Upgrading Antennas – Factors to Consider
Although it’s tempting to think that throwing amplifiers at the problem of weak signals will be a quick and “best” fix, experience has shown that using higher-gain antennas is often simpler and more cost-effective in improving problem WLAN connections. Consider the following points:
Amplifiers boost both signal and noise. Although this isn’t really a problem in the transmit direction, amplified noise can swamp out a weak wireless client signal.
WLANs are two way systems. It does little good to have an Access Point with a strong transmitted signal if wireless clients don’t have equivalent range.
For best results, amplifiers must be located as close as possible to the AP’s antenna to avoid losing the amplifier’s gain through loss in a long cable. This requirement can complicate an amplifier’s installation beyond the point where many home networkers will want to deal with it.
So let’s say that you’re convinced that using a higher-gain antenna is the way to go. Where do you start? First, your AP’s antennas need to be attached via connectors. Although you’ll find exceptions, consumer WLAN gear seems to have zeroed in on the two connector types shown below.
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RP-TNC | RP-SMA |
Figure 2: Popular WLAN Antenna Connectors (Photos courtesy of HyperLink Technologies) | |
Linksys uses the slightly larger RP-TNC connector across their wireless line, and the smaller RP-SMA is widely used by other manufacturers of consumer wireless gear. By the way, the “RP” in each connector-type’s name stands for “Reverse Polarity”. These are special versions of each connector type that have the gender of their center contact reversed from that of the non “RP” version. This is done to satisfy Part 15.203 of the FCC regulations which says in part:
An intentional radiator shall be designed to ensure that no antenna other than that furnished by the responsible party shall be used with the device. The use of a permanently attached antenna or of an antenna that uses a unique coupling to the intentional radiator shall be considered sufficient to comply with the provisions of this section. The manufacturer may design the unit so that a broken antenna can be replaced by the user, but the use of a standard antenna jack or electrical connector is prohibited.
Translation: “We don’t want folks changing antennas on their own and possibly violating FCC specs, so manufacturers can’t use ‘standard’ connectors”. Something obviously got lost in the translation, however, since “RP” based antennas and cables are now widely available, and it’s unlikely that the FCC is going to come knocking at your door to shut down your wireless LAN!
TIP: The entire FCC Part 15 Rules are located here.
Now that you have your old antenna removed and know its connector type, how do you select a new one? There are four main factors to consider:
Operating Frequency
Because of the way that radio waves work, antennas must be designed to work over specific frequency ranges. Generally, the higher the operating frequency, the narrower the frequency range of an antenna.
For 802.11b, you need an antenna designed for 2.4GHz operation. This antenna won’t, however, work for 802.11a purposes, even if you did manage to get it attached (remember that 802.11a gear doesn’t usually allow you to change the antenna).
Gain
As I showed earlier, the simple dipole that comes with your AP has a gain of about 2.2dBi. And no, the two antennas on your AP don’t provide a total 4.4dBi of gain, but are there to support antenna diversity, which can improve your WLAN’s performance through a different technique.
This is the main thing you’re trying to improve by changing antennas and you’ll probably spend the most time agonizing over this spec.
Radiation Pattern
This factor is as important as Gain in determining whether a specific antenna is right for you. It determines the directivity or coverage area of the antenna, and if chosen incorrectly can make your wireless connection worse!
Packaging/Mounting
It used to be that the antenna type determined the physical form-factor of an antenna. However, you can now get both omni and directional antennas in different physical forms. Especially handy for overcoming a spouse’s objections to your wireless improvement plans.
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