Why Does the Size of the Antenna Matter?

The continual push for smaller and smaller devices leads to the need for smaller and smaller antennas. Often, a designer requires a reasonably performing antenna in a very small size. The trouble is these tiny antennas do not necessarily provide the desired level of performance. As we shrink the antenna, the results often become increasingly worse. But why?

Resonant Frequency

To begin with, the resonant frequency of an antenna is size dependent. Granted, it is possible to “tune up” an antenna that is too small so that it will work at the desired frequency. However, the end result will have a narrower bandwidth. While this may be acceptable in some cases, we have to keep in mind that at the bandwidth edges there is a greater mismatch, resulting in lower gain.

Bandwidth

Some degree of bandwidth is required. As we keep shrinking the antenna and retuning it, the bandwidth becomes increasingly narrow. Furthermore, even in applications where almost no bandwidth is required, there is a practical limit on far we can keep shrinking things.

As the bandwidth gets smaller, component sizes and values become more and more critical. The designer eventually bumps up against manufacturing tolerances. At that point, it becomes impossible to build an antenna to the desired specs — there is no room for error. This puts a limit on how much we can shrink the antenna.

Other Design Concerns

As the antenna shrinks and the bandwidth of the antenna gets narrower, we also need to consider what effect external influences will have on the system. For example, if we are considering a small mobile device that someone carries on their person, how much will the person’s presence affect the antenna? Enough to severely detune it?

The gain of an antenna is related to the effective aperture. As an antenna’s size shrinks, so does the effective aperture and efficiency. And while we can, at least in some cases, reasonably tune it back up, the overall realized gain will be less than that for a full-sized antenna.