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 Accidental Cruiser in the West Indies

WIFI On Board! The Cantenna

Wifi on Board



Powerful, directional WIFI hub and antenna

Alfa hub passing packets

Alfa hub in action
USB cable below, RP-SMA connector above.

We are finding wifi in more and more anchorages this year. They range from free (spill over from some of the medical schools, hotels and even private homes) to expensive (most French islands). Both the free and for profit signals are too weak to really be reliable when you are sitting in your boat using the internal wifi. For years our solution was to use a wifi usb card with a usb extension cable. We would put the cable out the hatch and hope for the best. The problem is to try to get line of sight access to the wifi transmitter, a reasonably strong link and little enough noise. We often see cruisers perched on the bow with a towel over their heads to make it possible to view the screen in the Caribbean sun. This lacks dignity. Plus, all us old ham radio guys love to play with electronics

Powered Bridge vs. usb Hub

One approach is to mount a powered bridge outside. This is a little more complicated in that

  1. you must run power to it
  2. you must connect the gateway to the wifi signal
  3. you must connect the pc to the gateway.
  4. it is much more expensive

I also don't like hanging electronics out in the weather. The approach may get a slightly stronger signal but the awkwardness of searching for wifi signals, the expense and the unreliability doesn't make up for it.

Alfa hub passing packets
Cantenna witth camera stand mount.
Radiating element is the wire inside, N type Coax connector shown below

The second approach is to use a high power wifi usb hub that runs on power from your computer. It appears in your windows network control panel as a device similar to your internal wifi but allows you to use external antennas. When you are looking for signals, you do it in the same way you find wifi in the airport or at your favorite coffee shop. The difference is, the hub has a much stronger signal and can be connected to much better antennas. Furthermore, the antenna has no active electronics and can be placed out in the weather without damage. The usb hub costs less than $40 from amazon . The unit comes with a windows driver and a "rubber duck" antenna much like that on your handheld VHF. This works quite well and we used usb extension cables for several years to get the antenna and hub located high enough to get some range. You can also buy commercial passive antennas in a variety of shapes and sizes to improve the signal.

Better still, do it yourself in true cruiser style. You will get much better reception with a strong directional field. You will also find you get much better ability to actually connect to the signals you can see. This is because your signal is much stronger and the directional properties help to reject a lot of the noise found in the typical anchorage.

Build your own directional antenna

To play with wifi antennas you must have an wifi hub which connects to external antennas (ie not just to the built-in antenna). Your antenna will have a normal N type coaxial plug to which you will connect the coax cable running down to the hub. It will also be the place you solder the radiating element. You will need an adapter cable ("Pig Tail") runs from the wifi hub to the coax connector on the antenna. One end is the N type coax and the other matches the RP-SMA connector on the hub. You will want some way of mounting the antenna high in the boat but within reach for aiming. I used a camera mount which has multiple clamps and a big suction cup. The connector at the hub can be different if you use other than the alfa hub. Freeman Anderson and Bird give good close-up pictures of the various connector types. I just bought the whole pig tail and cable from them which they describe as a "cantenna kit" for less than $20. Click here for details.. My hub connects to the computer with a normal usb cable . The hub requires no external power source but this antenna will work with powered wifi hubs for increased power and probably longer cable runs

Building the Cantenna:

Basically the antenna is a kind of wave guide. You empty a tin can of a reasonable size. 3 or 4 inches across and as long as possible. We used an old fruit juice can. You cut off one end and clean the can. A coax connector is mounted on the side of the can and a very short radiating element is soldered on the inside of the connector extending about half way into the can. Exact location and element length for a given can are derived by a formula given here

I mounted a piece of aluminum angle bracket on the base of the can to allow for mounting a clamp and giving me the ability to point and rotate the can because I can never convince Susie to just stand there holding the can while I browse the net. Most women are funny that way. Trust me on this.

Using the Cantenna:

Close-up of cantenna
Close-up of cantenna with a simple clamp mount

Your wifi is a small very high frequency transmitter/receiver whose wavelength is measured in inches. It will pass through plastic (like a fiberglass hull) but does not pass through metal or mountains. My impression is that the signal passes very well over unobstructed expanses of water but is very badly affected by people and boats and heavy rain between you and the wifi hot spot. This antenna is very directional and you must experiment with where you place it. I use Netstumbler which shows actual signal strength to help aim the antenna. Your wifi hub will come with similar software to help you aim the can. Generally I like clamping the tin can to the shrouds as the boat is usually pointing toward shore when anchored in the lee of the Caribbean Islands. Most signals seem to be vertically polarized and I usually leave the cable hanging down so that the element is pointing straight up. Give other angles a try. The antenna is very directional and boat swing at anchor can be a problem in high winds. Under normal circumstances, it works great. There are no active components outside the boat and, as you can see from the picture, I often leave the antenna out side even when it rains.


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