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Super Wi-Fi antenna

Sep 10, 2008


Practically every marina worldwide now provides Wi-Fi access, but you may be assigned a berth that is either completely out of Wi-Fi range or where the signal is marginal. If you are on a mooring or at anchor you can lose your Wi-Fi connection completely each time the boat swings around. Commercial high-gain external omni antennas such as the Netgate ANT-H2415-Pro are readily available for $130 or so, but if you are reluctant to part with your hard-earned cash, here is a cost-effective solution (around $35) that will greatly extend your Wi-Fi range and earn extensive bragging rights on your electronic prowess.

On the Internet you will find lots of detailed information on antenna design and construction. The actual construction of the antenna requires careful attention to detail, as the dimensions are critical, but it is generally within the capabilities of most do-it-yourself boaters who can measure and solder.

The omni-directional antenna is the design of choice for a boat, as you do not have to aim the antenna at the Wi-Fi site. In the parlance of the antenna engineer, the omni antenna has no nulls.

To get started, google “homemade omni Wi-Fi antennas” and a number of sites will appear. Review several of them to get a general idea of what’s available. I selected the antenna described in www.wireless.gumph.org/articles/homemadeomni.html, due to its apparent ease of construction and use of readily available materials. The antenna described on the gumph site is a collinear dipole design. This means that each segment of one-half wavelength cumulatively adds gain to the Wi-Fi signal to produce an effective gain of 6 dB or more, depending on the number of segments. While the plans show an antenna of eight segments, I added four more segments, hoping for a further increase in gain.

Print out the instructions from the Web site and follow them very carefully. Don’t despair at the details — it’s surely not rocket science. Here’s a somewhat digested version plus a few helpful hints:

Obtain 3 feet (or 1 meter) of RG-213 marine-grade coaxial cable, 6 inches (150 mm) of RG-58U marine-grade coaxial cable, three small FT-50A-67 ferrite toroids (www.amidoncorp.com), one USB Wi-Fi adapter, such as Netgear WG111v2, or equivalent, one-inch (25-mm) PVC conduit and fittings, hand tools, 60/40 rosin core electronic solder, etc.

Lay out the length of the RG-213 coax on a flat surface and carefully work it into as straight a length as possible before you do any cutting at all. Any curvature will be much harder to cure when the coax is cut into the shorter segments for the antenna, and curved segments will degrade the performance of the antenna. Use only marine-grade coax, such as Ancor, as it is tinned and will not corrode as quickly in the marine environment as will bare copper coax.

Cut the RG-213 coax into segments to the lengths specified in the instructions: 11 segments of 49 mm and one segment of 74 mm. Measure twice and cut once! Compare all of the sections and discard or rework any sections that are too short or too long. After a few trial cuts with a hacksaw yielded quite crude results, I hit upon the idea of using a tubing cutter (a new one with a sharp cutter wheel!) to cut the segments to length. A sharp hobby knife was used to cut away the 6 mm bits of shield and dielectric on both ends of each segment down to the center conductor. A wire stripper was used to remove the dielectric from the center conductor. Cut away 31 mm of the braided shield from the 74 mm segment to form the one-quarter wavelength top segment, leaving the dielectric and center conductor.

Use the hobby knife to very carefully remove a small V-shaped section of the outer sheath on both ends to gain access to the braided shield, taking special care not to damage the shield. Mark a dot where the point or apex of the V-shaped cut should begin and mark a corresponding dot exactly in line with the first dot on the other end of the segment. This will insure that the connected segments will line up exactly, rather than take on a twisted shape, which will degrade the antenna performance. Tin the center conductors and exposed shields prior to soldering the segments together.

Assemble the segments with a jig as described in the instructions. I placed the segments over a dimension guide to insure that the spacing was exact at 40.5 mm — the most critical dimension.

Examine the completed antenna to insure that no strands of the shield or center conductor have gone astray to create a short circuit. Double-check your work with your digital multimeter set on “continuity” or “ohms” to check for open circuits or short circuits: continuity between the center conductor of the lowest segment and the tip center conductor; open circuit between the braided shield of the lowest segment and the braided shield of the tip, as per the instructions you downloaded from the Web.

The commercial omni antennas have a coax connector at the base or have a length of coax (the “flylead” in the Web instructions) attached to the base, both of which require the use of a Wi-Fi card in your laptop or desktop computer that is capable of accepting an external antenna. If you already own a Wi-Fi card with a connector for an external antenna, such as the Proxima model Orinoco Gold PCMCIA card, then you have the choice of terminating your new antenna with a coax connector and either purchasing an already-made coax cable with suitable connectors, or you could simply attach a length of coax to the end of your new antenna and connect it directly to the Wi-Fi card in your computer. But, since coax connectors and coax cables of any type or length incur a loss of signal, I reasoned that a design that would use USB and eliminate the coax cable and connectors would be good for at least two reasons: 1. minimal signal loss between the antenna and the computer; and 2. I could avoid the purchase of a Wi-Fi card with external antenna connector and instead use a very cheap USB Wi-Fi adapter.

A Google search of “external Wi-Fi antenna hacks” produced a site that showed how to modify a low-cost USB adapter to accept an external antenna. Have a look at www.xp11.com/g54/g54.htm and www.nathanlipscy.com/wireless_usb_hack/ to get a general idea. All makes of USB Wi-Fi adapters are very similar in design, so it’s easy to apply the modification to any of them.

Another Internet search resulted in the online purchase of a Netgear WG111v2 USB Wi-Fi adapter. Netgear is a known brand, and this item is cheap (about $12 — cheap enough for experimentation!). It is a simple exercise to peel off the label and then pry off the plastic housing to reveal the bare circuit board. Discard the plastic housing, as it will not be needed. Identify the patch antenna to which the external antenna will be connected — it’s on the end opposite the USB connector. Use a sharp hobby knife to reduce the patch antenna to a small square section nearest the lead from the RF section. (The RF section is what’s under the shiny metal cap.) Scrape off the protective coating over what’s left of the patch antenna and the ground plane. The ground plane is the outer perimeter of the circuit board. A magnifying glass will no doubt be of great help in this task. Tin both areas in preparation for attaching the connecting coax. Work quickly to avoid overheating the adapter.

Connect your new super Wi-Fi antenna to the USB adapter with a short length (two inches or 50 mm) of RG-58U (small-diameter 50-ohm coax). Solder the RG-58U braided shield to the ground plane of the adapter and the center conductor of the RG-58U coax to the adapter antenna lead. Slip the three FT-50A-67 ferrite toroids onto the RG-58U coax and then solder the other end of the center conductor to the braided shield of the lowest antenna segment. Solder the RG-58U braided shield to the center conductor of the lowest antenna segment. The ferrites absorb RF energy on the outer surface of the braided shield and prevent reflected RF power from damaging the Wi-Fi adapter. The antenna will work just fine without the toroids, but I can’t say for how long!

With completed antenna in hand, visit the local hardware store and hunt through the plumbing and electrical sections to find a length of one-inch-diameter (25-mm) PVC conduit. Conduit is thinner than pipe so the signal attenuation will be slightly less. I found a piece with the enlarged end for joining which handily accommodated the USB Wi-Fi adapter. Aesthetics are an important consideration here! A PVC pipe coupler or union, a pipe cap for the upper end and a pipe plug for the lower end completed the parts list for the watertight housing.

Drill and cut the PVC pipe plug as shown to create a channel for the USB cable. Drill the hole slightly larger than the USB cable. A dollop of silicone rubber (RTV) will seal the cable channel and the top cap when you have completed the assembly. The Netgear WG111v2 USB Wi-Fi adapter comes with a 5-foot (1.5-meter) USB cable, but I substituted a 16-foot (5-meter) USB cable for maximum mounting flexibility. Choose a length suitable for your application.

The completed antenna can be permanently mounted to a rail or can be hung from an eye installed in the top cap of the antenna, or just use a length of light twine. If you install an eye, seal it with a dab of silicone rubber (RTV) to prevent water getting in.

The inexpensive Netgear WG111v2 USB adapter’s power output is a measly 50 milliwatts. While you will be able to receive signals from distant Wi-Fi access points, you may not be able to connect with them due to the low-power output. Higher-powered Wi-Fi devices are available up to 500 milliwatts if you need additional range, and the antenna will easily handle maximum legal power. Shop around on the Internet for the best deal on Wi-Fi devices.

Performance testing of the omni antenna consistently produced significant gains over that of my Netgear WN121T, which is a costly directional antenna and is not weatherproof. In fact, when my WN121T was unable to find any networks at all, my super antenna found six and I was able to easily connect to and use the Internet. Total cost of the homemade omni system was less than $35.00.

Harry Hungate and his wife Jane Lothrop are currently Wi-Fi-ing aboard their Corbin 39 Cormorant in Southeast Asia, having departed Annapolis, Md. in 1997.

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