Why you are using capacitive hat in short antennas?

Answer 1

Capacitive hats are used to alter the distribution of the current on a short antenna so that the antenna current is not concentrated near the ground. A whip or short antenna can be modeled as a series of LC circuits in which the capacitance per unit length is highest near the base and the inductance per unit length is highest near the top. A capacitive hat simply adds capacitance to the antenna. It basically makes the antenna seem longer, but maximum current is still at the base.

The reason why you'd want to make your short antenna longer, is to obtain the antenna's resonance frequency. The length required to do this is a quarter or a half of the wavelength of the radio wave that it's receiving. At resonance, the electrical resistance of the antenna drastically drops, which effectively increases the Q factor, or quality factor, of the resultant output signal. The term Q factor is simply a measurement of how good the clarity and stability of the output signal is, the higher the better when dealing with radios and televisions.

Of course, instead of using a capacitive hat on your antenna, you could always physically lengthen the antenna itself. However, considering that commonly-used radio waves can be up to 10 m (33 ft) long, your antenna would have to be about 8 feet long, which would make using your radio a bit awkward.