Wireless Antenna GuideChoosing the correct antenna type
2014-11-07 (updated: 2015-04-13) by Philip
Tags: Wi-Fi, antenna, Yagi
One of the most important considerations for any radio communication, including 802.11 Wi-Fi networks, is the antenna, or "aerial". Installing an external omni-directional, or directional antenna with some good gain, at a proper location can make a huge difference in reception, wireless coverage area and wireless speed. This is not always feasible, or even necessary with built-in arrays and non-removable, or internal antennas on some residential equipment. Still, it is important to understand the basic concepts to help you in choosing the right equipment for maximum coverage and minimal interference. We will focus on covering most common antenna types for higher range wireless data communications (in the 900MHz, 2.4GHz and 5GHz bands).
It is important to understand that higher gain antennas simply change the shape of the radiation pattern, making it flatter, or more directional at the expense of gain in other directions. A higher gain omni-directional antenna (most common type on routers/access points), for example, will simply provide higher gain in the horizontal plane, at the expense of lower gain in the vertical plane.
So why is a higher gain antenna useful, then ? Consider this:
Increasing antenna gain is better for wireless networks than increasing TX (transmit) power. Simply replacing a wireless router with one that has higher TX power usually does not help increase wireless coverage. This is due to the fact that 802.11 standards use "positive acknowledgement" protocol, where each data packet transmitted by your router needs to be acknowledged by the client (via a reply to the router). If no such acknowledgement is received, the sender will retry until a timeout is reached and the connection is dropped. So, while your new high power router can send packets further, and your clients may be able to "see" it, they still can't reach it to acknowledge any packets, and unable to establish a connection.
If you replace the antenna(s) with higher gain ones, however, you amplify both the transmitted and received signals on your router, which is necessary to establish a two-way connection. Wireless LANs are two-way systems. It does little good to have a router/access point with a strong transmitted signal if wireless clients don't have equivalent range to reach it back.
The antennas used for wireless networking (or any other radio transmission) can be classified as either omni-directional, or directional, depending on their gain pattern. The most common type of WiFi antenna is a "base" omni-directional antenna, typically used in residential NAT routers and access points.
Omni-directional antennas have a 360 degree donut shaped radiation pattern to provide the widest possible signal coverage perpendicular to the antenna (in the horizontal plane if the antenna is pointed vertically). A typical "rubber duck" antenna, as found on most residential routers/access points with external antennas, has vertical polarity. If aiming at it with a directional antenna, it is important to match the polarity for the best possible signal.
Yagi antennas - consists of multiple parallel dipole elements in a line, typical directional gain of up to about 17dBi. Used as rooftop TV antennas, and, more recently in ptp wifi applications.
Patch/Panel antennas - flat, square, or rectangle antennas with typical gain of 10-20dBi. Patch antennas can have multiple polarities/frequencies built onto the same antenna.
Reflector grid/dish antennas - highly directional with typical gain up to 24dBi, with parabolic reflector that is either a metal grid, or sometimes similar to satellite dish reflector.
Sector antennas are another type of semi-directional antennas that have a fan-shaped (sector-shaped) radiation pattern, somewhat wide in the horizontal plane, and relatively narrow in the vertical direction. They are typically used in cellular base station towers. Typical sector antennas have 60, 90, or 120 degree beamwidths.
Cantenna - a the other end of the "spectrum" from commercial antennas (some pun intended), the cantenna is a homemade directional waveguide antenna, made out of an open-ended metal can. Typically used as a DIY antenna to increase the range of/discover Wi-Fi networks. The original Pringles potato chips can tube used is too narrow to produce considerable gain in the 2.4GHz band, some other designs using bigger diameter cans (with calculated diameter, length, etc. to match the wavelength) work better, and can achieve up to 10-12dBi gain.
Antennas function by transmitting or receiving electromagnetic waves that are of a particular frequency. The frequency is simply the number of complete wave cycles per second, or a measure of how fast the wave oscillates. Wireless networks operate most commonly in the following bands:
2.4GHz band (2401MHz - 2495MHz) - 802.11b/g/n Wi-Fi networks
There are a couple of other notable bands used for long range backhaul for networks and point-to-point wireless bridging:
900MHz band (902MHz - 928MHz) - sometimes preferred for point to point wireless bridges where there are some obstructions (no direct line of sight - NLOS).
Antennas are designed to operate in a specific frequency. Dual-band antennas combine multiple separate elements to be able to operate in more than one frequency.
If there is one important aspect of the different frequencies one should remember, it is that lower frequencies (i.e. = 2.4GHz). Higher frequencies, however, offer higher throughput.
In the U.S., Effective Transmit Power for radios is limited by the FCC to 36dBm. However, the FCC has an exception in place for fixed point-to-point wireless links in the 2.4GHz and 5.8GHz bands, allowing much higher EIRP than other bands.
Generally, antenna polarization describes the way the electric field of the radio wave is oriented. The more common antenna types are linearly polarized, either vertically, or horizontally. Vertically polarized antennas have their electrical field plane oscillating perpendicular to the ground. This is typical of most omni-directional "rubber ducky" dipole antennas. Horizontally polarized antennas have their linear plane parallel with the ground. There are also circular polarized antennas where the electric field also spins along an axis in a corkscrew pattern, however, they are much less common in wireless networks.
Some more advanced antenna designs can also have cross/dual-polarization. Dual polarized antennas usually have one vertically, and one horizontally polarized connectors, combining the characteristics of two separate antennas into one. This allows for two separate signals to be transmitted perpendicular to each other to double the capacity of the link. It is sometimes used in 802.11n antennas to enable multi-threaded communication (device can receive on one polarization and transmit data on the other simultaneously. It can also be used to transmit two signals at the same time. The advantage of this is that multi-threaded communication can be achieved with the use of a single antenna, instead of using two independent (and opposite polarized) antennas to achieve the same result.
For example, an interesting design is offered by the Ubiquiti airMAX AMY-9M16 900MHz Yagi antenna - it is a dual polarity (two Yagi antennas in one) that can transmit both horizontally and vertically polarized signals simultaneously.
There are also dual-band and multi-band antennas that can operate in different frequencies. For example, L-com makes a directional "2.4/5GHz Four Element, Dual Polarized" flat panel antenna that has four separate connectors: horizontally polarized 2.4GHs, vertically polarized 2.4GHz, horizontally polarized 5GHz, and vertically polarized 5GHz. Theoretically, you can connect 4 of your MIMO wireless device antenna connectors to it and transmit/receive four separate signals in both frequencies at the same time.
For single omni-directional antenna devices, pointing the antenna vertically offers the highest gain in the horizontal plane.
For dual omni-directional antenna setup, point one vertically, one horizontally (or at least 30 degrees offset from the first) to offer a different plane/reception. For triple omni-directional antenna devices, make sure to have one antenna pointing up to cover the horizontal plane, and the other two offset at considerably different angles, i.e. at least 30 degrees offset from the central antenna.
Directional antennas, as the name suggests, have their gain oriented in one direction. They are often used for long-range high-frequency communication such as p2p wireless Wi-Fi bridges in the 2.4GHz and 5GHz bands. For long distances, the antenna needs clear line of sight to the other end of the connection to be able to receive any signal. It is important to understand that the higher the gain of an antenna, the more directional its signal is, and more critical antenna pointing becomes. To correctly aim the antenna, you can either use visual orientation (easier with Yagi antennas), or you can actually connect a wireless access point, make small directional adjustments and peak the actual RF signal level (easier with panel antennas).
Some interference from trees and few walls can be overcome, however, it is important to note that long-range directional Wi-Fi requires line of sight (LOS). If elevating the directional antennas on both sides of the connection, one must also consider RF LOS (radio-frequency line of sight), which is different than visual LOS and dependent on the Fresnel zone - an oval shaped radiation pattern between the two antennas that needs to be free of obstructions. This Fresnel zone area varies depending on the frequency of the link and the distance. For 2.4GHz, over 1000 feet (300m) the Fresnel zone is 11 feet (3.4m). You would need an oval shaped area with that diameter in the center mostly clear for RF line of sight.
Antenna connectors and cables
If you plan on using any length of cable between the antenna and your transceiver (access point), make sure to use proper quality antenna cable, LMR200 or better (LMR195 is ok for very short runs). Do not use thin "antenna" cable from eBay without information on its exact specs! Any length of those cheap unknown cables will introduce loss greater than any gain from your antenna, just don't use them. Please also note that proper antenna (and Ethernetcables are 50 Ohm (not your regular 75 Ohm RJ-6 or RJ-59 video coax cable).
Below are common antenna cables and their loss per 100 feet at 2.5GHz
Any splices or other connections add about .2dB of insertion loss.
Note that there can be quite a bit of difference in attenuation in the variations of the same "standard" cable, (at 2.5GHz) i.e.:
For other specific cable types and lengths, you can try the Coax Cable Attenuation calculator