This tip covers how to make some basic improvements in WiFi signal coverage by changing the antenna radiation pattern.
WAP antenna basics
The antennas supplied with most Wireless Access Points (WAPs) are simple and inexpensive. They are referred to as “rubber duck” or “rubber ducky” style or sometimes a “stick”, due to their short and “rubbery” plastic construction. They are used in a lot of RF equipment (2-way radios, receivers, etc.). The origin of the name is obscure, but who cares.
The insides are small and simple, consisting of a folded dipole made from the small coax cable that runs into the plastic shell. The shield is “folded back” over the coax, usually formed by a small metal tube. The coax center conductor that sticks out and the shield are both a Â¼ wavelength at 2.4 GHz (about 1.25″). This is a bare minimum antenna, but for short distances it does the job.
When a WAP has two antennas, it’s capable of diversity reception/transmission. There are actually two receivers and transmitters in the box. The WAP compares the signal strength it receives from both antennas and uses the best one. The selection is constantly changing as the remote device moves or there is more than one remote device.
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There are two things you need to understand to optimize the signal reception/transmission from a WAP: polarization and gain. Bear with me, when you’re done reading this, you’ll feel much better.
Polarization refers to the orientation of the RF signal in space. All radio signals are made up of a propagating (traveling) interconnected electric field and magnetic field. These two fields are oriented 90Â° from each other. By convention, the orientation (vertical or horizontal or anywhere in between) of the electric field defines the polarization of the radio signal and the antenna that created it. The “stick” antennas supplied with WAPs polarize the transmitted signal in the same way they are oriented, i.e. vertical polarization if they are placed vertically. They can be oriented an any direction, but most remote devices assume they will be oriented vertically and in 99% of your installations this is the way you want them oriented. We’ll see why that’s a good thing shortly. FYI – FM radio transmission are oriented vertically (see car radio antennas) and television transmissions are oriented horizontally (TV antennas are horizontal).
Antenna gain is the ability of the antenna to direct or focus the transmitted radio signal in a certain direction. Antennas are not amplifiers, they do not increase the power coupled to the antenna, they just focus it the same way a flashlight focuses light. This has the effect of concentrating power in one direction but removing it from others. Again, the amount of power focused in one direction is referred to as it’s gain and can be expressed in decibels (like amplifier gain).
If an antenna has a “gain” of 3 dB (twice the power in some directions), you have to know what the 3 dB is referenced to. There are several possibilities, but most WAP and WAP antenna manufacturers seem to have chosen to measure the gain referenced to a theoretical isotropic antenna. The above diagram illustrates what isotropic radiation would look like. This is an antenna that radiates equally in all directionsâ€“a creature that doesn’t exist because you can’t physically make an antenna that does that, but you can calculate what it would do and then compare any real antenna against it. So a gain that is referenced to an isotropic antenna is written as dBi. If you don’t see a letter after the dB, then the number is worthless…not that they’re that good anyway. Be very skeptical about manufacturer claims about antenna gain. There are no real standards for measurement and manufacturers can claim about anything they want and it would be difficult at best to disprove.
OK, where was I…So the stick antennas supplied with most WAP’s have about a 3 dBi gain. The gain is achieved by the radiation pattern of the dipole. It looks like a donut pattern centered around the antenna (see figure below left and below). The dipole spreads the energy out more horizontally around it’s vertical axis. This is a good thing, since everything we want to communicate with is mostly in a horizontal direction in the house.
This is why you want place the WAP in the center of the home. Also, if you are installing in a 2-story house, it’s best to put a WAP on each floor since they don’t radiate well up or down.
If you want to see a cool animation of how RF radiates from a dipole, go to:
Simple things you can do to improve performance
Now that you understand how the stock WAP/antennas work, it’s easier to understand some things that you can do to improve the default antenna radiation pattern.
First, there are the standard good practices:
Keep the WAP as centered as possible in the living spaces where remote/portable devices will be located. Keep the antennas vertical! You want the signal to spread horizontally.
Also, mount the WAP above eye level. There is just less stuff higher up to absorb the RF. If you put the WAP down low, the signal has to pass through furniture, people, dogs, etc. Keep it away from AC ductwork. This means not mounting it too high.
Visualize the donut radiation pattern from the dipole antenna and you can “visualize” where the weak spots are going to be. As I mentioned above, in a multi-story house, mount a separate WAP on each floor since they don’t radiate up or down near where they’re mounted.
To insure the best signal to / from laptops, use a separate WiFi card with an external antenna. The internal antennas in laptops are usually unpredictable and in metal cases can be very poor performers.
Changing the Gain and Direction
You can increase the horizontal gain over the stock antenna by purchasing a larger, higher gain vertical antenna.
The Linksys HGA7S (left) is a typical example. Linksys claims a 7 dBi gain for this antenna. It’s probably a co-linear dipole. Inside, there are two folded dipoles, one on top of the other. This antenna will increase the horizontal spread while making the vertical spread smaller.
When you need to “point” most of the RF in one direction, for example when the WAP has to be mounted in one end of the house, you can easily make a simple metal reflector to mount next to the WAP antennas. The metal acts as a “mirror” to the RF that would be radiated in the direction of the reflector.
The above diagrams show the dimensions for the one or two antenna version. The reflector can be made from aluminum, sheet metal, brass, etc. You can get small sheets of metal from the hobby section of Ace Hardware and cut it out with scissors. The important distance is the 1.25″ from the antennas to the plate.
This will focus almost all the RF away from the reflector. It’s good for focusing all the RF energy in one general direction in the house without making the pattern too narrow. The resulting RF pattern is show below. The spead is not too narrow, probably between 60 and 90Â°.
The photo below shows a reflector I made from a thin sheet of aluminum. Took me about 1 hour total to cut, drill, and bend.
I cut the reflector to mount directly on a D-Link WAP that wall mounts. The two holes in the reflector fit over the antenna connectors to hold it in place. The reflector doesn’t need to be the height of the antennas because the tops of the two antennas you see have nothing in them! The real antenna part is 2.5″ high starting about 1″ up from the bend joint in the rubber ducky. I guess making them bigger makes them look more “powerful”.
This reflector works great giving at least a 6 dB signal increase in the “forward” direction. The reflector can be used to actually mount the WAP to the wall. You could just mount the WAP a sufficient distance off the wall and mount a square of aluminum on the wall “behind” the antennas.
Thing to avoid.
Beware of high gain directional antennas unless you really know what you’re doing. Tthere are a lot of high gain (>10 dBi), very directional antennas on the market. They work fine but are designed to concentrate the RF in a narrow field, typically less than 30Â°. They’re usually intended to establish a WiFi link between two remote buildings. This might be fine if you have a remote building on your job site, but it’s not very practical within or around a single building.
Avoid mounting the antenna separately from the WAP using an extension cable. The signal loss in the small size coax cable used to reach the antenna can be >10 dB, basically negating any advantage of the remote antenna. If you have to mount the antenna separate, use the shortest length of cable you can. Try to keep it less than 4 ft.
If you have any questions about anything in this TIP, drop me an e-mail at firstname.lastname@example.org I will actually answer it!
Copyright 2008, Training Reels