External Antenna Access Point is better in wireless coverage?
External Antenna Access Point is better in wireless coverage?
It is often perceived that the access points will provide better wireless coverage or higher transmission strengths if the devices has external antennas rather than built-in internal antennas. This is one of the common customer facing questions when we are designing which access points to be used in the project. For this blog, we are going to look into antenna radiation patterns for us to tell the difference of these access points. In fact, access points with integrated or internal antennas doesn’t mean compromise in wireless signal coverage.
Before we get to the comparison of external and internal antenna models, we need to understand there are different types of antennas, and these antennas are designed for different purposes.
Omni-directional antenna radiate an RF signal equally in all directions. A perfect omni-directional antenna would radiate RF signal like a theoretical isotropic radiator.
Semi-directional antenna radiate an RF signal in a specific direction. It is used for short-to medium distance communication.
Highly-directional antenna radiate an RF signal also in a specific direction but with more focused and narrow beamwidth. It is used for longer distance point to point communications. For example, bridging networks between two buildings that are far from each other.
To identify the RF signal characteristics of the antennas, we can leverage the antenna radiation chart that illustrates the radiation pattern of an antenna. These information can usually be found in the datasheets of antennas or access points by referring to Azimuth and Elevation chart. Below are the examples:
A radiation pattern has both horizontal and vertical coverage area. Azimuth chart shows the signal pattern from looking down from a ceiling at a vertical polarized antenna; while the Elevation chart illustrates the signal pattern looking at the same antenna but from a side view. In either chart. The antenna is placed at the center. The degree symbols along the outer ring of the chart indicates the coverage pattern around the antenna from 0 to 360 degrees.
Besides, you will also find a number of concentric circles originate from the center of the chart showing different number of decibel (dB) levels e.g. -10, -20 & 0.
These charts are often misinterpreted because of this representation of dB mapping of antenna coverage.
The outer ring of the chart usually represents the strongest signal of the antenna. However, the chart does not represent distance or any level of power or strength;
It represents only the relationship of power between different points on the chart.
Decibels are essentially a logarithmic measurement of the gain or loss that the antenna provides. 3 dB of attenuation reduces absolute power to 1/2 of the original power. And 10 dB of attenuation reduces absolute power to 1/10 of the original power.
After understanding how to read the Azimuth and Elevation chart, let us go back to our topic and compare the radiation pattern for the access points with internal or external antennas. In the example below, we will compare two models from Cisco Aironet 2700 Series which are both 802.11ac dual band with 3x4 MIMO supporting 3 spatial streams. In order to better see the difference, we leverage iBwave software tool (http://www.ibwave.com) which support 3D Wi-Fi planning that provide us the 3D visual of antenna RF pattern.
Since both models are with Omni-directional antenna types. They radiate in all directional and we can see the radiation pattern is circular. Another observation we have is that the access point with external Omni-directional antenna is more like a doughnut shape pattern; while the internal antenna model is more like a heart shape pattern. However, the signal coverage difference is not that huge. This is because both access points also have their transmission power strictly related by the law and local regulatory.
AIR-CAP2702I-x-K9 (Internal Antenna Model)
AIR-CAP2702E-x-K9 (External Antenna Model)
Thus when we are generally designing the WLAN for indoor environment, we will propose the access points with built in antenna. It makes more sense to us as we do not need to maintain additional hardware components. When we have other specific deployment requirements e.g. like the time we would need to focus the RF signal towards a specific direction, then we would consider external antenna model through usage of different external antennas.