How to Extend the Range of your Base Station with an Antenna

Range Extension Discussion:

The first question you have to ask is: Do I want to create a point-to-point network or a roaming network?

Point-to-Point networks consist of two base stations that only talk to each other over large distances. A house several miles from the telephone grid might benefit from such a system, as high-speed Internet systems like ADSL depend on a low distance from the local switch. You will need a high gain, directional antenna and might even require an amplifier to overcome the distance.

Roaming networks consist of a single or multiple base stations that allow mobile clients to connect to them. This is what your Apple Airport base station does out of the box. While it sits somewhere in the house, you and your laptop can roam about and connect anywhere within range of the transmitter to your home network and potentially the Internet beyond. However, the Airport base station may not give you the range you want so that you can sit in the garden and surf the 'net at the same time.

Antennas are a passive means to increase the sensitivity of your network in one direction at the expense of sensitivity in another. Active means of range extension are electronic amplifiers that boost the signal strength. However, amplifiers are expensive and still require an external antenna. So if you can design a system without the need for an amplifier, you're probably better off financially. Furthermore, there are some legal issues you'd have to cover (more on that later)

First some basics. I am no antenna expert. In fact, I still distinctly remember that day in Sophomore year of college when I had to decide a engineering major. Looking at the math requirements for the electrical engineering major alone frightened me off - two courses more and I'd qualify for a Math minor! Thus, while I have compiled the following to the best of my abilities, I'm no RF engineer and anything you do as a result of this page is at your risk alone.

The decibel (db) measure is used quite frequently in engineering because it simplifies calculations a lot. It allows a linear representation of data series that are are growing with the power of two. Every time you double power, the db measure goes up by 3: For example, while a 500mW transmitter has a output of 27dbm, its 1W cousin puts out 30dbm. Thus, the dbm measure is a conventient way to measure the transmitting power of a amplifier design. Currently, the amplifiers found in wireless PC cards have a output between 15dbm (30mW) and 22 dbm (200mW).

Similarly, dbi are used to calculate antenna gain, a measure of how much more sensitive an antenna is over a isotropic antenna (an antenna that radiates equally in all directions). A isotropic antenna has zero gain, while highly directional atennas can achieve gains of up to 24dbi.

When the time comes to calculate the effective gain of a system, simply add dbi and dbm. Thus, it is fairly simple to calculate the total transmission strength of your system. Thus, the nearly isotropic antenna found in Apple base stations has a net output of 15dbm (35mW) + 0 dbi (isotropic antenna) for a net output of 15db. Note, that this is the transmission strength and that it says nothing about the receive sensitivity of the system. While the Hermes chipset used by Lucent is apparently quite sensitive in reception mode, the Prism chipsets used by other Wi-Fi card manufacturers are less so.

I'll start with a discussion of passive means of increasing range. Adding an antenna does not influence the current draw on your system. An antenna merely makes the Lucent WaveLAN card inside the base station less isotropic, i.e. less omni-directional. The simplest way to increase your network range is to retrofit a OEM antenna like the Lucent Extender antenna which is plug compatible with the Lucent card inside all Apple Airport Base Stations. However, the carrier housing inside the current generation base station (i.e. version 2.0 or "Snow") does not allow the plug to fit without modification- more of that topic can be found on the Snow page.

Furthermore, for every connector in the system, you lose 2.5db of signal strength and cable losses can also be significant. For example, while high-quality RG-8 coax cable loses "only" 16db per 100 feet, the thin coax wires found on adapter cables, etc. can lose up to 75 db over the same distance! I've assumed that your cables aren't too long (like the standard 18" / 60cm adapter cables) and hence will not materially influence your calculations. The following graphs show the ways you can mix and match amplifiers and antennas to increase your network range.

The simplest and least expensive way to increase the range of a first generation or "graphite" Apple Airport base station from the 15dbm it has out of the box is via a Lucent antenna like the one pictured above. Since the antenna is soldered to the cable, the connection losses are limited to about 2.5db. The gain of the antenna is 5dbi, so you come out ahead by 2.5 db. This net gain of 2.5db equates to about 75% more theoretical range in open environments and 25% more indoors/through walls, etc.

If you want to limit yourself to just adding an antenna and you own a first generation Apple Airport Base Station (a.k.a. version 1.0 or "Graphite"), then using a Lucent antenna is probably your best bet. They are relatively cheap and come in gain settings up to 15db. I mail-ordered my 5dbi gain entry-level Lucent extender antennas for about $70 (Lucent part number 848072633). They can be found at MacMall, MacWarehouse, Fab-Corp, and many other fine on-line stores. Lookup the latest prices at ZDNet. In the US, you can attach an Lucent antenna with up to 30dbi of gain for point-to-point use (though I have yet to see a high-gain directional antenna with an MC Cable plug on it). For any other use, an antenna with a gain of up to 17.5 db is permissible.

For those with a later base station (a.k.a. version 2.0 or "Snow"), the 90 degree plugs Lucent uses on its antennas do not fit unless you make some pretty substantial alterations. With the newer ABS, I'd buy a standard antenna meant for 2.4GHz use coupled with a "MC Card" or "Orinoco" to "Male-N"cable adapter from companies like HyperLinkTech (part # CA-WL2CABLE4A - $19.95), or Fab-Corp ("Orinoco to N-Male Pigtail" $22).

Adding an external antenna with a standard N-type plug causes the transmission losses to double to 5db because you now have two connections. Thus, unless the antenna has a higher gain than 5db, there is no point adding an external antenna, other than positioning it in a better location.

This approach is more expensive than the Orinoco add-ons but using an adapter cable that converts to the N-type connector standard also opens up a universe of antennas from many different companies.

Standard antennas and amplifiers only have N-Female type plugs on them. Furthermore, I have yet to find a non-Lucent vendor who ships antennas with a soldered pigtail that has a MC Card connector on it. With the above setup, you can legally attach an antenna with up to 30dbi of gain for point-to-point use and 20 dbi for roaming use.

Antennas can then be further categorized as directional and omni-directional.

For roaming use (i.e. what home users usually want) go for "omni-directional" antennas. Instead of focusing the antenna on a specific spot like a Yagi or patch antenna, omni-directional antennas radiate in all directions equally when their gain is zero. Thus, the field of coverage would look like a sphere with the antenna at the center. As the gain of an omni-directional antenna is increased, the sphere flattens like a pancake. The flatter the pancake, the greater the horizontal range. However, in return for high gain, you lose vertical coverage. (See sketch below)

How antenna gain affects the reception of your Airport Base Station

Here are Apple's thoughts on how Apple Airport Base Stations orientation affects range

For point-to-point use, go for directional antennas such as "Yagi" or parabolic antennas. They can be basically thought of as beam-emitting devices. How well the beam is focused depends on the "gain" of the antenna. The higher the gain, the longer the range. However, the spot that the antenna can send signals to and receive them from also gets smaller. High gain Yagis allow the inexpensive connection of buildings where a clear line of sight exists. Experiments in Australia have allowed amateurs to cover over 20 miles! Most home users of Apple Base Stations have no use for such antennas due to their narrow beam-like coverage.

Furthermore, Graphite Apple Airport Base Stations or their Lucent RG-1000 series equivalents will not talk to each other wirelessly (i.e. bridge from station to station) without additional software that can be bought from Winncom, who sell the software package by Karlnet. You can also call Samuel Hendricks at 1-888-946-6266 x103 (Thanks Morgan l'Argent!). And if someone can get back to me with the proper item number, I'd be really grateful. Unfortunately, the Winncom site is only semi-functional and many product descriptions are missing.

"Patch" or "Sector" antennas are also quite directional in that they are supposed to cover certain "sectors" - like an area 45 degrees wide, with a certain vertical coverage as well. They are used in buildings where they can be mounted on walls and point "inward" into the building. They are usually directional but not point to point, so their output has to be less than 30db. Below is a sketch of a directional antenna as compared to a zero gain transmitter found inside the base station. The blue field indicates the field of coverage, while the black box is supposed to signify the transmitter. Top views are followed by side views.

Note how the zero-gain system radiates equally in all directions, while the highly directional antenna is only sensitive on one thin axis. This is why directional antennas are used for point-to-point applications, while omni-directional antennas are reserved for roaming use.

And now for the power user section:

Power users might opt to add a two-way amplifier to their antenna. With amplifiers you can easily reach the legal limits of unlicensed transmission. Amplifiers are usually quite expensive and work better at boosting the outgoing signal than amplifying the incoming signal. Thus, they are most useful in applications where you don't upload as much from mobile clients as you download to them.

The nice thing about such amplifiers is that you retain your previous coverage, and just extend the range. If you were to start with a zero gain sphere shown above, then active amplification of the system would increase the volume of that sphere and hence increase range in all directions. However, keep in mind that you still have to stay within legal limits.

See Bill Wiekings excellent WaveLAN power cascade page for more on this subject and how to calculate the different gains in your system and what legal antenna/amplifier options are possible. There are some more excellent antenna/amplifier information resources available from these web-sites:

Tim Pozar from the Bay Area Wireless User Group on FCC regulations for 2.4GHz systems
Trevor Marshall on how antennas can increase the security of your wireless LAN system
Green Bay Packet Radio has some excellent resources including home-built amplifier designs. Their low-cost network set-up page is an excellent resource.
Check out Jean Tourrilhes' comprehensive LINUX support for WLAN site.
Last but not least, the folks at Guerilla.net make for an interesting read.

Furthermore, as I mentioned, amplifiers are expensive and their performance uneven. They start at around $300, require additional cables, etc. HyperLinkTech will not sell these amplifiers by themselves unless you are a military user or they are exported. Instead, they sell US consumers entire kits. These kits include an antenna and adapter cable and depending on application and your budget may also include an amplifier. Thus, unless you know what you're doing, you might be best off buying a kit.

If you have any further questions or comments, please e-mail me.

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