Re: Ground reflection

"Wimpie" wrote
There is another sign that even an AM broadcast station does not have
maximum radiation at 0 deg. elevation. When you check your field
strength (E or H) graphs up to 60 miles (where earth curvature can be
neglected) you will notice that the field strength falls of faster
then with 1/distance. That would not be the case when max radiation is
at 0 deg. elevation.
_____________

No matter the relative value of its h-plane radiation, the resulting ground wave field from an AM broadcast station is subject to losses not only by 1/distance, but also as a function of earth conductivity and the frequency. Only for free space paths does the field decay by 1/distance, alone.

If the h-plane ERP of an AM monopole did not match the peak ERP of a 1/2-wave dipole when driven with the same tx power, then the monopole could not generate the same field as the peak, free-space field of that dipole, when the monopole is measured in its far field, but close enough to it for groundwave propagation loss due to the conductivity of the path to be small.

This was proven in the Brown, Lewis and Epstein study in 1937, where at 0.3 miles for 60-90 degree verticals they measured an equivalent field strength of better than 190 mV/m at 1 mile for 1 kW of radiated power. The peak, free-space field from a 1/2-wave dipole for those conditions is about 195 mV/m.

RF

.

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