지향성 안테나 [指向性-, directional antenna]

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A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources.

Directional antennas like yagi antennas provide increased performance over dipole antennas when a greater concentration of radiation in a certain direction is desired.

All practical antennas are at least somewhat directional, although usually only the direction in the plane parallel to the earth is considered, and practical antennas can easily be omnidirectional in one plane. The most common types are the yagi antenna, the log-periodic antenna, and the corner reflector, which are frequently combined and commercially sold as residential TV antennas. Cellular repeaters often make use of external directional antennas to give a far greater signal than can be obtained on a standard cell phone. For long and medium wavelength frequencies, tower arrays are used in most cases as directional antennas.

 

The Shootout
My plan was to get relative performance measurements for various designs (including mine) of homebrew antennas for 802.11b (WiFi) wireless networks. To do this, I setup a wireless link and changed only the antenna- recording each antennas' performance under identical conditions. I didn't compare them to a commercial directional antenna as my only one has a male connector and I don't have the right cable to hook it up yet. The contestants were (click on each for design specifications).

802.11b Homebrew WiFi Antenna Shootout Greg's obsession de' jour In my efforts to add the words "wireless savvy" to my network admin resume, I've been reading books and web pages on radio propagation, antenna theory and design, and building wireless networks with 802.11 (WiFi). One of the first things that got me excited was the Pringles Can Antenna.  Published on the internet and in a fine book by Rob Flickenger, the net admin for O'Reilly, this design for a do-it-yourself, VERY inexpensive antenna made from a recycled junkfood container is as cool as the other side of the pillow.  It seems that everyone is building and using these.  The various community wireless network groups all talk about them and folks are reporting that they do the job. A friend of mine built his before me and looking at his finished antenna got me excited to understand the theory of how it works. Reviewing his plan, I came up with different spacing that he Rob did.  To see if I could improve upon the design, I built mine with corrected spacing.  While waiting for some wireless equipment to come in, I started looking for my next antenna project.  Oddly, the more I studied, the less I understood.  There seems to be quite a bit of confusion on how the Pringles antenna works and what design category it falls under.  The inner lining of a Pringles can looks metallic, but my tests show it not to be. The Pringles Antenna design, and some designs that pre-date it, seem to treat it as though it were metallic.  While folks are calling it a Yagi-Uda style antenna, the design of the driven element in the Pringles can antenna looks like a Waveguide style design. Waveguide antennas don't use the director assembly (the washery bits), and therefore are much simpler to build. An old tin can of the right size, about $5 in parts and 10 minutes of time are all that are needed. The math for computing correct sizing of the components in a waveguideWiFi antenna is simple. Formulas in hand, I started searching my cupboards for tin cans that fit the spec. I found myself staring at the products on the canned food aisle at the grocery store. I even went so far as going grocery shopping with a tape measure. "No no, this spaghetti sauce looks much better. It's about three quarters of a wavelength in diameter, hon!" What the huh? On Feb 11th, Rob, posted an article on his newest homebrew WiFi antenna - a tin can waveguide! Rob used a large, 39oz. coffee can and placed a quarter wavelength driven element a quarter wavelength from the back of the can. He reported good results - even better than the Pringles can design used by so many. For the antennas I was building, I was using different measurements based on the antenna design material I had been reading. Now I'm a late entry into this wireless stuff and the experts are going a different way than me. It's time to benchmark.

A commercial Lucent "range extender" omni directional A buddy's Flickenger-design Pringles can Yagi My modified design Pringles can A Flickenger design coffee can waveguide A coffee can waveguide with corrected radiator placement A Hunts 26.5oz. pasta sauce can waveguide that fell in the optimum size range A Nalley 40oz. "Big Chunk" beef stew can Waveguide Performance numbers and methodology

The Performance Summary
The results surprised me! In our test, the Flickenger Pringles can did a little better than my modified Pringles design. Both did no better than the Lucent omnidirectional. Now this is just on raw signal strength, noise rejection due to directivity still makes a directional antenna a better choice for some uses even if there is no gain benefit. The waveguides all soundly trounced the Pringles can designs. I mean they stomped them into the ground on signal strength - as much as 9 dB better. Every three dB is a doubling in power - that's three doublings (8x increase)!

Of the waveguides, the Nalley's "Big Chunk" took top marks. It was followed by the Hunts Pasta Sauce, my modified coffee can, and the Flickenger coffee can in that order. My three waveguide designs, which utilized the correct theoretical spacing, out performed the Flickenger Yuban coffee can handily. It seems that the design formulas for the waveguide design made a sizeable difference in performance. In the yagis, it didn't matter much. This could be because neither Rob's nor my designs are anywhere near right for optimum performance for a Yagi. I've decided that Yagi design is not for the timid or non-radio-expert.

With these results, I'm convinced that the waveguide design is the way to go for cheap wireless networking. The performance is good, the cost is very low and the skill required is minimal. If you can eat a big can of stew, you can make a high performance antenna.

The How To
Build your own Tin Can Waveguide WiFi Antenna (Cantenna). It's the easiest antenna design I know of.

Copyright 2003-2007 Gregory Rehm - All rights reserved.
For information about reproducing this article in any format,
contact the author: greg@turnpoint.net

 

http://www.turnpoint.net/wireless/has.html

 

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How To Build A Tin Can Waveguide WiFi Antenna for 802.11(b or g) Wireless Networks or other 2.4GHz Applications click on image to enlarge Got no dough for a commercial WiFi antenna? Looking for an inexpensive way to increase the range of your wireless network? A tin can waveguide antenna, or Cantenna, may be just the ticket. This design can be built for under $5 U.S. and reuses a food, juice, or other tin can. I am not an electrical engineer, nor do I have access to any fancy test equipment. I've built some antennas that worked for me and thought I would share what I learned. I have no idea if this is safe for your radio or wireless network equipment. The risk to you and your equipment is yours. Building your Cantenna is easy, just follow these steps. Collect the parts Drill or punch holes in your can to mount the probe Assemble the probe and mount in can Collect the parts: You'll need: A N-Female chassis mount connector. Four small nuts and bolts A bit of thick wire A can These vendors can supply the parts (the wire and can you provide yourself). The Connector A N type Female Chassis-mount connector. one side is N-female for connecting the cable from your wireless equipment, and the other side has a small brass stub for soldering on wire. These can be found at electronics stores internet suppliers (see the list below under "Connect your antenna..." If you shop around, you should be able to find these for $3-$5. Nuts & Bolts You'll need them just long enough to go through the connector and the can. I've used #6x1/4" stainless. If your N-connector is a screw on type, then you won't need the nuts and bolts. Wire You'll need about 1.25" of 12 guage copper wire. This wire will stick into the brass stub in the N-connector. A Can This is the fun part. You're looking for a can between about 3" and 3 2/3" in diameter. The size doesn't have to be exact. I made a good antenna with a Nalley's "Big Chunk" Beef Stew can that was 3.87" in diameter. Others have reported good results with big 39oz. coffee cans that are 6" in diameter. The pringles can is really too small for good performance, however. Try to get as long a can as possible. The old fashioned fruit juice cans should work well. Click on image to enlarge

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Drill or punch holes in your can to mount the probe

The N-connector assembly will mount in the side of your can. You need to put holes in the right place to mount the connector. The placement of the hole and connect is very important. It's location is derived from formulas that use the frequency that the antenna will operate at and the can diameter. To make life easy on you, here's a calculator to figure it out for you. Can Diameter Cuttoff Frequency in MHz for TE11 mode MHz Cuttoff Frequency in Mhz for TM01 mode MHz Guide Wavelength in Inches inches 1/4 Guide Wavelength inches 3/4 Guide Wavelength inches Enter the diameter of your can above and click on the calculate button. 802.11b and 802.11g WiFi networking equipment operates at a range of frequencies from 2.412 GHz to 2.462 GHz. Ideally, with your can size, the TE11 cut-off frequency should be lower than 2.412 and the TM01 cut-off should be higher than 2.462. It would be good, also, if your can is longer than the 3/4 Guide Wavelength. If your can is a little off in length or diameter, don't despair, experimentation is fun! You want to mark the location on the can where you will put the hole for the connector. The 1/4 Guide Wavelength number tells you how far up from the bottom metal end of the can to put the center of the hole. Open only one end of your can, eat the contents, and give it a good washing. You'll probably want to remove the label too. Use a ruler to measure up from the closed end 1/4 Guide Wavelength and mark the can with a dot. If you've got a drill, select a bit that matches the size of the center of your connector. You may want to start with a small bit and work the hole larger and larger. You could even start with a hammer and nail, then use drill bits. If you don't have a drill, start with a nail hole and use a file to get the hole to the required size. If you're using a bolt on connector, make four more holes for the bolts - you can use the connector as a drilling guide.

Click on image to enlarge

Assemble the probe and mount in can Now you'll need that bit of wire. You'll need a soldering iron or a friend with one as well. Cut the wire so that when it is stuck in the connector as shown, the total length of both the brass tube and wire sticking out past the connector is 1.21". Get as close to this length as you can. When you've got your wire correctly sized, solder it into the connector keeping it as straight and upright as you can. When it's cooled, bolt or screw the assembly into your can. Put the heads of the bolts inside the can and the nuts on the outside to minimize the obstructions in your antenna. Your Done! Connect your antenna to your wireless card or access point To use your cantenna, you'll need a special cable commonly called a "Pig Tail". The pig tail connects your wireless card or access point to you antenna. one end of the cable will have a "N" Male connector (just right for connecting your your cantenna), while the other end will have a connector appropriate to your card or access point. For a good picture of a pig tail, take a look at: http://www.seattlewireless.net/index.cgi?PigTail You'll want to have a wireless NIC or access point with an external antenna connector. Otherwise, you may have to hack into the one you have to hook up the cable. I wouldn't recommend this unless you're good with a soldering iron and electronics. For this reason, I like the Agere Orinoco cards which have a nice antenna connector. Pig Tails can be hand made if you have the right tools, but it's probably easier to get a pre-made one. Try: Fleeman Anderson & Bird Fleeman Anderson & Bird has a "cantenna kit" for sale that includes the connector and pigtail.  Choose one of the "cables" links from the menu and look towards the bottom of the list. Hyperlinktech Antenna Systems Hook up your cable, point the antenna at a friend's, and see how far you can stretch you network. Be sure to let me know (greg@turnpoint.net) how it works. This antenna has linear polarization. That means that how you rotate the antenna will affect the strength of your signal. Usually, you will want to put the connection straight down, but experiment with rotating the can while watching the signal strength on your PC to get the best performance. For more information, check out these resources: The ARRL Antenna Handbook A Tin Can Design with good theory information. A free on-line book on microwave antennas "Juice Up" your Tin Cantenna with a surplus satellite dish. Spreadsheet for optimum can size calculation Go to the Homebrew WiFi Antenna Shootout Go to the Wireless Home

Click on images to enlarge

Copyright 2003-2007 Gregory Rehm - All rights reserved.
For information about reproducing this article in any format,
contact the author: greg@turnpoint.net

지향성LAN안테나.doc

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