I’ve read for a while about cantennas, and I bought one on ebay from a guy who makes a lot of them. Following is our email exchange. If someone could help answer my questions, I would really appreciate it. I really want this to work and not return it, but the cantenna doesn’t seem to be helping.
Me:
From my reading on cantennas, I thought they were very directional. I get almost exactly the same readings on inSSIDer through a 360 degree swivel of the cantenna. I’m very confused. It doesn’t seem to be helping. What is the body made of? Is is microwave reflective? I thought the point was to screen out the competing signls and clean up the intended signal. As for it being 100x stronger [20dbi increase, his ad said], I’m asking if it’s 100x stronger than just the antenna element alone, not in the can.
Some feedback from you would help.
Him:
Signal bounces all over the place,you cant screen out other signals.
Me:
Then what is the point of the can, if not to screen out other signals?
Either the cantenna isn’t constructed properly or there’s something wrong with the way you are measuring it. You could be picking up all kinds of reflections from the environment and may not be measuring the true signal strength at all.
This page shows the calculated antenna pattern of a cantenna compared to actual measurements from a Pringles can cantenna and a nearly identical cantenna made out of copper:
ETA: Cantennas are directional but as that page shows the pattern isn’t anything close to a pencil beam type of pattern. You should get a relatively uniform signal strength around the front of the thing, tapering off as you move to the sides and dropping off to almost nil back and to the right or left, then increasing again as you get right behind the antenna (but not as strong as out the front).
Note that proper test equipment an anechoic chamber makes much better measurements than a laptop in an uncontrolled environment. You might get better measurements from inSSIDer if you get further away from the cantenna, especially if you get out near the limits of its operational range. I’ve never played around with inSSIDer though so I don’t know how accurate it is. That might depend a lot on the hardware installed in your computer.
One more quick edit - a cantenna won’t completely screen out unwanted signals. It just picks up better towards the front of the can than it does in other directions. Signals from other directions will still be received. They will just be a bit attenuated, depending on the angle.
I even went to the trouble of wrapping the can in a few layers of aluminum foil, and making a slow 360 with the cantenna showed basically no change in any signal strength, according to inSSIDer.
So this seems to say that this cantenna is supposed to give effectively 100x increased performance over a perfectly omnidirectional antenna. I’m getting networks on inSSIDer that Windows doesn’t seem aware of (how Windows is missing them, I have no idea, since the signals are going through the wifi USB adapter). However, there seems to be no directionality to the performance, and no way to sign on to these other networks, as Windows doesn’t list them in the available wifi networks in the system tray.
I’d appreciate any other help. I don’t want to return this, I want it to work, but the seller is seems like he’s almost trying not to be very helpful. Thanks.
I don’t see how wrapping the antenna in foil would do anything useful.
I would expect the actual performance of the cantenna to be somewhere in the range of 8 to 12 dBi. 20 dBi seems a bit high for a simple cantenna, unless it has a feed horn attached to the front of it or some other way of increasing its gain.
Even a cheaply constructed cantenna will have about 8 dBi of gain and will be directional to about 30 to 45 deg or so. If you are measuring the same signal strength in all directions then either it’s not a proper cantenna or you are measuring it wrong. I can’t tell anything about the cantenna’s design or construction from your post, but I do have a lot of doubts about your signal strength measuring methods.
First of all, in your particular environment the signals could be bouncing off of all kinds of things. Maybe you are measuring the same signal strength in all different directions because the signals are being reflected so much that they are actually coming in with roughly the same signal strength from all directions.
If all of your signals are coming from fairly close by, then that could be giving you an incorrect reading on your inSSIDer display. I’ve never used inSSIDer but it is my understanding that it is just displaying the signal strength RSSI number from the wifi chip, and your typical wifi chip these days doesn’t put out an accurate RSSI for higher signal strengths. On some chips the granularity of the RSSI isn’t very good either. What sort of numbers are you actually getting?
As I recall, I was getting from -56 to -97. The open one I found stayed at -91 after appearing at first as -87. If there’s a better program to use, please recommend one. I also don’t understand how to use this cantenna in practicality, as the only way to connect to a wifi network seems to be through the Windows wifi utility, or whatever we’ll call it.
This whole thing has been so disappointing. If it’s not pretty directional, I can’t see what the point is. I don’t know why Windows doesn’t see the same networks. I can’t understand how I could have so many microwave reflections in a wood framed house.
Any chance you can take some pictures of the cantenna and your setup showing how you are testing this? It would be helpful if I could see how the cantenna was constructed.
You say you have this cantenna hooked up to a USB wifi adapter… Is there any chance Windows isn’t actually using that USB wifi, and is instead using a separate, internal wifi?
That would sure go a long way to explaining the results you’re seeing.
I will try to post pictures. The problem at the moment is that the cantenna is at the location I can’t get internet at. I will try to post something to Twitter or something from my phone.
The extra can made about 3 points difference for the Henk network, which is just next door in this triplex, according to inSSIDer. Turning the cantenna 90 degrees did almost nothing, which is why I raise the questions about its directionality.
I turn off the laptop wifi with the button, then put in the USB adapter. This then finds networks, and I know it’s acting as the antenna, because I can really tell the difference when I raise it up another yard on its USB extension cord. The downloads get much better. The adapter only shows a signal when I screw in its antenna, and then with the antenna raised to be perpendicular to the RF, I get about 4 points worse (-72 on Henk network) on inSSIDer than with the cantenna. So, so far I’ve spent $50 (with shipping) on something that barely outperforms a four inch metal stick.
Sorry to give you a big <shrug> about your ebay cantenna situation, and I know it might be frustrating to hear this, but you’d have been much better off focusing more on a better quality wifi adapter before worrying about the antenna.
I can’t recommend this specific product and seller enough - as far as reasonably-priced consumer grade equipment, this is the best there is, bar none. You’ll find the seller’s item descriptions to be very informative, and they’ll readily answer any questions you might have.
Based on the pictures, I would say that’s probably a reasonably functional cantenna. It should work, for what it is. Without making any actual measurements for size, it looks to have about the right dimensions. The tube isn’t shielding. It’s acting as a waveguide. The one thing I really don’t like about its construction is the seam in the tube. The closer to a symmetric tube it is the better. Still, I think it will work.
That said, it’s a pretty cheap little cantenna. The advantage of this type is that it gives you a reasonable gain (maybe 8 to 10 dBi) for a very cheap cost. You can throw one of these together yourself for less than 5 bucks worth of parts.
If you are willing to go up into the $50 price range you can easily do much better than this thing. I would get your money back for it if you can. Get something professional instead.
I don’t know where to find “professional”. Do you? What I wanted was to greatly increase my range so I can find an open network. What kind do you recommend? How can the metal not be rejecting the high frequency RF from the side?
Here’s another good source, with more antenna choices and good info. You’ll notice that they also specialize in the same wifi adapters I recommended and linked to above - there’s a good reason for that. (There’s lots of good info in that “Wifi Help” tab)
The main appeal of cantennas are that they are something simple that you can easily build yourself to somewhat improve directionality, but they are hardly the best choice performance-wise for something to buy. That would be the yagi, or maybe even better, a yagi cantenna, for the best of both worlds. That said, the directional panel antenna included with the USB adapter I linked in my previous posts may just be all you’ll need, all for under $30. (and a lot less cumbersome and ugly than a big 'ole rocket-launcher looking thing)
Seriously, focusing on the wifi adapter before you try upgrading the antenna is really your best bet. Listen to me now, thank me later…
A few points. 2.4GHz has a wavelength of 12.5cm or about 5 inches. Even at these relatively short wavelengths, antennas don’t behave in the same way as we might imagine light to behave. The metal of the can doesn’t block the signal from the side. The precise dimensions of the entire system work to reduce gain in some directions and increase it in others. And only at exactly the frequencies it is designed to work at.
If this was sound, a tone with the same wavelength would be 2.7kHz - which is close to F7. Almost an octave down from the top note on a piano. If you put your ear at the end of a short tube you don’t expect perfect rejection of sound from where the tube doesn’t point. You do however get weird effects where some frequencies will be rejected, and others curiously enhanced, due to the dimensions of the tube. The cantenna is working in a similar manner.
Similarly, the radio signal can reflect and generally bounce around in a manner much like sound, although it does need conductive material to reflect off. If there is a clutter of metallic objects about it becomes very hard to predict or reason about the directionality of the energy.
One thing that does worry me about the pictured cantenna - if the roll of metal in the tube does not make good contact across the length of the cut edge, the operation of the system will be compromised in hard to predict ways. It needs to operate as a conducting tube. (Cutting slots in tubes is done to effect all sorts of interesting tricks in antenna and waveguide design. However at this level we are talking black belt antenna design.)
A critical point - a cantenna is polarized. If you rotate it 90 degrees about its long axis you should see a very significant change in signal strength. Most simple WiFi antennas stick upright and are thus vertically polarized. To be vertically polarized the cantenna’s feed point should be pointing down (or directly up). If it is on the side, the polarization will be horizontal, and in a perfect world, the antenna would pick up no vertically polarized signal at all. Of course nothing is perfect, nor do you always know the actual polarization of the signal you want to receive, but polarization will be important.
I didn’t notice that there were directional antennas in the ebay store you linked to. And 7dBi just isn’t enough to get me to spend money. I like the simplewifi.com site: thanks. I will consider their stuff. I also may try to get something somewhat parabolic and use that.
What is wrong with the USB adapter I have? I’ve seen it pull over 2MB/s, and it can use N-type. What will a more expensive one do?
The point of that ebay auction was that along with the much better USB adapter, there was the directional panel antenna as a bonus - all for a very reasonable price.
And there are two main things about that particular USB adapter that make it much better. The main thing is its 1000 mW transmit power, which is what could be fairly described as barely legal. If you can even find the specs for the transmit power of your adapter, my guess would be that it’s somewhere between a fifth to a tenth of that. That makes a big difference when you’re trying to cover distance.
The second unique thing about that adapter is the wireless RTL8187 chipset it contains. That particular chipset has a very sensitive receiver, so that it will be able to lock onto relatively weak signals. So, you see, before even worrying about the antenna, you start off with a wifi adapter that is top-notch in its inherent ability to transmit and receive signals.
Now, the slight drawback to that is that that RTL8187 chipset only supports 802.11 b and g, but not the newer N standard. But with the way the N standard works, the router/access-point you’re picking up must be set to “N-only” mode for there to be any real speed/signal-quality benefit to being N-capable. Most devices out there are not set to “N-only” mode because they still have to support various older devices that don’t support N.
If for some reason you really want/need support for N, than you’ll want to get this (AWUS036NH) adapter. Instead of typing a whole bunch more, I’ll urge you to read ALL of the details in that auction link, where they describe the pro’s and con’s quite thoroughly. But to sum it up, for range/distance and the ability to pull in the maximum amount of signals reliably, the (b/g only) AWUS036H model in the original auction I linked to is the way to go. If you were trying to connect to a specific wireless signal that you know to be set to “N-only” mode, than the link in this post would be the way to go.
(There’s lots of good info in that “Wifi Help” tab)
