WLAN-Antenna Project

Transcription

1 Report WLAN-Antenna Project PPS Antenna Tower Uetliberg FS 08 Mathias Aebersold Matthias Bucher Page 1 of 7

2 For this project we wanted to build a WLAN-Antenna which has different and for some usage better characteristics than the standard omni directional antenna. Before we started the actual process of building we had a look into the basic technologies behind WLAN, to understand what we were actually doing. This report is structured in the same way. Firstly we will give you an introduction into WLAN, secondly we will describe several existing types of antennas and our actual choice. Thirdly we will show and explain the building of it and at last the evaluation of the antenna. About WLAN WLAN (Wireless Area Network) is a technology to connect a device wirelessly to another device, mostly a router. This is often used to connect a computer to an existing network within a distance from 30 up to 100 meters, depending on the conditions. As wireless implicates it's done by transmitting data over the air by using radio waves. Technology behind WLAN The basic of WLAN may be already clear to mostly everybody, but the interesting point is what is really behind this technology. In this chapter we will try to give an introduction to the basics technology of WLAN. Before we go further, we have to distinguish between the several existing WLAN standards. Since the introduction of WLAN in 1997 there are at least seven different standards. But you will probably only use one or two of them, standard g or n, where n is the newer one. We will only consider standard g, but the basics are about the same in all standards. The radio signals are modulated with orthogonal frequency-division multiplexing (OFDM) and quadrature amplitude modulation (QAM). This sounds pretty complicated but with very little explanation it should become clear. OFDM is a scheme to divide an available frequence band in to a large number of subcarriers, which are separately used to transmit different parts of your data. In all these subcarriers QAM uses two carrier waves, which are out of phase. In this way they don't interfere with each other. We have now divided our frequency band into sub-carriers, in each of them we have two carrier waves, which are modulated with a simple amplitude modulation. These are the basic technologies behind WLAN, we will now go in to more details concerning the actual usage of WLAN. Page 2 of 7

3 Types Omni directional antenna This type of antenna is mostly used for access points with no further add-ons. Omni directional antennas transmit all around and cover a big area. One can use this antenna where the coverage is not specified, i.e. (home-) networks. Sector beam antenna In opposition to the omni directional antenna, the sector beam antenna covers only a specified sector. This can be useful for example in a coffee shop, where you want only the customers being supported with wireless LAN. The coverage-angle can vary a lot, depending on the settings and design of the antenna. Radiated beam antenna A radiated beam antenna has to be set up very precisely, because they are kind of point-to-point connection, this means, the transmitting angle is very small. This antenna is mostly used for connection over longer distances (i.e. between buildings). We chose this type for our project. Picture: Our BiQuad antenna Legal aspects Since radio communication is rather restricted, we had to deal with the legal aspects as well. It turned out that in most of the countries building an antenna is allowed as long it has less than the prescribed transmission power and uses the specified frequencies. For example in Germany, the limitation is 100mW which is equal to 20dBm. Conclusion: Before you consider using the antenna you have to get your antenna tested. Page 3 of 7

4 Our Choice We had to make a decision between different possible designs, so we listed some criteria for our antenna: high power gain, easy to build and cheaper than a commercial antenna. What we came up with was a BiQuad antenna based on a CD spindle. Page 4 of 7

5 The BiQuad-Antenna The BiQuad antenna is a popular design based on two (or more) squares of wire as radiator and a metal as reflector. This antenna is a radiated beam antenna as one can see in the simulation. The main power gain is in direction of the x axis. We wanted to know how the specifications we found in our tutorial were calculated. So we did some research in the internet and found some illustrations. There are two lengths that are important: First the distance between the radiator and the reflector and second the inner lengths of the squares of the radiator. The following graphic shows the SWR (standing-wave-ratio) depending on the distance radiator reflector (in λ). The best SWR (for full reflection: SWR = 1) is at λ which is equal to 16mm. Distance radiator - reflector In this graphic the SWR is shown depending on the length of the radiator. It is optimal when the length is about 29mm. Page 5 of 7

6 Building the antenna The tutorial can be found here: vallstedt-networks.de/?fotogalerien/quad Evaluation After successfully building our antenna, we wanted to know how well it worked. We tested it in two different environments in which it could actually be used. First indoor usage and secondly the outdoor usage. We measured the signal to noise-ratio, which is the best indicator to measure the quality of the connection. Indoor usage For this measurement we tested this antenna and the standard antenna in an environment in which you would mostly use a standard antenna. A smaller room in which you want to connect with your laptop to your WLAN. You simply want good coverage in small distance but in the whole room. For this situation the omni directional antenna is perfect, that s also why standard routers utilize this type of antenna. In the following matrix the result of our measurements are displayed Antenna Standard BiQuad No Antenna 1m direct -25dBm -20dBm -54dBm 1m sidewards -24dBm -29dBm 1m backwards -25dBm -30dBm 3m direct -51dBm -48dBm -59dBm 3m sidewards -51dBm -54dBm 3m backwards -50dBm -69dBm As you can see the standard omni directional antenna has the same high quality in every direction, which is already implicated by its name. No antenna at all has the same characteristics but at a lower level. We can see a big difference in the data of the BiQuad antenna. If the antenna is directly pointed to the device, the signal is better than with an omni directional antenna, but the further you point it to the side, the worse the signal gets. This measurement showed exactly what we expected. The standard antenna is suited best for this environment, the BiQuad can be used but the small gain in main beam doesn't make up the big loss in the other directions. Outdoor usage This is exactly where BiQuad antennas have their strength and where they are actually being used. Because of their directivity you can transmit over very long distances. In the following graph you can see the measurements we did pointing the antenna out of Page 6 of 7

7 Signal/Noise Ratio in dbm the window into the neighborhood. This simulates for example a connection between two buildings which is one of the usages of BiQuad antennas. 90 Signal/Noise Ratio vs. Distance Qualität BiQuad Qualität Standard Distance in meters There are several bumps in the curves which seem rather strange at first. But they are because of trees in the way between the antenna and the laptop. The important thing is that with longer distances the quality gap between the two gets bigger, meaning that you have a pretty good connection with the BiQuad. With a standard antenna you would have a really slow or no connection at all. At 60 meters the BiQuad quality was still good, but we couldn't go any further because of buildings in between the antenna and the receiver. But connections over 200 meters are reported in various articles in the internet. For this situation our BiQuad antenna is a big success, it does the thing we wanted it to be in a decent quality. Conclusion A BiQuad antenna is a very good and still cheap and easy to make antenna which is very good for connections over long distance. But at the same time it won't give you any speed gain in the traditional usage in a house or a similar situation. If you want to build one yourself have a look at the link given above or search in the internet for it. There are many other tutorials and reports about BiQuads. Page 7 of 7