Why the most common antenna types are fractional of λ?
I am new to antenna. The most common antenna that we come across in literature are fractional of λ could anyone plz explain why is that so
It is the easiest antenna, since it is made up of two a metallic wires, separated by a very short distance (gap region), where the feeding line is connected and where we the field is approximated to be the same of a parralel plate capacitor.
Starting from this approximation, it is possible to derive, by exploiting the equivalence theorem, the current distribution of the antenna, from which it is possible to calculate the radiated field.
The current distribution on the antenna arms is sinusoidal with a maximum in the gap and a null in the edge of the wire.
The length of the antenna can vary from a little fraction of the wavelength to generally half-wavelegth, on the basis of the radiation characteristics you need.
This antenna has a poor directive behavior and radiates a linear polarizzation (the field is given by one component, the theta one, in spherical coordinates, centered in the gap).
An euristic way to understand the idea at the basis of this kind of antenna, is to consider a bifilar trasmission line. In that case, the field is confined in a specific region of the space. Therefore, the easiest way to change the confining behavior of the trasmission line is to deform the structure making the wire ortogonal to the previous direction of the trasmission line.
Unless, it's poor directive behavior this antenna is very simple to realize and has an input impedence which is near the typical impedance of coaxial cable, therefore it is easy to match.
Furthermore, it has an isotropic behaviour that can be useful in many applications, such as mobile phone handset.
Bye
thanks for the info
4 more detailz on antenna, just search 4 the book of
"Antenna Theory" by Ballanus.
this book is very informative.