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This article will show you how to use wireless networking (WiFi) gear to make a network link of 10km or more. The Art of Long D M ention WiFi and most people – at least those who know what WiFi is – think of a wireless link between a couple of rooms in the home or maybe a couple of offices in a building. That’s mainly because that’s all they are used to and all that they expect. But WiFi can go much further than this: the current world record for a “naked” (ie unamplified) 802.11g (WiFi) terrestrial link is (we believe still!) 280km (see SILICON CHIP, February 2007). But that was using some pretty esoteric gear including large, high-gain dishes, equipment that would probably be illegal to use here. Even so, you could use off-the-shelf and legal gear in Australia to set up a reliable, fast 2.4GHz link of perhaps tens of kilometres. The link could be used for anything a Local Area Network (LAN) connection can be used: Internet access, file sharing, Voice over Internet Protocol (VoIP), Video Surveillance and many more applications. But isn’t WiFi rated only up to 100m? Most WiFi equipment has a com- VISIBLE LIGHT ment somewhere that the effective range is something like 100m or less. That figure assumes a lot of worst case conditions, such as: • you only use the small antenna that comes with the unit • the other end of the link (a laptop?) has no external antenna and • this is all operating inside a building with walls and people in the way. By changing some or all that, much greater ranges can quite easily be achieved. Art – or Common Sense? If you think you don’t already know WiFi Transmitting end Bright light bulb (ie, high power) A reflector behind the bulb to focus the signal in the direction we want Clean bulb and reflector so we don’t lose light Aimed at the receiving end Strong transmitter power An antenna which can focus the signal in the direction we want Quality antenna cable so we don’t lose valuable signal Aimed at the receiving end Path between the two ends No trees or other obstructions in the way Low light levels (ie, dark night) No trees or other obstructions in the way Low WiFi noise Receiving end A good eye! A lens to focus the signal from the direction we want – maybe binoculars or telescope Clean Optics Aimed at the transmitting end A sensitive receiver An antenna to focus the signal from the direction we want – maybe a compass or GPS co-ordinates to help aim Quality antenna cable, so we don’t lose valuable signal Aimed at the transmitting end Table 1: it’s easy to see the similarities between visible light and WiFi signals when you compare them like this! 8  Silicon Chip siliconchip.com.au By Rob Clark, Terry Porter and Robyn Edwards (VK6XRE) www.Freenet-Antennas.com Distance WiFi how to do this, think again. What if someone asked you to use visible light to make a signalling beacon over 10km from rooftop to rooftop? We’re sure you would conclude you needed something like that in first column of Table 1. The second column in the table shows what we need for our 10km WiFi link. Notice the similarities? Let’s go through them individually: Transmitter Power The transmitter/receiver unit in the WiFi world is referred to by the all encompassing term Access Point (AP). Obviously the stronger the transmitted signal, the further it will go. Power, at least as far as WiFi is concerned, is expressed in either milliwatts (mW) or dBm. dBm is often confusing to the novice but is simply a ratio of the power with respect to 1mW. A 1mW transmitter would therefore have an output of 0dBm; a 100mW transmitter would have an output of 20dBm. Most mass-market APs have low transmitter power – as they are for the ‘50m’ market. Powers of 15 or 30mW (12 or 15dBm) are common but these are usually too low for long distance WiFi. Avoid them. At the other end of the scale you can get high power APs with transmit powers of 100mW or more. Amplifiers can boost that even further but there are legal limits on how far you can go. See the “Keeping it Legal” box for more information. The Freenet Antennas UltraWAP AP is available in a number of power siliconchip.com.au Cable Type CFD200 CFD400 RG-58/U One Bare Copper Wire 1.12mm Celled Foam 2.95mm Sealed Aluminum Mylar Aluminum Tape Tinned Copper Wire (88% coverage) Polyethylene (PE) 4.95mm 0.037kg/m 12.7mm RG8/U JIS 8D One Copper Clad Aluminium 2.7mm Celled Foam 7.24mm Sealed Aluminum Mylar Aluminum Tape Tinned Copper Wire (88% coverage) Polyethylene (PE) 10.3mm 0.108kg/m 25.4mm 50W 83% 80.4pF/m (24.5pF/ft) 19.6W/km 16.0W/km dB/m 0.540 0.493 0.424 0.326 0.228 0.159 0.130 0.075 0.058 50W 85% 78.4pF/m (24.0pF/ft) 4.56W/km 5.41W/km dB/m 0.220 0.196 0.168 0.128 0.089 0.061 0.050 0.029 0.022 Physical Properties Mechanically similar to Conductor Qty Material OD Dielectric Material OD Shield Binder Braid Jacket Material OD Mass Minimum Bend Radius Electrical Properties Nominal Impedance Velocity of Propagation Capacitance DC Resistance Inner Conductor Outer Conductor Attenuation MHz 2400 2000 1500 900 450 (~70cm band) 220 150 (~2m band) 50 30 Table 2: typical properties for high quality, low loss antenna coax suitable for WiFi. September 2007  9 levels from 60 to 200mW. The 90mW unit is a good all-rounder for long distance links that remain within the ACMA limits for directional antennas. Antennas Antennas are analogous to lenses in optics. They neither create nor destroy energy but rather focus it into a smaller beam – giving the impression of more power. The focusing power of an antenna is called “gain” and is measured in dBi. This abbreviation stands for gain (in decibels) over a theoretical isotropic (point source) antenna. But don’t let that worry you: simply remember that the higher the gain, the more focused the beam and the more accurately it must be pointed. There are lots of commercially available WiFi antennas. There are even more home-brew designs available on the web (some excellent, some not!) and some great ones have been published in SILICON CHIP (see Stan Swan’s article in November 2002; Rob Clark’s in August 2003 and Stan Swan’s WiFry antenna in November 2004.) Antenna Cable Just like a dirty lens wastes valuable light, a lossy antenna cable wastes valuable WiFi signal. But there’s another wrinkle with antenna cable: the higher the frequency, the lossier a cable becomes. Cable that is perfectly acceptable for long runs at, say, 144MHz (the Above: PC (PCMCIA) WiFi adaptor and (below) USB WiFi adaptor, both with external antenna connectors. These usually perform much better than the more usual adaptors which have the antenna “built in”. 10  Silicon Chip Fig.1: download “NetStumbler” and run it on your laptop/notebook for a really good signal strength meter. On this screen grab, the red signal is WiFi noise while the green is the wanted WiFi signal. “two metre” amateur band) can be a poor performer at WiFi frequencies – 2.4GHz (2400MHz). As a rule, we must use short, low- Understanding dBm fusing. The dBm scale can be con positive the re mo the Just remember: nal. For sig the er ong str number, the than -70dBm er example, -50dBm is strong e. itiv because it is more pos bers: Here are some sample num dBm Power (mW) 10 0 -10 10 1 0.1 loss antenna cables. Less than 3m is a good rule. In some cases, this will necessitate installing the AP in a weatherproof enclosure close to the antenna, and running a weatherproof power/ethernet cable up to the external AP. The ethernet and power cables (or sometimes one cable serving both) can be much longer than the 2.4GHz cable without appreciable loss. Because cables look similar, don’t be fooled into believing they have similar performance. The popular RG-58 coax cable looks similar to CFD-200 but at 1dB per metre, has almost twice the loss. That loss could be critical. One more point – the standard cable used for all WiFi gear has a characteristic impedance of 50W (ohms). That means – don’t even think of using left over 75W satellite TV cable! Even if it is low-loss type, the impedance mismatch will cause you horrendous problems. Table 2 shows the properties of cable that is suited for WiFi use. The CFD200 is recommended for runs up to 3m (1.5 dB of loss). CFD400 is OK for runs up to 10m (2.2 dB). Cable loss can be partially compensated for with a higher gain antenna but remember that a bigger antenna boosts noise as well as the wanted signal so may not work in high noise (eg, urban) areas. Antenna Pointing The higher the antenna gain, the narrower the beam. That means we must accurately point both the transmitting and receiving antennas. Pointing by eye, especially over long distances, is usually out of the question but can work in some cases. The best way to point is to: • Aim the transmitting end as best as possible (By eye? By compass? By GPS co-ordinates?). In practice the transmitter is an AP (in Access Point mode) connected to the antenna. • Go to the receiving end and connect a signal meter directly to the antenna. Move the receiving antenna until the maximum signal strength is seen. Tighten the bolts. siliconchip.com.au The FreenetAntennas UltraWAP V2 is only 125 x 85 x 32 mm. (excluding the removable antenna). Think of it as a WiFi Ethernet Modem. • Have a buddy at the transmitting end slowly move his antenna. Tell him when signal is at a peak. Tell him to tighten his bolts. So what is this Signal Meter? No, you don’t have to go out and buy some very expensive test gear (and test gear for 2.4GHz is always expensive!). All you need is a laptop computer and a freeware program called Netstumbler (www.netstumbler.com). This software shows a running plot of signal (green) and noise (red) on the laptop screen. Connecting the antenna (b) the right polarity – there are male and female types and to the uninitiated, they look much the same. Obstructions A WiFi signal behaves much like visible light. It does not bend, nor penetrate most solid objects – but it does pass through untinted glass very well. In addition, 2.4GHz loves to heat up water – microwave ovens (which use 2.4GHz waves, albeit at a dramatically higher level) work the same way. Keep in mind that tree leaves will cause a significant loss of signal if you are expecting WiFi to pass through them because tree leaves are largely composed of water. A brick wall or wooden paling fence may only pose a minor problem to WiFi when dry – but will act like a solid barrier in the rain! WiFi Noise With few exceptions, in Australia any device capable of transmitting intelligence requires a licence to not only operate but to own. ‑Fortunately, WiFi is one of those exceptions. The good news is that you do not need an individual license for every installation because there is a shared range of frequencies in the 2.4GHz band allocated for WiFi. 2.4GHz is a very, very high frequency (not to be confused with The ‘Rootenna’ is a 14dBi antenna with a built-in enclosure for the AP. Two of these antennas will make a legal, 15km link with 130mW APs. The manufacturers got the name from the Kangaroo pouch. The black cable is weatherproof CAT5e and carries both 12VDC and the ethernet data. ers, remote-controlled toys and even But how do you connect the laptop wireless doorbells! You may have to the antenna? Good question! Most put up with them interfering with laptops do not have an external anyour WiFi. tenna connector; they use an internal One WiFi user we know has an AV antenna. extender so he can watch TV in anProbably the easiest (and most Keeping it Legal other room. But when it is turned usual) solution is to buy an extra on, the WiFi signal disappears The Australian Communicat ions and Media Authority WiFi interface for your laptop that (ACMA, www.acma.g completely. Even swapping chanov.au) is responsible for the laws comes with an external antenna in Australia for this tec nels doesn’t help much. The only hnology. connector. Fortunately, these are solution is to turn the offending In the frequency band use d by 2.4GHz WiFi equipquite cheap these days. ment (2.400-2.484GHz), AV extender off. the bottom line is that you do If your laptop has a PC slot not need a licence if: Even if you don’t suffer interfer(they were originally called PCence from these types of devices, • You are using DSSS (Direct Sequence Sprea d MCIA slots but are now becomSpectrum) equipment. (W you will still have to put up with iFi is DSSS). ing obsolete), you can get cards all the other WiFi transmissions • Your EIRP is less than 4W. to suit with external antenna in your area. In city areas it’s not sockets. uncommon to find ten, twenty or Today, the most common solution VHF!) – in fact, until a few years ago, it more WiFi setups within range is to get a USB WiFi interface with was thought fairly worthless. It’s way of yours. an external antenna socket. Again, above radio and TV station frequenWhile WiFi protocols are designed these are quite inexpensive. But in cies – in fact, it operates in the same using military technology that makes both cases you may also need variarea of the spectrum as radar and space it almost impossible to ‘jam’ (two ous cables, called pigtails, that adapt communications. WiFi links can co-exist on the same the laptop antenna connector to the The bad news is that there are now frequencies and not garble each other’s antenna you have. huge numbers of domestic devices data), the penalty is that both links A word of warning here: be careful also using 2.4GHz – everything from will run slower. when buying a pigtail to ensure it is (a) the previously mentioned microwave So how do you do your best to the right type – there are several and ovens through to AV signal extendeliminate unwanted WiFi noise? siliconchip.com.au September 2007  11 UltraWAP runs at full speed. So we can quote the Receiver Sensitivity as: -83dBm <at> 3.3kbps, and -63dBm <at> 23.5kbps. The significance here is getting a link to ‘work’ is one thing, but getting it to run as fast as possible will mean stronger signals are required. That may require a more powerful transmitter, a bigger antenna, better quality cable or combinations of these. Designing our Link Fig.2: “real world” receiver sensitivity performance measurements for the UltraWAP V2 Access Point. This graph tells us that the received signal must be better than -83dBm to work at all and better than -63dBm to work at a reasonable speed. Beware of Access Point manufacturer’s spec sheets which give a simple, bland sensitivity specification. Experience has shown that this figure is often little more than a twinkle in the designer’s eyes! • Use low-noise antennas. The better antenna designs reject as must signal coming from off-axis as possible. The higher the gain (ergo, the narrower the beam) the better an antenna will be at ignoring off-axis noise. • Use a quiet channel. WiFi has a number of pre-determined channels of operation. If all your neighbours are on channel 1, you might want to use Channel 11. Netstumbler will tell you what other signals are out there and their channel. • Use horizontal polarisation. Most urban WiFi noise comes from the home AP – which mostly use vertically polarised rubber ducky antennas. The simple act of turning the antennas through 90° for horizontal polarisation, (if you can) will automatically reject much of the urban noise. Transmit TX Power 19dBi grid antenna +19.0 dBi 10km -120.4 dB 19dBi grid antenna 5m CFD200 +19.0 -2.7 dBi dB UltraWAPV2 (worst case) -83.0 dB Signal Margin (>5 for reliable link) 14.7 dB Effective Isoptropic Radiated Power (EIRP) [<= 36 by ACMA rules] 35.8 dB Free Space Loss Reception Antenna Gain Cable Loss dBm dB Propagation 5m CFD200 (ignoring connectors) + 19.5 -2.7 Antenna Gain Computed Results Deaf people can not hear as far as those with good hearing. That’s called sensitivity and WiFi is much the same. A sensitive receiver can “hear” weaker signals, which generally means signals from further away. Unfortunately, you cannot rely on the manufacturer’s quoted sensitivity. While some quote receiver sensitivity that is accurate, some quote receiver sensitivities that are wishful thinking. Some are realistic – for example, the Freenet Antennas UltraWAP AP has “real world” receiver sensitivity measurements as shown in Fig.2. This tells us that if the received signal falls below -83dBm, we get no data through. If the signal is -63dBm or better, the 90 mW UltraWAP Cable Loss RX Sensitivity Receiver sensitivity Table 3: fill in the gaps on the chart for your installation, add the red, green and yellow figures (taking into account minuses!). Here they come to -68.3dB, which is almost 15dB better than the worst-case receiver sensitivity of -83dB. So this link will work!. The greater the difference, the faster it will run. 12  Silicon Chip So how do we put all this together? We do what is called a Link Budget. The table below is the link budget for our 10km link. It is based on the online version available at http://store. freenet-antennas.com/linkbudget. php How are the results computed? To get the Signal Margin, we simply add all the red, yellow, and green numbers (= -68.3dBm) and compare to the minimum signal needed (-83dBm). -68.3 is 14.7dB more than -83.0. See the box on Understanding dBm. EIRP (Estimated Isotropic Radiated Power) is the sum of the red numbers: in this case 35.8 dBm. How do we interpret these results? 1. We are legal. The EIRP is below the ACMA limit of 36dBm. 2. It will be reliable. For a reliable link, we need a signal margin of 5dB or more. This allows for things like rain and other problems. As we have more signal than we need, it means the link will run faster than the slowest speed. Table 3 predicts that we would still have 5db of margin if our RX sensitivity was about -73dBm. Fig.2 predicts that with a -73dBm signal we will see a data throughput on our link SC of 12Mbps or better. Special Offer for SILICON CHIP Readers! Freenet Antennas (www.freenet-antennas.com) specialise in long distance WiFi networks. They not only have the components required but also have a free design service. Purchase anything not already on special from Freenet-Antennas before December 31 2007 and you’ll receive a 10% discount if you mention this article! siliconchip.com.au