Impedance transformation of antenna

Hi all,

What does impedance transformation mean with regards to antenna?
Is this transforming the input impedance(Z11) of an antenna to the desired value?
Can an L section using lumped components manipulate the resonant frequency of an antenna?

My next question is, what is meant by loading an antenna with inductive or capacitive elements? How can it be done? Is it possible to produce these effects, by modifying the (original)antenna geometry?

Regards, Vineeth

For most folk, this means to alter the driving point impedance by adding some sort of matching scheme.
This is usually a network of matching components/devices , between the the energy source and the antenna driven point.

Stretching the meaning a little, it might also mean additional antenna structure changes, elements, dimensions, feed matching structures that are also radiating parts, etc. to "transform" an inconvenient impedance into one that might suit a transmission line.
Yes - that is the normal purpose of impedance transformation matching schemes.

Yes it can - if the lumped element is deliberately an unscreened part of the radiating structure, but it need not be so.
A non-radiating matching network only changes impedance. The rest of the antenna then resonates at its own frequency.

An example of antenna where the matching scheme also affects resonance is a loading coil at the base of a vertical antenna where the length of the coil is a significant part of the antenna size, and fed from a another component to a tapping part way up. Its an L-Network, but part of the network is also radiating.

Usually, the antenna structure has it's own resonant frequency that is not altered by the matching network.
it is often more convenient to have it this way, reducing the variables in the scheme.

The example above answers part of that. A example monopole rod above a ground plane will have an impedance between its base, and the ground plane. If the length is shorter than the resonant length (1/4 wavelength), then the impedance appears capacitive. Longer than resonant, then it will appear inductive. A pole physically too short to fit a quarter-wave long current distribution can be padded with a inductance, to bring the whole thing to perform (electrically) like a much longer antenna, but will radiate only from the current from the upper part of its structure, the currents in the short loading coil unable to deliver radiating fields into the space around it.

As you ask such typical questions, I would recommend you to read the ARRL Antenna book where your questions are answered. An exact answer will sound too theoretically and in antenna design you will need a B.S. degree to understand.