Antenna (bandwidth, Q-factor , permitivity)

[Garmi]

Hi,

I have a confusion relating to those three parameters in antenna design. It is clearly understood that bandwidth inversly relates to Q . However, adding dielectric to the design means more energy is stored (by definition) than in vacuum, means higher Q, means lower bandwidth. This is explained when we chose material with low K in patch antenna to achieve higher BW.
But doesn't this mean in vacuum I should get higher BW as less energy is stored?? why though better BW is seen with
adding dielectric than in vacum??

Make my question more direct :
I have a slotted waveguide antenna. My bandwidth has increased from 220 MHz to 325MHz when I just load dielectric (2.2 K) to fill just 1/4 the height? Why :?::?:


Thank you

 

[FvM]

The relevant parameter for antenna bandwidth isn't permittivity, it's antenna size in relation to free space wavelength.

 

[Garmi]

Thank you. But you did not provide an answer. It is well known that adding dielectric will reduce the electrical size of antenna. My question how we relate this to Q and Bandwidth??

 

[vfone]

This difference in length is due to fringing fields around the antenna, which makes the printed antenna seem longer. For example, when designing a patch antenna its lengths typically are reduced by about 4% to achieve resonance at the desired frequency.
With the increase in height of the dielectric material, the fringing fields from the edges increase, which increases the extension in effective length, however decreasing the resonance frequency of the antenna but increasing the bandwidth. Another option to increase the bandwidth of the antenna is to decrease the dielectric constant Er.