[Solar-general] TAREAS CONCRETAS: urgente

Mar Mar 15 14:49:55 CET 2005

Antenna on the Cheap (er, Chip)  

 Rob Flickenger

Jul. 05, 2001 05:20 PM
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Like many would-be 802.11b hackers, I'm increasingly obsessed with
pushing more bits further and faster for less cost (I believe the
unofficial goal of our community wireless project is to provide
infinite bandwidth everywhere for free. Of course, there are problems
with approaching infinity, but it's still fun to try!)

The work that Andrew Clapp and others have done is helping to
demystify the ancient black magick of Resonance (i.e. antenna
building). And so, over last weekend, some friends and I decided to
give it a go for ourselves.

(standard disclaimer): Anything you do with your gear is YOUR
RESPONSIBILITY. This is a stupid idea that will probably ruin your
radio, set your house on fire, bring the FCC to your door, ruin your
crops, and send famine and pestilence across the land. And as the
operator, it is YOUR RESPONSIBILITY to not take the word of some
raving lunatic on the web with funny colored hair, and find things out
for yourself. Your mileage will vary. I'm probably lying. You have
been warned.

Anyway, our first run was a direct rip-off of Andrew Clapp's terrific
original design (knowing next to nothing about antenna construction,
it's helpful to start off with a working known good.) By using PVC,
all-thread, washers, some cheap copper tubing, a Pringles can, and
some scrap cardboard, we were able to make a prototype shotgun yagi in
a matter of hours. Having a couple of other excited alpha geeks around
can help move construction projects along very quickly.

Once this was up and running, we looked at the design, and of course
speculated about ways to optimize it. While a directional antenna
showing between 12 and 15db gain is impressive, it's also pretty
large, physically. We realized that, if we were careful, we could fit
a full wavelength inside the Pringles can itself (at a reduced total
gain), but make the entire antenna much more compact.

In about 45 minutes, we had the collector rod built, the locknuts on,
and the whole thing in place. The result: A Pringles can that pulls
about 12db!

Parts list:
All-thread, 5 5/8" long, 1/8" OD $1.00 
two nylon lock nuts $0.10 
five 1" washers, 1/8" ID $0.10 
6" aluminum tubing, 1/4" ID $0.75 
A connector to match your radio pigtail
(we used a female N connector) $3.00 
1 1/2" piece of 12 gauge solid copper wire
(we used ground wire from house electrical wiring) $0.00  
A tall Pringles can
(any flavor, Ridges are optional.) $1.50 
Scrap plastic disc, 3" across
(like another Pringles can lid) $0.00 
Total: $6.45 

Of course, buying in bulk helps alot. You probably won't be able to
find a 6" piece of all-thread; buy the standard size (usually one or
two feet) and a 10-pack of washers and nuts while you're at it. Then,
you'll have enough for two, for about $10.

Tools required:
Ruler
Scissors
Pipe cutter (or hacksaw or dremel tool, in a pinch)
Heavy duty cutters (or dremel again, to cut the all-thread)
Something sharp to pierce the plastic (like an awl or a drill bit)
Hot glue gun
Soldering Iron

Construction time: about an hour

Front collector construction:
Mark and cut four pieces of tubing, about 1.2" (1 15/64"). Where did I
get this number? First figure out the wavelength at the bottom of the
frequency range we're using (2.412 GHz, or channel 1). This will be
the longest that the pipe should be:

  W = 3.0 * 10^8 * (1 / 2.412) * 10^-9
  W = (3.0 / 2.412) * 10^-1
  W = 0.124 Meters
  W = 4.88 inches

We'll be cutting the pipe to quarter wavelength, so:

  1/4 W = 4.88 / 4
  1/4 W = 1.22"

Now figure out what the shortest we'll ever use is (2.462 Ghz, or
channel 11 in the US):

  W = 3.0 * 10^8 * (1 / 2.462) * 10^-9
  W = (3.0 / 2.462) * 10^-1
  W = 0.122 Meters
  W = 4.80 inches 
  1/4 W = 1.20"

Practically speaking, what's the difference between the shortest pipe
and the longest pipe length? about 0.02", or less than 1/32". That's
probably about the size of the pipe cutter blade you're using. So,
just shoot for 1.2", and you'll get it close enough.

Cut the all-thread to exactly 5 5/8". The washers we used are about
1/16" thick, so that should leave just enough room for the pipe,
washers, and nuts.

Pierce a hole in the center of the Pringles can lid big enough for the
all-thread to pass through. Now is probably a good time to start
eating Pringles (we found it better for all concerned to just toss the
things; Salt & Vinegar Pringles get to be almost caustic after the
first fifteen or so.)

Cut a 3" plastic disc, just big enough to fit snugly inside the can.
We found another Pringles lid, with the outer ridge trimmed off, to be
ideal. Poke a hole in the center of it, and slip it over one of the
lengths of pipe.

Now, assemble the pipe. You might have to use a file or dremel tool to
shave the tips of the thread, if you have trouble getting the nuts on.
The pipe is a sandwich that goes on the all-thread like this:

Nut Lid Washer Pipe Washer Pipe Washer Pipe-with-Plastic Washer Pipe Washer Nut

Tighten down the nuts to be snug, but don't overtighten (I bent the
tubing on our first try; aluminum bends VERY easily.) Just get it
snug. Congratulations, you now you have the front collector.

Now for the can:
By now you should have eaten (or tossed) the actual chips. Wipe out
the can, and measure 3 3/8" up from the bottom of the can. Cut a hole
just big enough for the connector to pass through. We found through
trial and error that this seems to be the "sweet spot" of the can.

On the Pringles Salt & Vinegar can, the N connector was directly
between Sodium and Protein.

Element construction:
Straighten the heavy copper wire, and solder it to the connector. When
inside the can, the wire should be just below the midpoint of the can
(ours turned out to be about 1 1/16"). You lose a few db by going
longer, so cut it just shy of the middle of the can

We were in a hurry, so we used hot glue to hold the connector in
place. If you have a connector that uses a compression nut and washer,
and you're really careful about cutting the hole, you could use that
instead.

Now, insert the collector assembly into the can, and close the lid.
The inside end of the pipe should NOT touch the copper element; it
should be just forward of it. If it touches, your all-thread is
probably too long.

Now, just read FCC Part 15.247, connect your pigtail, aim carefully,
and have fun!

Unfortunately, I don't have access to any of the necessary equipment
(spectrum analyzer, power meter, or even an SWR meter) to properly
evaluate the characteristics of the antenna. SWR in particular would
be a really good idea to measure, as we're not sure how much power is
feeding back into the circuit (too much and you can easily blow your
transmitter.) With the extremely low power output (15dbm) of the
Orinoco cards, I don't think this is too much of a danger, but
remember, anything you do with your equipment is your responsibility,
and at your own risk!

Without the proper (multi-thousand dollar) tools, how were we able to
estimate antenna performance?

Using the Link Test software that comes with the Orinoco silver cards,
you can see the signal and noise readings (in db) of a received
signal, and your test partner's reception of your signal. As I happen
to be 0.6 mile LOS from ORA headquarters, with very little noise on
the channel between, we had a fairly controlled testbed to experiment
with. We shot at the omni on the roof, and used the access point at
ORA as our link test partner.

To estimate antenna performance, we started by connecting commercial
antennas of known gain, and taking readings. Then, we connected our
test antennas and compared the results. We had the following at our
disposal:

two 10db, 180 degree sector panel antennas
one 11db, 120 degree sector panel antenna
one 24db parabolic dish
a couple of Pringles cans and some tin foil

Here were the average received signal and noise readings from each, in
roughly the same position:

Antenna Signal Noise 
10db A: -83db -92db 
10db B: -83db -92db 
11db: -82db -95db 
24db: -67db -102db 
Pringles can 
(shotgun): -78db -99db 
Pringles can 
(internal): -81db -98db 

The test partner (AP side) signal results were virtually the same.
Interestingly, even at only 0.6 mile, we saw some thermal fade effect;
as the evening turned into night, we saw about 3db gain across the
board (it had been a particularly hot day: almost 100 degrees. I don't
know what the relative humidity was, but it felt fairly dry.)

Yagis and dishes are much more directional than sectors and omnis.
This bore out in the numbers, as the perceived noise level was
consistently lower with the more directional antennas. This can help
alot on long distance shots, as not only will your perceived signal be
greater, the competing noise will seem to be less. More directional
antennas also help keep noise down for your neighbors trying to share
the spectrum as well. Be a good neighbor and use the most directional
antennas that will work for your application (yes, noise is
everybody's problem.)

When trying to aim a yagi (like our little can), keep in mind that
they have large side lobes that extend up to 45 degrees from the
center of the can. Don't point directly at where you're trying to go,
aim slightly to the left or the right. We also found that elevating
the antenna helped a bit as well. When aiming the antenna, hold it
behind the connector, and SLOWLY sweep from left to right, with the
Link Test program running. When you get the maximum signal, slowly
raise the end of the can to see if it makes a difference. Go slowly,
changing only one variable at a time.

Remember that the can is polarized, so match the phase of the antenna
you're talking to (for example, if shooting at an omni, be sure the
element is on the bottom or the top of the can, or you won't be able
to see it!) You can use this to your advantage to try to eliminate
some noise on a long distance link: slowly turn both ends of the link
from vertical through horizontal, and stop at the point that you see
the most gain (and lowest noise.)

We haven't looked into weatherproof housing for the can; sinking the
whole thing into some 3" PVC should do the trick. Of course, at $10
for two, it might just be more economical to replace them when they
fail.

Apparently, antennas of comparable gain cost upwards of $150. Over a
clear line of sight, with short antenna cable runs, a 12db to 12db
can-to-can shot should be able to carry an 11Mbps link well over ten
miles.

Have fun!

Rob Flickenger is a long time supporter of FreeNetworks and DIY
networking. Rob is the author of three O'Reilly books: Building
Wireless Community Networks, Linux Server Hacks, and Wireless Hacks.

Return to weblogs.oreilly.com.

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On Tue, 15 Mar 2005 10:44:02 -0300, Felix Zolezzi
<felix.zolezzi en gmail.com> wrote:
> el programa en cuestion es el Network Stumbler, es una "papita" y la
> placa de red es una Cisco Aironet
> 
> 
> On Sat, 12 Mar 2005 21:52:15 -0300, Sebastian Bassi <sbassi en gmail.com> wrote:
> > On Sat, 12 Mar 2005 08:48:41 -0600, maram <maramsis en gmail.com> wrote:
> > > On Sat, 12 Mar 2005 11:31:02 -0300, Felix Zolezzi
> > > <felix.zolezzi en gmail.com> wrote:
> > > > Creo, que en los dos casos son ideas en las que me plegaria, pero en
> > > > wifi para depurarlohace falta un tema de seguridad, ya que he
> > >
> > > totalmente deacuerdo, el servidor que valida los usuarios de la red,
> > > debe estar muy bien protegido. Y actualmente estamos lidiando con ese
> > > tema... para evitar sobretodo los ataques de denegacion de servicio..
> >
> > Das de alta por MAC address y listo. Eso hacen los ISPs locales que
> > son wireless.
> >
> > > otro problema importante es el ataque especifico con interferencias
> > > provocadas... ya que a varios proveedores no les gusta la competencia
> > > y menos si es libre/gratuita...
> >
> > Eso si que es un gran problema. Aca (Balcarce) los 2 ISP que hay
> > (pagos) wireless se viven pisando, o al menos uno le hecha la culpa al
> > otro. Tienen un acuerdo de uso (canal 1 a 6 para uno y del 7 para
> > arriba el otro), pero no se que legalidad tiene, que pasa si viene
> > otro, si no se necesita algun permiso para usar las distintas
> > frecuencias, etc.
> >
> >
> > _______________________________________________
> > Solar-general mailing list
> > Solar-general en lists.ourproject.org
> > http://lists.ourproject.org/cgi-bin/mailman/listinfo/solar-general
> >
> >
>