# 47 CFR 73.190 - Engineering charts and related formulas.

(a) This section consists of the following Figures: 2, r3, 5, 6a, 7, 8, 9, 10, 11, 12, and 13. Additionally, formulas that are directly related to graphs are included.

(b) Formula 1 is used for calculation of 50% skywave field strength values.

Formula 1. Skywave field strength, 50% of the time (at SS 6):

The skywave field strength, Fc(50), for a characteristic field strength of 100 mV/m at 1 km is given by:

The slant distance, D, is given by:

Note (1): If |FM| is greater than 60 degrees, equation (1) is evaluated for | FM| = 60 degrees.

Note (2): North and east are considered positive; south and west negative.

Note (3): In equation (7), k = −1 for west to east paths (i.e., bR >bT), otherwise k = 1.

(c) Formula 2 is used for calculation of 10% skywave field strength values.

Formula 2. Skywave field strength, 10% of the time (at SS 6):

The skywave field strength, Fc(10), is given by:

(d) Figure 6a depicts angles of departure versus transmission range. These angles may also be computed using the following formulas:

Computations using these formulas should not be carried beyond 0.1 degree.

(e) In the event of disagreement between computed values using the formulas shown above and values obtained directly from the figures, the computed values will control.

- 47 CFR 73.187 — Limitation on Daytime Radiation.
- 47 CFR 73.189 — Minimum Antenna Heights or Field Strength Requirements.
- 47 CFR 73.185 — Computation of Interfering Signal.
- 47 CFR 73.183 — Groundwave Signals.
- 47 CFR 73.150 — Directional Antenna Systems.
- 47 CFR 73.182 — Engineering Standards of Allocation.