how to control the bandwidth of an antenna?

resonant frequency is determined by size and dimension
radiation pattern is determined by type of antenna

so how to control the BW of the antenna?
in other words, control Q factor? but how?

Please help~ thx

Hi,
it's depending on the type of antenna. Antennas in resonance (e.g. dipole, patch...)are usually narrow, antennas that are not resonant (e.g. Vivaldi, spiral antenna) are broadband.
The size influences the bandwidth as well e.g. if you shorten a dipol and match it with a coil it get pretty narrow in bw. Use very thin material for antennas make them narrow, very thick material makes them broadband.

...there are many differences and no general rule... really depends on the type.

Ciao, Bodo

a increase in the permittivity of the dielectric constant would results in decrease of the bandwidth of the resonant frequency. This occurs from the fact that increasing the permittivity lowers the resonant frequency,

Q that is given by the equation: Q=(resonant frequency)/Bandwidth

Thus for a constant Q, the decrease in the resonant frequency will cause a corresponding decrease in the antenna bandwidth.

it is possible to achieve a broader bandwidth by using parasitic elements in antenna design.
The gain and bandwidth of an antenna are related with the following equation
(Gain *BW) /(volume)=K, where K is constant of proportionality

So for a given gain the bandwidth is directly proportional to the volume of the antenna. Hence, increase in the volume results into increase in the bandwidth.

I am addiding to the cooments of rula:
* lowering the substrate permitivity.
* increasing the thickness of substrate.
* cutting slots in the patch (thus lowering the quality factor)
* Using fractals.
* using photonic band gap.
And finally check the book
"Broadband Microstrip Antennas Girish Kumar and K.P. Ray Artech House Inc.

hi hamid,
what is photonic band gap?
and how to use it?

Thx

Photonic band gap cristals are periodicaly placed holes on the dielectric substrate of MSA, which can limit the amount of surface waves that are confined in that region. Please, search for the "photonic band gap cristals" on the internet, and yo'll find useful stuff in this field. Any other questions, please let me know.