Friday, November 24, 2017

Antennas: Why They Matter and When Do You Want to Use APs with Internal vs. External Antennas

Here is a practical guide on when you want to use APs with internal antennas vs. APs with external antennas.   For the purpose of this blog, I'll be using two indoor APs, the EnGenius EAP1300 (internal antenna, ceiling mount) and the EnGenius EAP1300EXT (external antenna, wall or ceiling mount) for demonstrative purposes.  The content, however, applies to any vendor that has APs of comparable specifications with both internal and external antennas, for both indoor and outdoor applications.

What are Antennas

Antennas serve to shape and focus the radio signal in particular directions.   Antennas, therefore, act like a lens for RF frequency.   Every radio system (Wi-Fi, cellular, cordless, walkie-talkie, etc.) requires antennas on both the transmitter and receiver to shape and focus the signal.   Antennas are passive devices and work in both directions - i.e. an antenna equally increases the radio's ability to talk (transmit) and to hear (receive).  

The signal gain (i.e. strength) of an antenna is measured in "dBi", or decibels relative to an isotropic radiator.  An isotropic radiator is defined as a point-source of RF signal where the energy radiates spherically equally in all directions.  Such an antenna cannot physically be built, but it serves as a useful mathematical reference, as such an antenna has no gain, or a gain of 0 dBi.   

Antenna gain, therefore, is based on their deviation from a perfect sphere. A typical "rubber duck" dipole omni-directonal antenna typically has a doughnut-shaped pattern with the antenna sticking through the hole of the doughnut.  As this shape is "roughly spherical", these antennas typically have fairly low gain (i.e. 2 - 3 dBi).  The gain of such an antenna can be increased by lengthening it.  When doing this, you are increasing the energy propagated horizontally by stealing it from the energy propagated vertically.   One can also make directional antennas, which serve to focus the bulk of the RF energy in one particular direction.  Such antennas have very high gains as they deviate dramatically from a perfect sphere.   

Examples of increasing antenna gain.   Top:  Low gain dipole.  Middle:  High gain dipole.  Bottom:  High gain directional.  
A couple of notes on directional antennas: 
  1. Just as antenna designers cannot build a perfect sphere, they cannot build a perfect cone.   As a result, there is some amount of RF energy that is projected and received in the other directions.  These are known as backlobes and sidelobes, as seen in the figure above.   If two neighboring antennas are placed very close together, they can interfere with each other, thus a certain amount of separation distance (at least a few feet) is generally recommended when placing directional antennas next to each other.  
  2. The beamwidth is defined by where the energy of the antenna drops by 3 dBi (i.e. half) of the peak.   Thus, while the gain of the antenna is less beyond this beamwidth, it is also generally not zero, which needs to be accounted for in Wi-Fi design.

There are various types of antennas that can be constructed, as shown below.

Examples of different antenna types, with their typical potential for beamwidth and gain.
Like lenses, antennas are tuned to work at particular frequencies, as the length of the element is a function of the operational wavelength.  In Wi-Fi, this means that you will often see separate 2.4 GHz and 5 GHz antennas; these may look identical on the outside (i.e. the plastic radome that covers the antenna), but are actually different on the inside.  Some antenna manufacturers are able to make "dual-band" antennas that work at both 2.4 GHz and 5 GHz frequencies, though such dual-band antennas generally require a compromise on the gain of the antenna for each frequency.

APs with Internal Antennas

EnGenius EAP1300
(indoor 802.11ac wave 2, 2x2:2, internal 5 dBi / 5 dBi omni-directional antennae)

An indoor ceiling mount AP with internal antennas is generally designed to be mounted on a ceiling, with most of the antenna energy being projected outwards (horizontally) and downwards (vertically).  Such a device can naturally be mounted on the wall, but then the area of coverage changes to project most of the energy both up and down (vertically) and outwards primarily in one direction (horizontally).   Since the internal antennas are fixed, the area of coverage is also fixed based on how the AP is physically mounted.


Differences in coverage area when mounting an AP on a ceiling vs. on a wall.
(Figure source:  Ruckus Wireless™ ZoneFlex™ Indoor Access Point Release 9.5 User Guide)

For most indoor Wi-Fi deployments in environments such as schools, apartment buildings, hotels, offices, etc., the built-in internal antenna of a ceiling mount AP, like the EnGenius EAP1300 depicted here, provides sufficient coverage for the desired area. It also satisfies aesthetics constraints, as people generally do not like seeing external antennas, especially in most indoor environments. Additionally, for 802.11n/ac features like MIMO to work properly to achieve faster speeds, the relative alignment of the antennas to each other is critical.  For an AP with internal antennas, this alignment is fixed at the factory and cannot be altered.

Naturally, however, when you select an AP with an internal antenna, you lose the ability to change that antenna in your design, such as providing larger areas of bi-directional coverage in particular directions.   

APs with External Antennas




EnGenius EAP1300EXT

(indoor 802.11ac wave 2, 2x2:2, external 5 dBi / 5 dBi omni-directional antenna)

When an AP has external antennas, the antennas can naturally be adjusted to fit the coverage area.   A vendor will typically supply omni-directional dipole antennas.   

In this example, both the EAP1300 and EAP1300EXT include 5 dBi antennas.   Accordingly, the gain is the same, and the effective coverage area will be "approximately equivalent" for both models when configured with the same transmit power settings.  I say "approximately" because there will be some subtle differences in the antenna pattern between the internal and external antennas.  

As mentioned above, aesthetics are often a reason to not go with external antennas.   More importantly, however, the MIMO capabilities of 802.11n/ac take advantage of phase offsets between the antennas.  This necessitates that the multiple antennas are at a fixed separation distance from each other so as to be out of phase with each other.   With internal antennas, this phase offset is fixed at the factory and cannot be changed.  For external antennas, the relative alignment of the antennas can be easily altered, either during install or during the AP's normal lifecycle, which will corrupt the MIMO performance and therefore the ultimate performance of the AP.   

The primary advantage of an AP with external antennas, such as the EnGenius EAP1300EXT, is that you can replace the included antennas and use either higher gain omni-directional or directional antennas.  The antenna that you select will be highly dependent on the particular application.  Typically, external antennas are useful for applications where additional range is critical in a specific direction, such as when covering warehouse aisles or an outdoor area with limited AP / antenna mounting options.  In such applications, external sector or patch antennas are usually suitable. For MIMO APs, sector and patch antennas also have their individual antenna elements pre-aligned and fixed by design internally, thus meeting both aesthetic and MIMO constraints.

Takeaway Message

I generally only recommend using an AP with external antenna ports in cases where the dipole antennas packaged with the AP are NOT actually going to be used, but instead are going to be replaced with an external directional antenna, such as a sector or patch. If omni-directional coverage is the requirement, APs with internal antennas are typically the most suitable.

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