Paradigm Lost: Nikola Tesla's True Wireless Page 14

tends to be focused along the ground. In fact, if the current in amperes along the antenna is \( I_0 \), then elementary antenna theory establishes that the strength of the electric field is at a distance \( r \) from the antenna, above the earth, is given by

\[ E_\theta = \frac{I_0 120 \pi h}{\lambda r} \sin\theta \quad \text{for} \quad 0 \leq \theta \leq \pi/2, \]

where \( h \) is the length of the antenna, \( \lambda \) is the wavelength in use and \( r \) is the distance of the observer from the antenna.⁴⁰ All the distances are in meters. The meanings \(\theta\) and \( E_\theta \) should be evident from Fig. 9.

Observe that directly above the antenna corresponds to \(\theta = 0\), so that \(\sin\theta = 0\), which indicates there is no radiation normal to the earth, while along the earth \(\theta = 90\) degrees, and the radiation is maximum, which might suggest a wave gliding along the surface of the earth, provided we are close enough to the antenna to neglect the earth’s curvature. This result would certainly have been known well before 1919. The book Robison’s Manual of Radio Telegraphy and Telephony for