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Wireless Basics

Introduction

Some *really* big antennas

Photo courtesy of Wikipedia

Part 1 of this series provided an explanation of WLAN basics and described common wireless problem symptoms and their probable causes. Part 2 showed you how to perform a site survey to get information about your wireless environment, while Part 3 described the principles behind increasing wireless coverage and talked about improvements on the AP end. This time, I'm going to cover the hows and whys of upgrading antennas.

NoteNOTE: Please read references to access points (AP) or wireless routers as applicable to both kinds of products unless otherwise noted.

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 WLANs. 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? Unless you use the trick of adding parabolic reflectors to your existing antennas, your AP's antennas need to be attached via connectors.

Many Linksys wireless routers, including its ubiquitous WRT54G, have upgradable antennas attached via RP-TNC connectors. Many of D-Link's 802.11g wireless routers also have connectorized antennas, but use smaller RP-SMA connectors. Figure 1, which is taken from the Citrus Cables RF Connector Identification Chart, shows the jack version of both connectors.

RP-TNC Jack RP-SMA Jack
RP-TNC Jack
RP-SMA Jack
Figure 1: Popular WLAN Antenna Connectors (not to scale!)
(Photos courtesy of Citrus Cables)

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 widely available, and it's unlikely that the FCC is going to come knocking at your door to shut down your wireless LAN!

Tip! TIP: The entire FCC Part 15 Rules are located here.

Connectorized antennas were relatively common with 802.11g routers and APs. But for draft 802.11n gear, manufacturers are moving toward internal or hard-attached antennas. (See Where Have All the Antennas Gone?)

Hard-attached antennas don't necessarily mean that you have to break out your soldering iron, however. Because draft 11n routers often have miniature UHF connectors inside (usually Hirose U.FL) to attach the radio board (or radio section of the main board) to the external antennas that are hard-attached to the case.

Figure 2 shows what U.FL connectors look like and this Wikipedia photo can get you really up close and personal with the connector.

WRT610N 2.4 GHz radio with UF.L connectors
Click to enlarge image

Figure 2: WRT610N 2.4 GHz radio with UF.L connectors

Another miniature UHF connector that you might run across in current products is Murata's SWF series.

Murata SWF connector

Figure 3: Murata SWF connector

It tends to be used on client cards, however, such as the Linksys WPC100 shown in Figure 4.

Linksys WPC100 board with Murata SWF connector
Click to enlarge image

Figure 4: Linksys WPC100 board with Murata SWF connector

Of course, connectorized antennas are easier to deal with and changing them doesn't involve voiding your router's warranty. But if you're really determined to move up to higher gain antennas, it's nice to know that it will only involve buying (expensive) "pigtail" connectors vs. soldering.

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