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## Topband: Power Coupling in the USA

 To: Topband: Power Coupling in the USA bobnm7m@cnw.com (Robert Brown) Fri, 3 May 2002 20:25:51 -0700 (PDT)
 ```John, In regard to your question about power coupling and take-off angles (TOA) of antennas, power coupling is greatest when the E- field from an antenna is parallel to the geomagnetic field and the least when the two are perpendicular to each other. Good examples of the two circumstances are, first, propagation in all directions from a vertical antenna at one of the magnetic poles where the field is vertical and, second, E-W propagation from a vertical antenna at the geomagnetic equator where the field is horizontal and in the N-S direction. Numerically, power coupling is calculated by finding the degree to which the radiation field at a radiation angle has an E-field component parallel to the magnetic field. It involves the elliptically polarized waves propagated in the field and the geometry of the waves relative to the antenna. In short, the numerical factor by which power coupling affects the signals entering or leaving the lower ionosphere is largely geometrical in origin. Of course, there is an infinity of antenna situations and a similar degree of multiplicity when it comes to magnetic field inclinations and radiation angles. By way of illustration, the vertical variation of signal intensity from a simple vertical antenna over a perfect ground plane and power coupling are given below for propagation of signals to the north and south of Omaha, NE, where the field points 4.7 degrees east, is at 20.4 degrees with the zenith and directed down into the ground:. Angle RF Coupling Coupling (deg) (vert. ant) (to North) (to South) 3 0.997 0.963 0.748 6 0.989 0.971 0.737 9 0.976 0.981 0.722 12 0.957 0.991 0.704 15 0.933 0.998 0.685 18 0.905 1.000 0.666 21 0.872 0.996 0.647 24 0.835 0.985 0.629 27 0.794 0.969 0.612 30 0.750 0.947 0.596 33 0.703 0.921 0.582 36 0.655 0.893 0.568 39 0.604 0.864 0.556 42 0.552 0.834 0.546 45 0.500 0.804 0.536 Couplings are given as numerical factors, not in logarthmic notation, and the relative signal strength from the vertical at a given radiation angle is the product of the RF and coupling factors. It is seen that the coupling is greatest to the north at 18 degrees elevation, close to the inclination I of the magnetic field. To the south, the coupling factor at that angle is 0.666, because the field to the south is tilted AWAY from the direction of propagation. In any event, the angular variation is slower for the power coupling than the radiation pattern. That is because power coupling depends just on differences in angles whereas the radiation intensity depends on interference, because of path differences between direct and ground-reflected rays. 73, Bob, NM7M ```
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