Coaxial Fed Monopole Antenna
in CST, can someone please explain to me? many thanks
A monopole antenna is inherently an unbalanced antenna, so it it usually easy to connect a coaxial feed - but we take it step by step.
Imagine first a horizontal ground-plane
The "pure" monopole looks like one half of a dipole, stood vertically, and driven by a source at the base, between the end of the monopole, and the ground-plane.
Now consider "top-loaded". If the monopole is shorter than any odd-number multiple of a quarter-wavelength, but longer than an even number of quarter-wavelengths, then the monopole impedance at the generator point looks partly capacitive..This is fixed by adding a some inductance loading to tune out the capacitance at the wanted frequency. It can be done at the base or at the top. Commonly, if too short, a base loading coil is used.
"Top loading" is more common for the other case, where the monopole is longer than the odd number multiple of quarter-wavelengths, and the impedance looks inductive. In this case, the "top loading" consists of adding capacitive structures to the top of the monopole, sometimes called "capacitive top-hat".
These top-loading has much effect on the attempt to feed the antenna at the base with a coaxial cable, and there are many tricks to finding a combination that will directly match a 50-Ohms cable. These can include feeding at some other point than the base, or coupling to the monopole with short structures at the base (like "gamma match") among others.
For the present, consider the case where the top loading has been designed to tune out whatever reactive component the monopole had, so we are left with a pure resonant monopole, tuned to a frequency we want, and the feed point impedance is purely resistive. In theory, one can directly connect a coaxial cable to the monopole, with the outer conductor connected to the ground-plane, though it is likely be a disappointment will follow when the monopole impedance turns out not be conveniently be 50 Ohms.
This is where some kind of matching network .. a transformer .. something, is needed to get efficient energy transfer between the monopole and the cable