Inside Wireless: 𝜼 - Antenna Radiation Efficiency
Channel: RF elements s.r.o.
Date: 24/08/2022

In this Inside Wireless we talk about antenna radiation efficiency, or 𝜼. It is one of the basic antenna parameters anyone working with antennas should know about. Videos referenced in this one that are good to watch as a prerequisite:

Reflection loss: https://youtu.be/jgjaFzsYd88
Antenna gain: https://youtu.be/errdGMyQF6k
RF elements Products: https://rfelements.com/products

Radiation efficiency is closely tied to antenna gain and tells us what part of the RF signal energy delivered to the antenna is radiated into the free space. It spans values from 0 to 100 %, where 100 % would be the ideal. In reality, because of the material and manufacturing imperfections, we can only approach this value. There are three main components of the signal power loss decreasing antenna radiation efficiency: Reflection, or mismatch loss; conductor loss, and Dielectric loss.

The main cause of loss within an antenna structure itself varies depending on the antenna type - the full-metal antennas are affected mainly by the conductor loss, while with PCB based antennas the dielectric loss is dominant.

REFLECTION LOSS
The reflection loss is caused by the impedance mismatch of the antenna input port and the cable feeding it. The bigger the difference the more power is reflected from the antenna port decreasing the antenna radiation efficiency. Check our older IW episode for more details on this topic.

CONDUCTOR LOSS
Electrical currents flowing through the metal antenna parts with particular resistance cause some of the RF signal power to be lost in the metal. In practice, it depends on antenna type, size, and operating frequency what the conductor loss will be but for most antennas, the conductor loss increases with frequency due to the increasing skin effect and at very high frequencies, higher surface roughness further amplifies the conductor loss.  

DIELECTRIC LOSS
Materials such as PCB substrate on which antennas are often etched have so-called loss tangent, which is a measure of how strongly particular material absorbs the power of RF signal passing through it resulting in the material heating.

In practice, it is hard to compute or separate the conductor and dielectric loss but they can be determined experimentally. To determine the radiation efficiency of an antenna, we can express the losses as efficiencies - being inversely proportional to the losses with values from 0 to 100 %, where 100 % would be the ideal value for each of them.

Antenna radiation efficiency that relates the gain and directivity is defined as the product of the combined conductor and dielectric loss because these two components are tied to the materials an antenna is made of. This is the official definition of antenna efficiency according to IEEE Standards. For more information about gain, check our previous video through the link in the corner or description.

This gain does not include losses arising from impedance mismatch, in other words, reflection loss, or polarization mismatch. Realized antenna gain includes the impedance mismatch on top of the materials losses and considers the chain of the feeding transmission line and the antenna together.

Ideally, antenna radiation efficiency would be 100 % meaning Gain would be equal to Directivity and all the power at the antenna ports is radiated into the free space and none is lost in the materials. But since real-world materials always introduce some loss, we can only approach this value. The same goes for the realized gain - there is always some mismatch between a feeding cable and an antenna, so realized gain is always smaller than the absolute gain.

0:00 - Intro
0:41 - Efficiency loss components
1:04 - Reflection loss
1:22 - Conductor loss
1:49 - Dielectric loss
2:03 - 𝜼 in practice
2:50 - Realized gain

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