Upgrading Antennas - Making the Choice
The art of antenna selection can get fairly involved when you're trying to get a long-range outdoor link to work. But for indoor or short-range (house to workshop, across a street, etc.) use, selection is fairly easy if you keep two rules of thumb in mind:
Rule of Thumb #1: It takes a 6-dB (dBi) increase in gain to double the range over what you get with a simple dipole antenna. Doubling would be a best case because your WLAN includes obstructions and other effects that reduce the actual range-boosting effect.
Rule of Thumb #2: The higher an antenna's gain, the higher its directivity or area of coverage will be. This effect is similar to what happens with binoculars or telescopes. The higher the binoculars' power, the narrower its field of view will be.
TIP: An antenna's directivity is also commonly referred to as its "beamwidth"
With these rules in mind, let's look at the types of antennas made for indoor use and their merits for specific applications...
TRENDnet TEW-IA04O 4dBi Omni Antenna (Photo courtesy of TRENDware) | Type: Dipole Gain: 4 dBi Pattern: Omni | This is a typical omni-directional desk-mount type antenna, with slight gain over a normal 2.2dBi dipole The primary reason you'd use this would be for flexibility in locating the antenna separate from the AP itself, with minor performance improvement from the 4dBi gain. |
HyperGain HG2404CU 3.5dBi Ceiling Mount Antenna | Type: Dipole Gain: 3.5dBi Pattern: 90° vertical , 360° horizontal beamwidth | This antenna's radiation pattern extends in all directions around it, but in a 90° "cone" below it. Good for keeping signals for escaping to upper floors, or above a roof. With no significant gain advantage over a dipole, its primary use is for aesthetics and controlling signal leakage. |
TRENDnet TEW-IA06D 6dBi Directional Antenna | Type: Directional "panel" / "patch" Gain: 6dBi Pattern: 80° beamwidth, horizontal and vertical | The 6dBi gain should provide noticeable performance improvement. This antenna's directional characteristics will also help keep the signal inside the area in front of the antenna. The 80° horizontal and vertical beamwidth would provide good coverage for remote rooms, but may miss areas in the same or adjacent rooms. |
HyperGain Range Extender 8 dBi "Range Doubler" Omni Antenna | Type: Dipole Gain: 8 dBi Pattern: Omni | Appropriately named, this omnidirectional antenna's 8dBi gain meets our first rule of thumb for range-doubling. But its 16 in size might not be to everyone's taste, and remember that you'll need two of these bad boys if your AP has dual antennas! |
| Type: Directional "panel" / "patch" Gain: 8dBi Pattern: As shown | Although it has the same 8dBi gain as the omnidirectional model above, this "patch" style has a smaller 4.5 x 4.5 in size. The trade-off is its directional pattern which has relatively wide horizontal coverage for indoor use, but perhaps too narrow a vertical pattern for multi-floor coverage. This type of antenna is about as powerful as you'd want to go for general indoor coverage. |
| Type: Directional "panel" / "patch" Gain: 14dBi Pattern: As shown | The 14dBi patch is more for outdoor use in point-to-point applications, or to reach a remote omni-directional station. Although the high gain is tempting, its beamwidth is too narrow for general indoor coverage. |
To sum up, you'll need a minimum gain of 5dBi and no more than 8dBi to see a noticeable performance improvement. The choice of omnidirectional or "patch" types depends on antenna placement relative to the coverage area and any signal containment requirements. Now that wasn't so hard was it?
Before we leave the subject of antennas, let me show you why laptop wireless cards have such poor performance. As most any laptop user knows, their WLAN adapter card is highly directional, but Figure 3 shows just how much!
Figure 3: PC card antenna gain plot
(click on image for a larger view)
From Antennas Enhance WLAN Security by Trevor Marshall
Used by permission
This plot shows the relative sensitivity of a typical 802.11b PC card adapter, with the key points being:
- this is definitely not omni-directional performance
- the adapter would perform better if the laptop's body were vertically oriented!
The reason for the second point is simple. Notebooks orient a WLAN adapter card's antenna in a horizontal plane, while most access points' antennas are vertically oriented. This simple fact itself accounts for the significant performance improvement from built-in WLAN notebook adapters because of their better orientation (usually vertical when the laptop screen is raised) and design (from not having to be squeezed into a 1 x 1.5 in space in the adapter itself).
Now that you're an expert on antennas, let's explore an wireless performance improvement alternative that has become available to consumers within the past year or so.