Dear Vinod Ji

Design Frequency is the center frequency with needed bandwidth.Operating frequency is the frequency at which the communications are being made with the total bandwidth occupied by the carrier signal with modulation.Usually bandwidth of the antenna will be wider than the bandwidth of the signal so that more than one center frequency the antenna can be put in to effective use.

OK, I agree with Prof. Sundaram.

Usually a device or antenna is designed for a given frequency. However the antenna will be used for some specific communications that require a central frequency and a bandwith. The bandwidth is dependent on the modulation or the multiplexation. The bandwidth is centered around the operating frequency. Antenna designers should pay attention to both: frequency and bandwidth. For instance a narrowband antenna is useless in 3G were a high bandwidth is required.

Another example: An antenna is designed to operate at 900MHz with a bandwith of 40MHz. The design frequency is 900MHz and the operating frequency is the interval 880MHz-920MHz.

Regards,

-Enrique-

The design frequency is the specifications of the antenna at which it resonates but operating frequency is the usable bandwidth for wireless communications !!!!!!!!!!1

Design frequency is the frequency considered to decide the dimensions of antenna. (Precisely, dimensions are decided based on wave length, in tern frequency).

However, either due to material characteristic tolerence/variations or certain considerations are missed/not taken care during design, hence antenna resonates at different frequency. The frequency at which antenna has optimum result is basic operating frequency. More over, let the antenna is designed for VSWR=1.2 and design frequency is 110MHz. Actual antenna offers VSWR=1.1 at 112 MHz More over, within the frequency band of 108 MHz to 116 MHz it offers VSWR=1.2 or less. The antenna can be operated at any frequency within this band. Thus, design frequency and operating frequency can be same or different. 

Sundaram and Enrique are in my opinion correct.

Dr. Amipara has explained technically. Still if any confusion, please do write me at [email protected]  

For any system, the design starts with some specific goals so that the designed system may meet those goals. For antennas, design frequency is the frequency centering which it is meant to operate. However, no design is perfect and thus the actual centre frequency of operation often becomes different. That precisely is the operating frequency. Usually, this frequency is the one where best impedance matching with the feed network is obtained. However, it is neither to be confused with bandwidth (the entire frequency range over which a tolerable matching e.g. VSWR

Dr. Vinod, 

The design frequency is the specification which you try to achieve in simulations, and operating frequency is the one which you get through measurements often shifted due to fabrication variations.