antenna and the repeatability of the device assembly, but some shifting is inevitable. Some shifting of the frequency response will also occur during normal use when the device is near other objects, a secondary or temporary enclosure, the user's hand or head and so forth. Therefore, excess bandwidth should be designed in from the beginning of the product design.
Figure 1. Typical Return Loss of an Embedded Dual-Band PCB Antenna
The bandwidth of an antenna over which the return loss is acceptable is directly proportional to the volume the antenna occupies, so very small antennas can produce inadequate bandwidth, especially in the 850 MHz band where the effective volume is smallest relative to the frequency of operation.
3.2 Antenna Efficiency
For most antennas, but especially for an embedded antenna (or an external antenna mounted on a small device), efficiency is arguably the most important performance parameter, assuming the antenna produces an acceptable return loss
Antenna Fundamentals – Technical Brief