo I R^2 / [ 2 (Distance^2 + R^2)^3/2, Biot-Savart Law, magnetic field of solenoids to determine H Field Strength for distance (R).
Rearrange the power density and field strength equations, S = EIRP / 4∏R^2 = E2 / 3770 = 37.7 H^2 from FCC, OET Bulletin 65 (1997) pp 9, 19-21 to solve for E Field Strength for distance (R).
Utilizing the repetition of a sequence of operations calculate H and E Fields at 1 cm intervals moving outward from the antenna until both fields are less than their respective MPE.
Repeated for controlled and uncontrolled.
Always include 4.1 dB (2.56) for ground reflection factor based on FCC, OET Bulletin 65 (1997) pp 20-21 and let the user increase losses should they desire a free space solution.
Duty Cycle is a direct percentage of power.
Minimum power of 5 W limit was subjective.
Maximum power of 1500 W limit is the highest allowed by FCC Part 97.
Minimum frequency of 3 MHz limit is MPE break point.
Maximum frequency of 30 MHz limit is MPE break point.
Loop Circumference are most common sizes used by HAMs, 5 – 25 is approximately 0.5 M to 2 M diameter loops.
Cable losses of 0 to -10 dB was subjective.
Worst case used for Radiation Loss as it does not include DC resistance or capacitor losses.
The gain or loss of a magnetic loop antenna is a debated topic as the answer depends on a dozen variables. Worst case maximum value was used, 2.15 dBi (gain of dipole) as a loop is an infinitely small dipole. Modeling does show low angle 2.1 dbi gain is possible when the loop approaches /3 at 2 M height above ground, P. DeNeef, AE7PD, “Effects Due to Ground For Small Transmitting Loop Antennas.” QEX, July/August 2018, pp 15-17.