A coaxial cable is made of two brass conductors. The spacing between the conductors is filled with Teflon $$\left( {{\varepsilon _r} = 2.1,\,\,\tan \,\,\delta = \,0} \right)$$. Which one of the following circuits can represent the lumped element model of a small piece of this cable having length $$\Delta \,\,z\,\,?$$

Consider the 3 m long lossless air-filled transmission line shown in the figure. It has a characteristic impedance of $$120\pi \,\Omega $$, is terminated by a short circuit, and is excited with a frequency of 37.5 MHz. What is the nature of the input impedance $$({Z_{in}})$$?

The directivity of an antenna array can be increased by adding more antenna elements, as a larger number of elements

A coaxial capacitor of inner radius 1 mm and outer radius 5 mm has a capacitance per unit length of 172 pF/m. If the ratio of outer radius to inner is doubled, the capacitance per unit length (in pF/m) is _____.