Two identical antennas are placed in the $\theta=\pi / 2$ plane as shown in the figure. The elements have equal amplitude excitation with $180^{\circ}$ polarity difference, operating at wavelength $\lambda$. The correct value of the magnitude of the far-zone resultant electric field strength normalized with that of a single element, both computed for $\phi=0$, is
- $2 \cos \left(\frac{2 \pi s}{\lambda}\right)$
- $2 \sin \left(\frac{2 \pi s}{\lambda}\right)$
- $2 \cos \left(\frac{\pi s}{\lambda}\right)$
- $2 \sin \left(\frac{\pi \mathrm{s}}{\lambda}\right)$