Efficiency is a measure of how well an antenna radiates. For small portable devices, efficiency is crucial to ensure battery longevity. If an antenna is inefficient, more transmitting power must be supplied to compensate. Either the battery must be made larger to provide the necessary power, thereby enlarging the device, or the user will have to live with shorter battery life.
In addition, there is frequently a legal limit on how much power a device can transmit. This means that once this cap is reached, increasing the antenna’s efficiency is the only way to increase the radiated signal.
Furthermore, an inefficient antenna will not receive as well as an efficient one, potentially leading to poor coverage.
For high-power transmitting applications, efficiency can be important for entirely different reasons. An antenna with poor efficiency turns power into heat. Either the power is reflected back to the transmitter at the risk of damaging it, or the antenna itself converts the power into heat, and may burn up… quite literally at times.
Efficiency Vs. VSWR
Efficiency is not to be confused with VSWR.
VSWR is a well-recognized antenna parameter that tells us how much of the power sent to the antenna is reflected back. Obviously, then, an antenna with a poor VSWR reflects back most of the signal, which means it is inefficient.
However, just because an antenna has a good VSWR does not guarantee that it is efficient. A 50-ohm load has an outstanding VSWR across a wide frequency range, and yet does not radiate at all, turning the power into heat instead. This is one reason, incidentally, why resistors can be used with an antenna to increase VSWR — at the expense of efficiency.
Efficiency vs. Gain
Gain is not a direct measurement of antenna efficiency, either. Gain is a combination of efficiency and directivity. Directivity measures how much power is concentrated at the peak area of the antenna’s radiation pattern. Efficiency simply tells us how much power is actually radiated regardless of where it is radiated.
The perfect antenna would have an efficiency of 100%; then the gain would simply equal the directivity. However, no antennas are 100% efficient, which means the gain is less than the directivity.
Building Efficient Antennas
The most obvious way to increase an antenna’s efficiency is to use low-loss materials. For example, a dipole made with copper arms would be more efficient than one made of nichrome resistance wire. The reason is obvious; the resistance of the arms is eating up power.
Therefore, highly conductive material for the antenna elements is desirable. However, if the antenna is mounted with or in lossy dielectric materials (such as a poor-quality PC board), the antenna’s efficiency will suffer, for this material is absorbing power like a resistor. An extreme hypothetical example of this would be wrapping a dipole’s arms in carbon-based RF absorber. The carbon tends to absorb and convert the RF energy it receives to heat.
However, for an antenna to radiate efficiently it also needs to be the right size. Electrically small antennas are simply less efficient. To be sure, we can tune them up and bring back some efficiency (often at the expense of bandwidth), but an electrically small antenna will never be as efficient as its full-sized counterparts.
Conclusion
VSWR is a useful parameter, but does not accurately reflect how well an antenna is actually radiating. Efficiency is the measure of how much power is radiated.
For too-small antennas, the efficiency can be increased by careful tuning and design, but the end result will still fall short of the full-sized version of the same antenna and will have a narrower bandwidth, as well.
A worst-case efficiency parameter is often given as a minimum acceptable gain; however, as gain is not efficiency, it is important to keep in mind where exactly the power needs to be radiated to be used effectively. For many mobile devices, the power needs to be radiated in all directions to ensure coverage.
In the end, antenna design is a compromise between bandwidth, gain and/or efficiency, physical size, and VSWR. Skillful designing aided with simulation is a great help towards creating compact yet efficient antennas.
ARSI 