Fullscreen HTML5Med Res (??MB)HTML4

The Downpipe waveguide antenna mounted on a standard TV mast. Due to increased wind loading, guy-wires are used for stability. Homebrew Weatherproof 2.4GHz WiFi Antennas In the Nov 2002 SILICON CHIP, Stan Swan introduced us to the ‘art’ of making your own microwave antenna for 2.4 GHz (WiFi) networking, using a readily available ‘kipper can’ and a piece of bent wire. There are many homebrew microwave antenna designs available on the Internet; not all of which are weatherproof. This article shows how to make two, high performance, weatherproof WiFi antennas using readily available materials and common garage tools. W e’re going to show you how to build two antennas – the Downpipe Antenna and the AntCap Antenna. Now just in case the significance of those names has been lost on you, the Downpipe Antenna is a wide-beam antenna that is suitable for use at the center of a wireless network and is, in fact, made from a length of downpipe. The AntCap Antenna is, suprisingly enough, made from (you guessed it!) an ant cap. It is a narrow-beam antenna used to connect by ROB to either another AntCap (for a point42  Silicon Chip to-point link), or to a Downpipe antenna. A Quick Review of WiFi Networks The most common WiFi standard in use today is 802.11b, which specifies a 2.4GHz carrier, and a nominal 11Mbps data transfer rate. The technology is undergoing explosive growth and development which will make 802.11b obsolete very soon but new standards continue to use the same 2.4GHz frequencies so CLARK the antennas described in this article www.siliconchip.com.au will work equally as well. Most WiFi networks resemble old 10baseT networks, which had a 10Mbps hub, with all the computers connected into the hub. These networks were collision-based – so Ethernet packets sent simultaneously, would often collide and be resent. It can be mathematically shown that collision-based, 802.11-style networks have an effective upper limit on data traffic of 30% of the nominal speed. So, 10BaseT, hub-based networks saturated at 3Mbps. 802.11b networks are virtually identical, except the hub is replaced with an antenna and an Access Point (AP) and the computers have an antenna and some sort of wireless ethernet interface. 802.11b Networks resemble hub-based wired networks The Downpipe wide-beam antenna At the center of a WiFi network, there is generally an Access Point (AP) with a wide-beam antenna. APs usually come with a short stub antenna (or two) which are have a low performance (‘gain’), adequate for distances up to 100m. If you want to have a network that spans kilometres, you will need a higher gain, external antenna. The Downpipe is such an antenna. How does it work? If you cast your mind back to those physics lessons at school that you slept through, you may remember something about organ pipe theory and resonance. Well, that is the secret to the Downpipe! It is effectively a resonant pipe for 2.4GHz electromagnetic waves (fed into the pipe by a short stub antenna), which ‘leaks’ energy out the slots. The slots are spaced so that the leaked energy is in-phase and so that the impedance is 50Ω. That is pretty much all there is to it. The magic is working out where to place the slots. In fact each slot is half a wavelength from the next and the offset from the centre defines the impedance of each slot. Parts List – Downpipe Antenna 1 1 1 1 2m length (approx) 95mm x 45mm ZincAlume downpipe. 40mm length 1.5-2mm dia. solid copper wire Tube caulking compound Roll UV-stabilized, microwave-transparent tape. 50mm wide. (Norton Part Number AT232297 Cat No. 725 Barcode: 9310357501190) 2 V-Clamps, for mounting (Jaycar Cat LT-3235) 1 3mm (1/8") aluminium pop rivet 1 Female-pin N-connector, panel mount, screw-type. (LINK Connectors Part: B30-005. See: www.gordontech. com.au) www.siliconchip.com.au If you cut slots in one face, you get an antenna that radiates in a nominal 180° arc. If the slots are cut in both faces, you get a nominal 360° beam pattern – but the signal strength is 50% (3dB) lower. (In reality, the beam is not uniform in all directions and the purists will call these Sector Antennas, as they radiate mainly in an 80° beam from each face.) The nominal gain of a single-side, 8-slot Downpipe is 14dBi. A 2-sided, 16-slot Downpipe is 11dBi. Before we start . . . First of all, you’ll need the following tools: 1. Electric router, with 6.5mm bit, or Nibbling tool (hand operated, or electric) (eg, Altronics Cat T2355) 2. Hacksaw 3. Rivet gun 4. Drill, with 3mm (1/8") bit 5. Set Square (for nice 90° faces) And you will also need software to calculate the resonant frequency and wavelength. You can download an Excel file which will do it all for you: www.erlang-software.com/FreeNet/Waveguide/WaveguideCalculator.zip The Downpipe antenna radiates, and receives, RF energy through specially spaced slots. A length (or lengths) of UV-stabilised, microwave-transparent tape over the slots helps prevent little critters (spiders, ants, bugs, etc) taking up residence inside the antenna! August 2003  43 Let’s make one! 1. Check size. Note that while the downpipe has a nominal size, it is manufactured so that one end witha taper fits inside the next. The material will either slowly change size from one end to the other, or will be deformed at one end. 2. Select squarest end. Decide from which end you will work. The one with the straightest cut is a good choice. Mark TOP with a marking pen. 3. Measure and mark pipe. Mark “BOTTOM” at the approximate location of the bottom of the air column. This is approx. 815mm from the TOP. 4. Workout the average large ID of the air column. Take a few OD measurements between TOP and BOTTOM. Decide where a good average point would be. Measure the Average OD (e.g. 95mm). Measure the material thickness (e.g. 0.4mm). Calculate the Ave ID as (Ave OD) - (2 x thickness) (e.g. 95 - 2 x 0.4 = 94.2 mm) 5. Calculate your resonant frequency wavelength. Using the Excel file which you down-loaded earlier, select the “Wavelength Calcu- Scale drawing of a Downpipe antenna on 2.437GHz. The 10mm U-bolt mounting holes are on the “back” face. 44  Silicon Chip This table shows the dimensions of the antenna drawing at left, as calculated by the Waveguide Calculator Excel spreadsheet software. For different frequencies and antenna types it’s just a matter of plugging in the appropriate data. lator” tab and enter the Ave. ID in the Large ID cell shown. Note the calculated Lg (your wavelength), and Small ID. Confirm that your tubing has a small ID that is LESS than the number calculated. 6. Calculate the Dimensions for your antenna. Select the Antenna Dimensions tab in the Excel file. The wavelength (Lg) calculated in the step above should be automatically transferred to the correct cells in this spreadsheet. Table 1 shows an example set of calculations for 94mm (ID) downpipe, tuned to Channel 6. 7. Square-off the Top end. If necessary (ie, if you didn’t use the square-cut end thoughtfully provided by the manufacturer!), use a set square and a file/grinder to get a perfectly square top. 8. Mark and cut your downpipe. Using the Antenna Dimensions spreadsheet determine the TOTAL LENGTH value and mark then cut your downpipe to this length (eg approx 900-920mm for an 8-slot antenna). 9. Mark the position of all slots. www.siliconchip.com.au Cut the 6.5mm slots with your router or nibbling tool. If making a 360° antenna, the slots on the back are positioned such that you can see through both slots from front-to-back. 10. Mount the N-Connector. Mark the position. Drill and mount temporarily. Remove. 11. Make the feed. Solder a length of copper wire into the solder bucket of the N-connector. Cut so the length of the copper wire extends 31mm from the end of the metal shield of the N-connector. 12. Cut the bottom-reflector mounting slots.      Mark the bottom of the Air Column on the SMALL sides (only). Use a hacksaw to cut through the SMALL SIDES ONLY of the antenna at the bottom of the air column. The two resulting slots will be the thickness of the hacksaw blade. 13. Make the Bottom reflector. Use an off cut to make an L-shaped reflector, which slides through the two slots (step above). It should protrude about 1 mm on the far side. 14. Drill hole for rivet. Drill a hole for the rivet so that the bottom reflector will be held in place. Note: Keep the reflector as flat/straight as possible, to maintain antenna performance. DO NOT RIVET IN PLACE YET. 15. Make the Top reflector cap. Use an off-cut to make a ‘cap’ that fits neatly over the top of the antenna. Note: Keep the reflector as flat/straight as possible, to maintain antenna performance. 16. Drill V-clamp mounting holes. These holes go in the bottom 100 mm section below the reflector, on the face with the folded metal seam. See the mounting section of the to-scale drawing for details. 17. Clean all metal swarf from inside the antenna. 18. Mount the N-Connector/Feed assembly. Caulk around edges to make waterproof. 19. Attach Bottom Reflector. Slide bottom reflector in place, and rivet on one side. Caulk around the two slots to make waterproof. Do not waterproof the inside edges of the bottom reflector. You want any condensed water (or rain) to escape. 20. Attach Top Reflector cap. The Law The Australian Communications Authority (ACA, www.acma.gov.au) is responsible for the laws in Australia for this technology. In the frequency band used by 2.4GHz WiFi equipment (2.400- 2.484GHz), the bottom line is that you do not need a licence if: • You are using DSSS (Spread Spectrum) equipment. (802.11b is DSSS). • Your EIRP is less than 4W Are you allowed to pass internet traffic over a neighbourhood WiFi network? While it has not been tested in courts, the current interpretation of the laws is that you can only carry internet traffic for a fee if you have a Carrier License. But – it appears legal to extend an internet connection within the ‘same organisation’ so long as there is no fee. Of course copyright laws apply – regardless of what medium is used to pass a copyright protected work, such as music or video. Do not rivet... as you don’t want protrusions inside the antenna cavity. The top reflector is held in place by the UV-tape in the next step. 21. Cover slots, and top reflector, with UV-tape. Installation The Downpipe antenna gets its gain by compressing the beam into a very flat, pizza shape; generally aimed at the horizon. That is great if the other antennas wishing to connect to the Downpipe are at the same elevation (height), but can cause problems if the Downpipe is mounted way above the other antennas. Usually it is best to mount a Downpipe at the height of the nearby roof-tops. Alternatively, you might consider two 180° Downpipes mounted such that they are tilted down a bit. Downpipe antennas have thin, horizontal beams. Mounting them high is not always ideal. www.siliconchip.com.au V-clamps hold the Downpipe securely to a suitable pole/mast. August 2003  45 The AntCap narrow-beam antenna If you want to make a point-to-point WiFi link, or just connect to your neighbourhood AP, you need a narrow-beam (directional) antenna. There are many designs on the Internet but not all are weatherproof, or include pole-mounting brackets. The AntCap has both features! By the way, you will notice that the AntCap is really nothing more than a waterproof version of Stan Swan’s Kipper Can antenna, which is itself an implementation of the standard ‘BiQuad’ antenna. We trust Stan will not mind. How does it work? The radiating element is a simply a pair of loops; each one-wavelength in circumference. The diagrams below show how it works. Imagine a wave traveling around each loop, and imagine the wave crest being indicated with a “+”, a wave valley with a “-“, and the zero-crossing points with a “0”. Each diagram is a snapshot, a quarter of a wave period later in time than the previous. Where the fields line up, they are shaded red for “+”, and blue for “-“. As you can see, with the feed oriented as shown, the signal appears to oscillate in a horizontal fashion. This antenna is horizontally polarized. Next, we add a back reflector one eighth of a wavelength behind the feed, so that all the energy is radiated in the same direction and we have an antenna of about 12dBi gain. Let’s make one The tools you’ll need for this antenna include: · Drill · Drill bits · Screwdriver · Soldering iron · Rivet gun · RG58 Crimping tool The first thing to realise is that we only need to weatherproof the BiQuad feed, not the reflector. While it would not particularly matter if the back reflector did rust, many hardware stores (in Australia!) sell ready-made, galvanized antcaps for stopping white ants (termites) coming up the stumps or piers and into the house timbers . . . perfect for a homebrew microwave antenna! And if your hardware store does not sell antcaps, it’s As the wave travels around the loops, the signal appears to move from side to side 46  Silicon Chip Side-on view of the AntCap antenna showing both front and rear. Construction is very simple – basically it’s just a single PC board (the actual antenna) inside a weatherproof case, fastened to an antcap (the reflector) with a V-block/U-bolt mounting assembly on the rear. easy enough to make your own from a piece of light weight Zincalume or galvanized sheet steel. Step-by-step 1. Mark center of ant cap. 2. Place short arm of Bracket against the back of the ant-cap, with center hole lined up with the center of the antcap, and aligned ‘square’ with the edges of the ant-cap. Drill four holes to suit your rivets. Note: The bracket purchased from Bunnings has four small and one large hole pre-drilled on each face. 3. Drill two holes on long arm of bracket to suit your V-clamp. 4. Rivet bracket to ant-cap. 5. Place enclosure base over the center of the front of the An AntCap with the cover removed showing the BiQuad feed PC board inside the weatherproof box. www.siliconchip.com.au Parts List – AntCap Antenna 1 AntCap 125 x 125 x 50mm (Bunnings 1079234) 1 “Angle-Pergola” Bracket 88 x 63 x 36mm (Bunnings 1071032) 1 BiQuad PC board, 55 x 98mm, coded SC06108031 4 M4x20 screws (Bunnings 643277 [pack of 20]) 4 1/4" spring washer, 1mm thick (Bunnings 2446511 [pack of 50]) 1 V-Clamp, for mounting (Jaycar LT-3235) 4 1/8" or 3mm rivets 1 Tube of flexible, waterproof caulk 1 IP65-rated enclosure, 115 x 64 x 40 (Jaycar HB-6122) 4 M3 x 25 Nylon screws (Jaycar HP-0142) 4 M3 Nylon nuts (Jaycar HP-0146) 2 M3 x 20 Nylon spacer (Altronics H-1327 [pack of 100]) 1 N connector, jack, RG58, crimp (see www.gordontech.com.au Part No. B30-330C) 1 0.4m length RG58-9006 low loss external coax (Rob Clark www.erlang-software.com/FreeNet) And here’s a front-on view showing how the box containing the antenna PC board is fixed in the exact centre of the ant cap. This antenna has a narrow beam. ant cap. Align enclosure to be ‘square’ with the antcap. 6. Drill four 4mm dia holes (one in each corner of the enclosure) through the ant cap 7. Rotate enclosure 90°. Drill four more holes at corners. 8. Locate the top-half of the enclosure. The top has four brass mounting nuts embedded in the plastic (these are NOT the ones at the corners; they are ‘inside’ the enclosure. 9. Using a hot soldering iron, remove and throw away each of these embedded nuts. Insert soldering iron into the nut, and gently rotate it out as the plastic starts to melt. 10. Insert the supplied gasket into the groove in the enclosure. The gasket is too long; cut as needed. The PC board removed from the box, showing how simply the coaxial cable connects to each dipole. www.siliconchip.com.au 11. Drill two small (!!) holes in the PCB. Each hole goes near the center of the two short parallel tracks, near the center of the BiQuad. These holes are for the Coax connection 12. Drill 4 x 3mm holes in the PC board as follows: 15mm either side of the center axis, and inline with the ‘top’ and ‘bottom’ points of the BiQuad. (See figure) 13. Assemble the N-connector onto one end of the coax cable. 14. Locate the base of the enclosure. The base has the groove for the gasket. Drill a 5mm hole in one of the 64 x 40mm faces. 15. Remove 5mm of external insulation of other end of 9006 coax. Separate and fold back the shield. Remove 4mm of internal insulation. Twist the shield so that it resembles a piece of wire than can go through one of the small holes in the PC board. You may have to use only 50% of the shield wire or it ends up too thick. Solder the shield braid together to form the ‘wire’. 16. Cut the spacers so that you have four pieces, each 8.5mm long. NOTE: The objective is to have the BiQuad 15mm from the antcap (reflector). Using the parts specified here, the spacing is: Head of nylon screw: 2mm Thickness of enclosure wall: 3mm Spacer: 8.5mm Thickness of PC board: 1.5mm Total: 15mm If you are using different size components, adjust the spacer length as needed. 17. Cut corners of PC board as needed to fit into bottom of enclosure 18. With PC board centered in the bottom of the enclosure, drill four 3mm holes through the bottom of the enclosure 19. Assemble PC board into enclosure using: four nylon screws, four spacers, PC board, and four nylon Nuts. Check that the PC board sits ‘flat’. Remove four nylon August 2003  47 References and URLS: www.erlang-software.com/FreeNet More antenna information and designs by the author, including the 6dBi Brick antenna, and the 29 dBi Satenna. Terminology 802.11b A wireless ethernet standard using a 2.4GHz carrier, and supporting 11Mbps www.nodedb.com/australia List of FreeNet nodes in Australia 802.11g An emerging wireless ethernet standard; 2.4GHz carrier, and 54Mbps www.qsl.net/n1bwt/contents.htm Online Microwave Antenna Book Channel 802.11b channels are in fact spread-spectrum frequency ranges; each 24 MHz wide, defined by their center frequency. The main channels in use are Ch1 = 2412MHz (2400 to 2424), Ch 6 = 2437 (2425 to 2449), and Ch11 = 2462 (2450 to 2474) melbourne.wireless.org.au/tib Cheap WiFi parts www.acma.gov.au/aca_home/legislation/radcomm/ acts/radcom/spread_2002.pdf The official word on licensing of WiFi (Spread Spectrum) equipment in Australia nuts, and PC board. 20. Thread the un-terminated end of the coax through the 5mm hole in the side of the enclosure. 21. Pass the ends of the 9006 coax through the two small holes in the PCB. The solder-side of the PC board should be facing out when done. 22. Solder the two ends of the coax to the PC board tracks. 23. Place some flexible caulking compound on the coax, just below the PCB 24. Using the four Nylon nuts, re-install the PC board on the four Nylon screws/spacers. As you do this, the coax goes back out the hole in the enclosure and should drag some caulking compound with it, making a waterproof seal. 25. Consider polarisation. The orientation of the enclosure dBi 0dBi is reference gain seen with an Isotropic (all directions) antenna. Every 3dB increase (approx.) represents a doubling of intensity. dBm 0 dBm equals 1 milliwatt. Every 3dB increase (approx.) represents a doubling of power. EIRP Effective Isotropic Radiated Power. For unlicensed WiFi use in Australia, your EIRP must remain below 4W (= 36dBm). If you have a standard 30mW (15 dBm) WiFi transmitter, then the maximum antenna gain you are allowed is 36-15 = 21dBi. Gain Antenna gain is measured in dBi. As antennas have no active components (eg amplifiers), they get their gain by focusing the signal into narrow beams. Much like a lighthouse appears to have a brighter light than it really has. Polarisation Imagine you could see a 2.4 GHz transmitter and it looked like a light beam. If it went side-toside, the beam is Horizontally polarized; up-and-down, Vertically polarized, and round in a circle, Circularly polarized. WiFi ‘Wireless Fidelity’. A catch-all name for standards-based wireless ethernet . 06108031 defines the polarisation. Keep in mind that with the enclosure VERTICAL, the antenna has a HORIZONTALLY polarized signal. The pictures show an antenna that is suitable for either a vertical mounting pole with a horizontally polarised signal, or a horizontal mounting pole with a vertically polarised signal. 26. Using the M4 screws, and the spring washers, assemble everything together. The screws pass as follows: – through the BACK of the ant-cap – through a spring washer – through the bottom of the enclosure – into the top of the enclosure SC Where do you get one? Same-size artwork for the PC board “Biquad” antenna. 48  Silicon Chip The parts are available from the places shown in the article. Alternatively, you can purchase individual components, or fully assembled antennas, from Rob Clark. See www.erlang-software.com/FreeNet/ForSale www.siliconchip.com.au