cantenna experiments

Trying to use what I had at hand, I put together a quick cantenna and
ran some tests. I'm describing the results here hoping that I can get
ideas for improvement. I built the cantenna following the instructions
at: http://www.turnpoint.net/wireless/cantennahowto.html

My client card is a Linksys WMP11v4. My router is a Netgear MR814v2. I
built a cantenna as follows:
diameter = 3.25 inches
length = 6 inches (one can) or 12 inches (two cans)
probe 2.5 inches from end of can
probe length = 1.2 inches (seemed short compared to the pictures but
that's what the instructions gave)
coax cable = 1 foot of RG-6 quad shield or 5 feet of same (bad, I
know, but that's what I had)
connectors = one or three F type connectors (again, what I had)

I attached the cantenna to the WMP11 and used Netstumbler to make
some measurements. The overall results were disappointing. The
cantenna didn't give me any improvement over the original rubber ducky
antenna that came with the card. I was able to see drops in the signal
level of up to about 20 dB depending on where I pointed the cantenna,
but even when pointed in the optimum direction the level was about
equal to level with the card's original omni antenna.

Thinking that I might be losing a lot of signal in the coax and
connectors, I shortened the cable from 5 feet to 1 foot and eliminated
2 of the connectors. But that made no difference. That surprised me.
Doesn't that imply that the problem is elsewhere? I know I should get
some LMR cable and the right connectors, but I wonder if it will make
a difference since shortening the lossy cable and removing connectors
didn't help.

I also tried changing the length of the cantenna from 6 inches to 12
inches by adding a second can but that made very little difference
(about 1 or 2 dB).

Another thing I noticed is that, without any modifications, a usb
dongle I had on hand (Zonet ZEW2501) worked much better than the
Linksys WMP11 card. The maximum signal level was about 12 dB higher
for the ZEW2501 than the WMP11. The ZEW2501 is tiny. The WMP11 is a
more serious looking piece of hardware, but it performs much worse
than the tiny dongle. Is there a lot of difference in the quality of
different manufacturer's radios?

Many thanks for help.

Confused,
Bruce

> I attached the cantenna to the WMP11 and used Netstumbler to make
> some measurements. The overall results were disappointing. The
> cantenna didn't give me any improvement over the original rubber ducky

What you haven't said is how far away from the AP you were when testing.
If you're in the same room and already have a good signal, the result
will be masked by reflections and the generally good signal that you
already have.

Try it once you've gone somewhere where the signal is really weak and
then see what difference you have. Or find a neighbours signal with the
cantenna and then take it off and see if it still works.

> than the tiny dongle. Is there a lot of difference in the quality of
> different manufacturer's radios?

Yes.

David.

David Taylor <(E-Mail Removed)> wrote:
>> I attached the cantenna to the WMP11 and used Netstumbler to make
>> some measurements. The overall results were disappointing. The
>> cantenna didn't give me any improvement over the original rubber ducky

....
> Try it once you've gone somewhere where the signal is really weak and
> then see what difference you have. Or find a neighbours signal with the
> cantenna and then take it off and see if it still works.

I agree with David. When I first set up for testing, I was too close, and
I saw very little difference in the NetStumbler signal. It's almost as if
the device doesn't report signal above a certain strength. If you move to
where the signal is in the 70-80 range in NetStumbler, you can see some
antenna improvements. If it's in the 40-50 range, you aren't going to see
much change at all.

It's also helpful to be outside, at least a few feet off the ground, and
have a clean line of sight to the AP.

A single 3.25" can should give you 8-10 dB improvement in NetStumbler.
The 20dB drop that you see by pointing it the wrong way is impressive.
Or maybe your cable shorted out when you twisted it that way ;-)

> probe length = 1.2 inches (seemed short compared to the pictures but

That is 1.21" of _exposed_ probe, including the exposed center conductor of
the connector. The wire also goes down into the connector a bit.

---
Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5

Thank you both for your replies.

Wow, was I wrong! Now I see a 30 dB difference in Netstumbler when I
aim the cantenna for minimum and maximum signal. I get about 10 dB
better than the maximum signal with the original rubber duck antenna.

Heres what happened: I decided to put up some pictures to ask if
anyone saw anything I missed. After taking the pictures, it was clear
that that the hot coax lead had lifted off the point where I soldered
it to the client card. It must have broken off due to moving the cable
around after I put the card into the computer since I checked it just
before. Slap me with a fish.

I'll put up the Netstumbler results and some pictures of the cantenna
in a little while. I'll post the web address in this post.

I am still puzzled by some things. I get a very strong signal from my
own two APs (in the same room about 20 feet away) but none from other
nearby APs. There are about three others in houses close to me that I
can pick up easily with my Zonet ZEW2501 usb dongle. But they don't
show up when I use the cantenna even if I point it in all kinds of
different directions and turn off my own routers to prevent
interference. Any ideas on why this is happening?

Thanks,
Bruce

bjs555 wrote:
> Trying to use what I had at hand, I put together a quick cantenna and
> ran some tests. I'm describing the results here hoping that I can get
> ideas for improvement. I built the cantenna following the instructions
> at: http://www.turnpoint.net/wireless/cantennahowto.html
>
> My client card is a Linksys WMP11v4. My router is a Netgear MR814v2. I
> built a cantenna as follows:
> diameter = 3.25 inches
> length = 6 inches (one can) or 12 inches (two cans)
> probe 2.5 inches from end of can
> probe length = 1.2 inches (seemed short compared to the pictures but
> that's what the instructions gave)
> coax cable = 1 foot of RG-6 quad shield or 5 feet of same (bad, I
> know, but that's what I had)
> connectors = one or three F type connectors (again, what I had)
>
> I attached the cantenna to the WMP11 and used Netstumbler to make
> some measurements. The overall results were disappointing. The
> cantenna didn't give me any improvement over the original rubber ducky
> antenna that came with the card. I was able to see drops in the signal
> level of up to about 20 dB depending on where I pointed the cantenna,
> but even when pointed in the optimum direction the level was about
> equal to level with the card's original omni antenna.
>
> Thinking that I might be losing a lot of signal in the coax and
> connectors, I shortened the cable from 5 feet to 1 foot and eliminated
> 2 of the connectors. But that made no difference. That surprised me.
> Doesn't that imply that the problem is elsewhere? I know I should get
> some LMR cable and the right connectors, but I wonder if it will make
> a difference since shortening the lossy cable and removing connectors
> didn't help.
>
> I also tried changing the length of the cantenna from 6 inches to 12
> inches by adding a second can but that made very little difference
> (about 1 or 2 dB).
>
> Another thing I noticed is that, without any modifications, a usb
> dongle I had on hand (Zonet ZEW2501) worked much better than the
> Linksys WMP11 card. The maximum signal level was about 12 dB higher
> for the ZEW2501 than the WMP11. The ZEW2501 is tiny. The WMP11 is a
> more serious looking piece of hardware, but it performs much worse
> than the tiny dongle. Is there a lot of difference in the quality of
> different manufacturer's radios?
>
> Many thanks for help.
>
> Confused,
> Bruce
Bruce, Get a Hawking Tech Omni Directional 6DB Antenna, I have one on
the outside of the house sitting on the window sill of the 2nd floor, it
has about a 26 inch cable feeding a NeatGear WG311 card, it works great
for me, I picked up 9 neighborhood AP't this morning.
I don't know if you live near a Micro Center, that are not too many, but
they carry Hawking Products. The Tech Omni Directional 6Db antenna is
only 5 bucks after a rebate, Hawking Tech Wireless G USB Network Adapter
6DB Antenna Combo 802.11G is the same deal, 5 bucks with a rebate, or
was when I bought mine. I have that adapter, and it works great. Go to
http://www.microcenter.com/ and check out these two devices, I didn't
read all the details, since There is a Micro Center store in my area
located in Fairfax Virginia. You might be able to get these product
thru an on-line order, but check on the rebate details for on-line orders.
FM

>Bruce, Get a Hawking Tech Omni Directional 6DB Antenna, I have one on
>the outside of the house sitting on the window sill of the 2nd floor, it
>has about a 26 inch cable feeding a NeatGear WG311 card, it works great
>for me, I picked up 9 neighborhood AP't this morning.
>I don't know if you live near a Micro Center, that are not too many, but
>they carry Hawking Products. The Tech Omni Directional 6Db antenna is
>only 5 bucks after a rebate, Hawking Tech Wireless G USB Network Adapter
>6DB Antenna Combo 802.11G is the same deal, 5 bucks with a rebate, or
>was when I bought mine. I have that adapter, and it works great. Go to
>http://www.microcenter.com/ and check out these two devices, I didn't
>read all the details, since There is a Micro Center store in my area
>located in Fairfax Virginia. You might be able to get these product
>thru an on-line order, but check on the rebate details for on-line orders.
>FM

Thanks FM! I've heard good things about Hawking and that's a great
deal. The cable and plug alone are worth more than that. An omni isn't
the best for long distances, I'm told, but I can't argue with success.
I'm definitely getting one of those for experiments.

Bruce

Ok, I put up pictures at:
http://www.patmedia.net/bseiler/CantennaPics/

I'd appreciate any comments or suggestions.

Bruce

> I'd appreciate any comments or suggestions.

Yeah, it's the wrong colour!

Why did you fart around soldering to the back of the board when you
could have just made up a short pigtail to convert from the SMA to N
type?

David.

>Yeah, it's the wrong colour!
>
>Why did you fart around soldering to the back of the board when you
>could have just made up a short pigtail to convert from the SMA to N
>type?
>
>David.

Haha. Didn't have the connectors and none in local shops. Didn't want
to wait for delivery. Also, I was trying to minimize the number of
reflections by using as few connectors as possible. But with using the
wrong impedance cable and all, the connectors might have helped as
much as hurt. Anyway, the board had nice big solder pins and I figured
"What could go wrong?". My bad.

I've done some more testing. I don't see any difference by using one
can instead of two. That is, shortening the length to 6 inches. Easier
to aim. Maybe there are differences on the order of a couple of dB
that I don't see in my ramshackle setup.

bjs555 <(E-Mail Removed)> wrote:
> Ok, I put up pictures at:
> http://www.patmedia.net/bseiler/CantennaPics/

Rearrange the F connector in the can so that the shielded portion is barely
exposed, just enough for the attaching nut. The exposed inner conductor of
the probe should be 1.21".

--
---
Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5