Because of the sheer length of the radio waves in certain bands, portable antennas or antennas in low space/stealth applications use a number of tricks or compromises to get resonance and radiating power sufficient to make contacts.
Two of these tricks and compromises used with vertical antennas are Loading and Ground Radials.
Loading is a technique where electrical length is added to the antenna by using coils of wire placed in the middle of the antenna. In effect, part of the antenna is compressed into a smaller space but the electrical current still “sees” a longer path.
There is a cost for this technique however, in the adding of more inductance to the antenna.
Loading most often is implemented at the base of the antenna for practical reasons– such as stability, but that is not the most efficient place compared to loading at the middle or top of the vertical.
Here are a couple of resources discussing how loading at different points in the antenna differ.
Base vs. Center Loading Vertical Antennas – Discussion of loading using different positions in the antenna at Ham.stackexchange.com
Center vs. Base Loading Antennas – Similar discussion of antenna loading positions, including the loading of fiberglass masts at worldWideDX.com
“The antenna ground plane acts as a simulated ground. It is found that for a monopole antenna like a quarter wavelength vertical, the ground acts as a plane to reflect the radio waves so that an image of the top half of the antenna is seen in the Earth.
It is possible to simulate this function by replacing the real earth with a conducting plane. To function as an antenna ground plane, the conducting surface must extend for least a quarter wavelength from the base of the antenna.”
Antenna Ground Plane: theory & design (At Electronics-Notes.com) – excellent discussion of how ground planes work, how impedance is altered depending on the angle of the radials.