Tony’s Cafe

I’ve been meaning to put up information regarding my experiences with 2.4GHz 802.11, amplifiers, and antennas. I have designed and implemented some installations for ship-to-shore use, to provide high speed TCP/IP communication to the Internet. There are some radio power restrictions to be aware of and ways to calculate the power output of a 2.4GHz system. Also, there are different places where you can purchase this equipment, to help keep down on the cost… which is always a factor.

Amplifiers and Antennas

An amplifier allows you to extend the range of your 802.11 signal, but that’s not all you’ll need. A proper antenna and cable is also needed when connecting your 802.11 card, access point, or bridge to an amp. There are two types of amplifiers: “indoor” and “outdoor”. Indoor amplifiers are usually placed immediately after a 802.11 device, with the antenna cable (and antenna) following. The more recommended amplifier is the outdoor or “mast mount” amp, as it amplifies the outgoing and incoming signals right at the most lossy point — the end of the cable that connects to the antenna. While indoor amps are usually a single self-contained unit, outdoor amps do have their drawbacks. They are usually two separate units; a DC injector which is placed at the beginning of the antenna cable run. The DC injector supplies the power to the outdoor amplifier at the antenna. Since the amplifier needs to be mounted close to the antenna, the amplifier will need to be enclosed in a weatherproof housing. When I implemented the ship-to-shore wireless connection, I used indoor amplifiers — I didn’t want to expose any mast mount amplifiers to the wind and sea spray of open waters. (Note: If you are going to use amplifiers or the radios on a boat, make sure you have clean AC power! Either use an AC outlet marked “CLEAN,” use a UPS, or make your own DC-to-DC hack. The dirty AC current with the alternator and engine noise will wreak havoc on the signal!)

The type of cable and length are important factors, too. The less loss, the better. For example, a cable-TV grade coaxial cable is more lossy than a LMR400 double-shielded coaxial cable. If you are using an indoor amplifier, the LMR400 is the way to go. Also, the shorter the cable, the less loss, of course. If you use an outdoor amp, you might be able to get away with using more lossy cable.

While the maximium wattage for a transmitter in the 2.4GHz ISM (Instructional, Scientific, and Medical range — an unlicensed frequency range that 802.11b/g utilizes) is 1 Watt, there are some constraints to the antennas you can use. The output at the antenna is the most important thing, ultimately. There are two different guidelines in the U.S. governing omnidirectional and directional antennas. The maximum power output for an omnidirectional antenna is fixed at 36dBm. For a directional antenna, it varies depending on your radio power and the total signal loss of the antenna cable. (“Radio” means the 802.11 network card, access point, or bridge.) The equations for determining your total power output of your 802.11 system is as follows:

You first need to convert the total power of your 802.11 radio from mW to dBm. If your amp has an Auto Gain Control (AGC) feature, it will be the maximum output of your amplifier. If your amp doesn’t have AGC, then you will have to add the power output of your radio to the power output of your amp.

Radio Power [dBm] = LOG10(Radio Power [mW]) * 10

Now, depending on your cable length and it’s rated cable loss per 100 ft., you’ll need to compute the total cable loss. The “1.5dB” below is the loss of the two connectors on the antenna cable (1.0dB total) and the “pigtail” connector (0.5dB — in case your amplifier does not have AGC) between the amplifier and radio.

Total Cable Loss [dB] = 1.5dB + ( Cable Length [ft.] * ( Cable Loss per 100ft. / 100 ) )

The total output power of your 802.11 system is:

Total Output Power [dBm] = Antenna gain [dBi] - Total Cable Loss [dB] + Radio Power [dBm]

If you are using an omnidirectional antenna, your total output power must be less than 36dBm. If you are using a directional antenna, your maximum power will have to be less than:

Directional Antenna Maximum Power Limit [dBm] = 30 + ( 3 * ( 30 - ( Radio Power [dBm] - Total Cable Loss [dB] ) ) )

I have also created an Excel spreadsheet that will do the calculations in case you don’t want to do them yourself.

Multiple Antennas

There are times where you may want to use multiple antennas at one radio. While you are limited in power per antenna, you can have multiple antennas to increase your range. For example, in the ship-to-shore situation, I had one 120 degree wide-angle antenna on the shore. (Since I mounted this antenna on a building on the shore, I was only concerned with signal in front of the antenna — out to sea — and not behind it. This allowed for less noise from anything behind the antenna.) On the ship, I connected two antennas to the radio. One 8dBi omnidirectional and one 14dBi yagi directional. Since I had two antennas, I could use two amplifiers — one for each antenna. To achieve this, I used a splitter at the radio. A splitter will split the signal allowing two (or more) cables to run to antennas. Remember that splitters also introduce loss into the power equation, so if you use my calculator above, be sure to add this loss in with the cable.

In this case, I ran two cables from the splitter; one to the omnidirectional antenna, and the other cable to the directional. (Note: I used an 8dBi antenna for two reasons — An 8dBi antenna has a greater vertical beam area, so if the ship rocks, I have less of a chance of losing the connection. The other reason is because using an 8dBi with a 1 Watt amp and 50 feet of LMR400 cable puts me within legal limts. Using a 15dBi omnidirectional antenna will put me outside of legal limits.) When the ship was close at shore, the omnidirectional antenna would provide a connection within about a 5 mile radius on a clear day. The directional antenna, when pointed back to shore, increased the ship’s distance to 13 miles on a clear day. Not too bad!

Since the 14dBi yagi directional antennas are relatively inexpensive (you can find them for 99 cents on eBay sometimes) it would be interesting to see if you could put three of them together. A three way splitter with three amps could be utilized to increase the beam area and range.

Equipment

I used Cisco Aironet 350 wireless bridges for that ship-to-shore project. They were great and even though 802.11b communication is half-duplex, the Cisco bridges performed beyond my expectations. I was able to use two MPEG-1 encoder/decoders by V-Brick for a high-quality ship-to-shore video conference, have a Quicktime broadcast from an underwater ROV (Remotely Operated Vehicle), check my e-mail, AND surf the web. I used Hyperlink Technologies’ antennas, amps, and cable. Not too shabby! You can also view a diagram of the design.

Cost and Purchasing

I was searching the Internet, looking for a low cost way to amplify a 2.4GHz signal, better known as the frequency Wi-Fi or 802.11 uses for data transmission. Hyperlink makes these amplifiers (1 Watt is the maximum for unlicensed use in the U.S.) but they are a bit on the expensive side. Hyperlink’s 2.4GHz amplifiers are excellent quality; I have implemented them a few times for ship-to-shore high speed TCP/IP communications in Long Island Sound and in Thunder Bay. Hyperlink’s 1 Watt amp is about $180, but you can build a similar home brew one for under $100 from the Low Cost Wireless Network How-To web site. BUT, if you’re lucky, you might find a Hyperlink amplifier at a low cost on eBay (as well as cables and antennas…)

This entry was posted on Tuesday, April 19th, 2005 at 9:41 pm and is filed under Wireless. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.