microstrip antenna design

[sonalik]

hello to all
i want to design a microstrip antenna which has the bandwidh of 869-890MHz. i am having difficulty in getting the entire band. which i found with rectangular patch, was very small range of bandwidth. suggest me a better design plz.

 

[WimRFP]

increase height of patch above ground plane. Doubling the height will double the useful bandwidth.

Use of an additional resonator so you get a pass band that looks like an over-critically coupled 2 resonator band pass filter. By using two resonators (that is the antenna element itself and an additional resonator), you can double the useful bandwidth.

using a seperate non-resonating resonator is the least difficult design, as the resonators do not couple via radiation or near field. Using a patch with double/multiple resonance is possible, but more complicated.

You may also use material with lower er (of course this increases the size).

Some references: "Electrically Small, Superdirective, and Superconducting Antennas" and "Compact and Broadband Microstrip Antennas".

 

[sonalik]

thank you for ur reply.
actually i am considering some parameters in my design. i have to design the antenna on apoxy glass having er=4.4 and the substrate height of 1.6 mm. i cant vary these values.
do u still have some suggestions. i will surely try the additional resonator.

 

[WimRFP]

Do you mean that the total heigth for the antenna construction is just 1.6 mm?

I really doubt whether such antenna will have reasonable radiation efficiency as the loss factor for standard FR4 material is high, and height/lambda is too small. In addition "er" is uncontrolled so can be anything between 4 and 5.

You need to have air between the ground plane and the PCB. You may elevate the PCB (without ground plane) above a metallic ground plane. This reduces effective "er", reduces the dielectric loss and increases the useful bandwidth. The requried other functions (matching, splitter, delay line, etc) can be routed on a PCB that is directly on de metallic groundplane.

If the antenna needs to have good circular polarization across the whole frequency band, consider a wilkinson splitter with 90 degrees delay line to create the circular polarization. If the available height for the antenna is relatively large, you may get the desired BW (VSWR=2) with a single resonator.