Tim Higgins reports the AirStation 54Mbps Wireless card with Antenna Interface ($119), delivers 54 Mbps on the 2.4GHz band. It works with any Type II or Type III CardBus ready PC notebook and the MMX connector extends wireless range and performance with an external antenna.

The G54 AirStation PCI Card comes with an Omni Antenna ($139), an MMX antenna jack and is bundled with a 2.2dBI omnidirectional antenna.

Buffalo Tech’s WLA-G54 AirStation ($189) is a 54Mbps Bridge Base Station featuring a Wireless Distribution System (WDS) Bridge/Repeater with integrated 4 port 10/100 switch that supports point-to-point and point-to-multipoint bridging and wireless repeating.

Broadcom-based, 54-G product outputs 32mWatts. You might plug in a +14dB panel ($37), a +18dB panel ($69), a +19dB panel ($49), or a +24dB grid antenna ($149) and see how far you could go.

A directional antenna, in the 2400-2483.5 MHz band used exclusively for fixed, point-to-point operations, “may employ transmitting antennas with directional gain greater than 6 dBi provided the maximum peak output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi”.

FCC Power Regulations from the Telex FAQ on 2.4 GHz

Q: How much power can I transmit on a 10 dBi omni at 2.4GHz and still be legal?

A: The FCC regulations for PtMP allows only 36 dBm (4 watts) EIRP. This is 30 dBm (1 watt) into a 6 dBi antenna. If you use a 10 dBi antenna, you must limit your transmitter (or amplifier) to 26 dBm (10 + 26 = 36 dBm). A 200 mW card (+23 dB) can drive a (+13dB) panel (+36dB total). If you figure a - 1.5dB cable/connector loss, you could legally drive a 14.5 dB panel. Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna. Refer to the following table:

A 2.4 GHz omni antenna is restricted to 4 watts EIRP. But, if you knock off -3db loss for cabling and a lightning protector, you’d still be FCC street legal using Broadcom’s 32mW (+15dB), feeding a +24dB dish (+39dB - 3dB loss = 36dB). You could put one on a roof or nearby hill, for example, and hit dozens of clients along a linear street or park.

Point-to-point links can use lots more power. A Subscriber Unit that only talks with a central AP is considered to be PtP and can use more power. A client using a 200mW (+23db) card can legally use a +23dB antenna. The FCC allows significant “passive” gain in client antennas.

Q: How much power can I transmit with in my 2.4 GHz Point-to-Point system?

A: According to FCC regulations, 2.4 GHz Part 15.247 point-to-point transmitters may use a 30 dBm transmitter with a 6 dBi antenna. For a 3 dB increase in antenna gain, the transmitter power output must be reduced by 1 dB. Power is measured at the antenna connector, so subtract any cable loss between the amplifier and the antenna. Refer to the following table:

A stock 200 mW PC card (+23dB) can feed a +24dB dish (in point to point mode) and still be street legal because you can go up to 63 watts (+48dB). A solid-state Soekris box with a couple of 200 mW cards feeding a couple of 24dB dishes, is, in fact, the basis of the Bay Area Wireless Metro Network. Each relay node costs about $2,000.

Things get dicey if you add an amplifer. YDI’s +18dB antenna has a built-in +14dB amplifier ($495). That raises a 32mW (+15dB) radio like Broadcom’s 54G to a total of +29 dB output power. Then add the passive +18dB antenna gain and you get a total gain of (29dB + 18dB) or +47dB gain. That’s over the FCC limit.

But not by much. Because we are using a high gain antenna, we can use more power. We are only required to knock off -4 dB (if my calculations are correct). So instead of 1 watt feeding an +18db antenna we must limit ourselves to about 400 mWatts feeding the antenna.

That’s not bad. YDI’s +14dB amplifer boosts the 32mW (+15dB) radio to about 800mW (+3dB too much). But cabling and a lightning protector will knock off about -3dB so we might be right on the FCC-legal limits. We might use the YDI amplified antenna without power reduction (in long haul, point to point networks). Stock. 54G.

How far can you go?

Calculating Wireless Range:
How far can YOU go? Young Designs has an online calculator for determining range. Michael Young recommends a minimum of +10 dB to +20 dB “fade margin”. Compute your System Operating Margin (fade margin) using his on-line calculator. For sensitivity start at -88dB for a 2.4 GHz radio (at 1 Mbps). For faster speeds, use a -70-80 dB sensitivity for starters.

Roughly speaking, you combine the losses (path loss, cable & connector loss) with the gains from the antennas, radio output and sensitivity of receiver. Using (dB) instead of milliwatts (mW) simplifies calculation because you can just add and subtract the numbers.

Power needs to be translated from millwatts (mW) to decibels (dB) in order to use the calculator. Here’s the dB translation you can substitute in the on-line calculators; 30 dBm (1 W), 27 dBm (500 mW) 24 dBm (250 mW), 23 dBm (200 mW), 21 dBm (125 mW), 20 dBm (100 mW), 17 dBm (50 mW) and 15 dBm (30 mW).

So figure this; add 15dBm (30mW) for a Broadcom radio and +14dB for the antenna amp. That totals +29dB. Then add +18dB for the antenna (take out -3dB for losses). Sensitivity goes down sharply with higher speed and 54Mbps is a real killer. Let’s estimate a radio sensitivy of about -80dB (at 12Mbps) as a compromise.

Other on-line calculators include the Cable loss Calculator, Wireless Link Calculator, Distance Calculator, Fresnel Zone Calculator, Beam width Calculator, as well as other free community networking software. Antenna suppliers include Hyper Link, Super Pass, Telex and YDI.

Plug in your own numbers. Long-range, point-to-point in the 2.4 GHz band probably shouldn’t be using stock Wi-Fi components due to the constraints of the protocol (it’s designed for local area networks, after all).

The 5 GHz band & the 802.16a

The 802.16 Wireless LAN standard provides a communications path between a subscriber’s site and the Internet or ISP. The 802.16 standard (summary), supports a Metropolitan Area Network (MAN), not a Local Area Network (LAN). It assumes a point-to-multipoint topology with a controlling base station that connects subscriber stations not to each other but to various public networks linked to the base station. It is a connection-oriented service with full Quality of Service.

According to FCC Part 15.407, 5.8 GHz UNII-3 wideband digital fixed point-to-point transmitters may use a 30 dBm (1 watt) radio with directional antennas with up to 23 dBi gain without ANY corresponding reduction in transmitter power. Maximum EIRP is 53dBm (200 watts).

It’s not hard to imagine someone offering a 5.8 GHz amplified antenna for the 5.8 GHz Atheros card. Atheros puts out about 40mW (+17dB) at 5.8 GHz. An amplified 5.8Ghz antenna might boost it another +13dB and the antenna gain for a panel might add another +23db.

That’s +53dB total. The FCC legal limit. Then add QOS. With 1 watt radio and a +23dB panel, you should get about 15 miles at 12Mbps (with a +16dB fade margin).

Perhaps 5 GHz, 802.16a backhauls could feed three-sector, 802.11g neighborhood nodes or Vivato (3-mile) scanning beams on rooftops.

Ring around your city. Welcome to the neighborhood. You’re on the net.

Posted by Sam Churchill on Wednesday, April 30th, 2003 at 4:48 am.