One of my latest obsessions
is building homebrew WiFi antennae.
Heinz Beans Cantenna
This is the first antenna I built. It's the
ubiquitous circular waveguide "cantenna":
I used the convenient calculator from Greg Rehm's
site. My can was obtained by going to my local 99centsonly store and buying
a can of Heinz beans, which happened to be the desired 3.25" diameter (the
resultant methane gas from consuming the beans was used to power my soldering
iron afterwards). Contrary to popular belief on the Internet, a can length of
3/4 the waveguide wavelength is not optimal. The ARRL Antenna book recommends
2-3 waveguide wavelengths instead. I've found that adding more cans indeed
increases the gain. A 4-can one is the longest I tried; I didn't write down the
gain testing results, but it was considerably better than the 1-can antenna.
Adding more cans helps launch the standing waves in the can better.
bmoore314 has some excellent info in this Netstumbler.com
thread, including info about adding a conical collector to it.
I've
written my own cantenna calculator in C++ (based on formulae from the ARRL
Antenna Book); the source code and Win32 EXE are available on my free software
page. While Greg Rehm's calculator fixes the operating frequency at Channel
6 (2437MHz), which is the center channel in the USA, mine lets you choose a
specific channel if you want to get maximum gain for a specific channel. For
instance, with my 3.25" (82.55mm) diameter waveguide, the optimal probe distance
from the rear of the can varies by over 5mm between Channels
1-11.
Experimenting with my calculator program I've found some
interesting information. As the waveguide diameter increases, the difference in
optimal position for the driven element between Channels 1-11 drops. So I tried
upping the diameter iteratively until the TM01 cutoff frequency started to go
too low to do Channel 11. From my studies, it seems that about 92mm is the
optimal diameter for the waveguide; the difference in the probe position between
Channels 1-11 is about 2.63mm, so the SWR curve across the WiFi band is flatter.
Toothpick Monopole
This is my first attempt at designing something
myself. I downloaded the EZNEC demo from http://www.eznec.com/ and started fiddling with
it. I still don't have a good grasp of how to model a real ground plane, but i
was able to get some plots and start tweaking things.
I started w/ a
quarter wave whip. In the US, Channel 6 is the middle channel at 2.437GHz. This
makes a quarter wavelength about 30.6mm, so I started with this and just
experimented w/ various lengths to change the pattern and SWR and ended up with
89mm. EZNEC shows SWR of 1.2-1.6 over the WiFi band and gain of 4.35dB max
assuming a perfect ground (which we don't have). Below are plots of my EZNEC
model:
Here is my prototype:
It's just a piece of 2mm dia. coat hanger cut to 87mm and
soldered into an N-female panel jack for 89mm length from the tip to the base of
the middle pin on the jack. Just for the hell of it I soldered on the ground
plane, which is the lid of a 3.25" dia. tin can. The tape is just to keep me
from shredding my fingers on the sharp edges. How well does it work? I was
amazed. walking outside with MiniStumbler, i can find my AP 120ft farther away
than with the ORiNOCO built in antenna. Inside the house, I went the the place
w/ the worst reception and the signal & SNR went up by over 10dB vs the
built in antenna. I haven't even begun to tweak the thing yet. not bad for a $4
antenna (the cost of the N jack).
The Mobile Mark 5dBi
antenna is the stumbling antenna of choice used by many Netstumblers.
outcast_one was kind enough to post some pictures on the Netstumber.com
website which were clear enough to get measurements from. Hope he doesn't mind
my re-posting them here:
From the above photos, I estimated the dimensions below:
wire: ~1.5mm OD
ground plane to coils: 34mm (9.5mm of that is under the black plastic
bump..wonder what's in there?)
length of coils: 13mm
coil ID: 5mm
coil OD: 7mm
coil spacing: ~3-3.5mm
coils to top: 51.5mm+13mm(plastic tip)..wonder how high the wire goes into
the tip.
Here is my implemenation alongside my toothpick for
comparison:
I used solid copper wire cut out of a piece of Romex...I forget
the gauge..it was all I had available; tried initially to bend a coat hanger but
the steel wire was too difficult to bend into the coils. Once again I used a
3.25" can lid as the ground plane; this is close enough to Mobile Mark's
specified 3" ground plane. A nicer implementation would be to use a discarded
hard disk platter (kudos to sparafina for that idea). I am worried that
the copper is too soft to stand up to high winds when attached to my car. When I
get a chance I will either encase the whole whip on a plastic tube or just
support the coil by inserting a suitable piece of plastic into it. Another idea
is to just fill the coil with hot glue.
My initial tests were not that
promising...the gain was about the same as my toothpick, except that the antenna
seemed less sensitive to polarization. However, stumbling with the antenna has
shown that on the average, I pick up AP's 1-2 car lengths farther away than with
the toothpick, and the SNR is often a little higher. Therefore, this antenna is
used in my current stumbling rig.
Trevor Marshall's BiQuad
Trevor Marshall has posted plans, as well as
NEC2 models for his biquad
dish feed. The antenna can also be used standalone.
I
fashioned the reflector from a discarded tin can. The reflector is 123x123mm,
with 30mm "lips" as specified by Trevor for standalone use. The driven element
is composed of copper wire I got out of a piece of Romex, with 30.5mm legs, and
is suspended 15mm above the reflector. The antenna as pictured above was a
complete failure and had horrible performance. Trevor explained to me
via e-mail that I messed up the feed (the photos on Trevor's site are grainy).
Here is my revised feed:
Instead of rigid coax as specified by Trevor, I just used some
more copper wire for the connections; I'm not sure how this affects VSWR, but
the antenna gave me about 3dB more gain than my Comtelco patch clone during my
initial tests.
Bazooka Cantenna
I've been trying to hook up my brother, who is a
professor at a local college to his campus network. He lives just on the edge of
campus, and although the IT Dept. has discussed putting an AP on his side of the
street, no progress has been made for several months. Therefore, I decided to
take matters into my own hands. There are tons of AP's just around the corner
and out of LOS from my bro's house, but his block is strangely completely devoid
of any signal.
Finally, one day I climbed up on his roof to see if I
could get LOS and a signal from a yagi on a hill which was pointed away from my
brother's house. I used my ORiNOCO card in my Jornada, pointed my biquad through
a tree, and amazingly got a 5dB SNR! Now we were in business, but the 5dB seemed
a little too weak for reliable communications, especially with the chance of the
tree growing denser foliage.
I decided a cantenna might be the way to
go, so I built a new one using 3 3.25" diameter cans...this makes the total
length about 1.75 waveguide wavelengths. The driven element is 30.75mm long and
mounted the 64mm from the back of the can. The conical collector is 7.25" in
diameter on the big side, w/ a 30degree flare. This was just a quick prototype
so I made the collector out of 2 coat-hanger circles, separated w/ four 4" long
coat hanger supports covered in aluminum foil. the final design will need to be
more durable to stand up against wind & hail. Here is what it looks like:
Before trying it on the target site, I did some testing with my
AP at home. Here are the SNR's I got across the street from my AP:
ORiNOCO built in: 26 dB 2 cans w/o collector: 36 dB 3 cans w/o
collector: 37-38 dB Trevor Marshall biquad: 39 dB 3 cans w/ collector: 43
dB (!)
This is the highest gain antenna I've built yet. In my excitement,
I dragged my Jornada off a table while connected to this %$* thing and it fell
on the floor. Lucky the card & Jornada are ok, but I broke off the end of my
pigtail.
The next morning, I climbed on my bro's roof armed w/ the
bazooka cantenna. Going back to the same place I got the 5dB SNR w/ the biquad,
the bazooka got 8dB. I fired up PocketIE on the Jornada, and was able to surf a
little - paydirt! Since it was daylight this time, I was able to try out more
places on the roof, and finally found one clear of the tree which yielded 12dB
SNR. Now we're in business; I've got a little more margin to play with so when I
hook up the long LMR-400 cable to get the signal inside the house I won't get
killed by attenuation.
To be continued after I get the rest of the
equipment to complete the setup...
In the meantime, I played with the
bazooka from the deck of my hillside house, and was astonished to find that it
picked up an AP I'd detected while stumbling on the freeway in my car (using my
Mobile Mark clone on the dash). Plugging the GPS coordinates in from the freeway
into Microsoft Streets & Trips, it turns out the AP is about 4 miles away!
Using the bazooka on at my house, the SNR was 8dB (signal ~-88dBm). As a
comparison, I also tried the biquad. Using the biquad, the signal is unstable w/
max 4dB SNR, and it catches the AP for only a second at time.
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