DIY cantenna - part 2

I'm still searching for the 'perfect' can for a cantenna. I've got the
N-connector and pigtail ready to go. So far, I haven't found one that
matches the dimensions on: http://www.saunalahti.fi/elepal/antenna2.html

The closest I've found, so far, is a V8 juice can that measures 4 1/4"
x 7". I'm hitting the grocery store with a ruler tomorrow. :) Also, he
says the sides and bottom should be smooth. I haven't found a can yet
that's completely smooth. They all seem to have 'ripples,' that I
assume increases metal strength? Could aluminum foil, glued to card
stock to prevent crinkles, be used to cover the inside to make it
smooth in lieu of a smooth sided can? This would also narrow the
length some what. Thanks again.

On Sat, 25 Aug 2007 02:49:04 -0700, sillyputty
<karmictaragem@2die4.com> wrote:

>I'm still searching for the 'perfect' can for a cantenna. I've got the
>N-connector and pigtail ready to go. So far, I haven't found one that
>matches the dimensions on: http://www.saunalahti.fi/elepal/antenna2.html
>
>The closest I've found, so far, is a V8 juice can that measures 4 1/4"
>x 7". I'm hitting the grocery store with a ruler tomorrow. :) Also, he
>says the sides and bottom should be smooth. I haven't found a can yet
>that's completely smooth. They all seem to have 'ripples,' that I
>assume increases metal strength? Could aluminum foil, glued to card
>stock to prevent crinkles, be used to cover the inside to make it
>smooth in lieu of a smooth sided can? This would also narrow the
>length some what. Thanks again.
My cantennas here at home are 83.5mm diameter and 186mm
length, and 156mm diameter and 180mm length. The probe of both is 31mm
copper wire, thick enough to fit +- snuggly into the hole in the
connector, then soldered in. Both cans have ripples all down the side.

The placement of the probe varies according to the diameter of
the can.
Both work pretty well, (the narrower tin maybe a little
better), for web browsing, but I sometimes get errors when downloading
big files. For real use I have a big parabolic. The cans are more like
an experiment.
I read somewhere that optimum would be a 90mm diameter can
with a length of 200mm or longer. Aim for 90mm.
I'd love to make one of stainless steel, to the best
dimensions .... mine rust. Will have to find time to go to a junkyard
:P
FWIW ....

sillyputty <karmictaragem@2die4.com> hath wroth:

>I'm still searching for the 'perfect' can for a cantenna.

One nice thing about can antennas is that the design is very
forgiving. The can can be fairly sloppy in construction (ripples,
edge rims, dents, dings, etc) and it will still mostly work. The only
items that are critical are:
1. Grounding of the coax connector.
2. Length of the probe.
3. Position of the probe relative to the bottom of the can.

>I've got the
>N-connector and pigtail ready to go. So far, I haven't found one that
>matches the dimensions on: http://www.saunalahti.fi/elepal/antenna2.html
>
>The closest I've found, so far, is a V8 juice can that measures 4 1/4"
>x 7". I'm hitting the grocery store with a ruler tomorrow. :)

Sigh. Good luck. See suggestions below.

I've watched some of the locals build cantennas. I try to talk them
into building better antennas, but the simplicity and price of a
coffee can antenna is just too easy to ignore. The one's that are
successful usually end up building several. How much V8 can you
drink?

>Also, he
>says the sides and bottom should be smooth. I haven't found a can yet
>that's completely smooth. They all seem to have 'ripples,' that I
>assume increases metal strength? Could aluminum foil, glued to card
>stock to prevent crinkles, be used to cover the inside to make it
>smooth in lieu of a smooth sided can? This would also narrow the
>length some what. Thanks again.

Yes, but it's not necessary. The ripples don't cause much of a
problem with the gain pattern. The ripples at the bottom of the can
might cause some tuning changes that result in the position of the
probe being not exactly where the calculations suggest, but that can
be compensated with a bit of tweaking (tune for max signal).

The aluminum foil covered insides will have a very minor benefit in
reducing the surface conductivity (skin effect) of the antenna at RF
frequencies. However, you won't notice the difference without a pile
of expensive test equipment.

If you're really looking for the ultimate can antenna, then I suggest
you visit the hardware store, not the grocery store. Purchase a small
roll of aluminum or copper flashing and roll your own can. Copper can
easily be soldering into shape. Aluminum can't be soldered and the
typical "conductive" epoxy is worthless at RF frequencies. So, I use
aluminum foil duct tape to finish the seams.

If you're going to roll your own, then I suggest you look into
building a proper horn antenna, which is what the waveguide beyond
cutoff type of can antenna is trying to simulate.
<http://users.skynet.be/chricat/horn/horn-javascript.html>

Now, if you're really cheap, as are most hams, you can build an
acceptable horn antenna out of cardboard, and embalm it in aluminum
foil. This is not terribly useful for small antennas, but really
works well with giant horns used for long distance experiments.
<http://www.seattlewireless.net/index.cgi/CardboardHorn>

Antennas have also be constructed from trash cans, step ladders, HVAC
ducting, roof flashing, plywood, chicken wire, barbeque grills, snow
dishes, kitchen cookware, and other abominations. The reason these
work is that at low gains and wide beamwidths (such as the typical can
antenna), the tolerances are VERY wide, thus making precision a
non-issue. However, if you're planning on building a fairly high gain
can or horn antenna, some precision will be required.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

sillyputty <karmictaragem@2die4.com> wrote:
> I'm still searching for the 'perfect' can for a cantenna. I've got the
> N-connector and pigtail ready to go. So far, I haven't found one that
> matches the dimensions on: http://www.saunalahti.fi/elepal/antenna2.html

I played with many different sized cans. I decided that two "13oz" coffee
cans soldered end to end were the best, based on signal testing, but in
actual use, one "3lb" coffee can was less sensitive to being pointed and
held exactly on target.

Can Diameter Length
3lb Coffee 6 7.5
Atkin's Bake 4 7
Hunt's Pasta 3.3125 5.5
Country Time Lemonade 5 9
Nalley's Big Chunk 3.87 6
15oz coffee 4 5.5
Stagg Chili 3.3 4.2
12oz Yuban 3.875 5.3875

http://www.turnpoint.net/wireless/cantennahowto.html


--
Clarence A Dold - Hidden Valley Lake, CA, USA GPS: 38.8,-122.5

On Aug 25, 7:58 am, Jeff Liebermann <je...@cruzio.com> wrote:

> If you're really looking for the ultimate can antenna, then I suggest
> you visit the hardware store, not the grocery store. Purchase a small
> roll of aluminum or copperflashing and roll your own can. Copper can
> easily be soldering into shape. Aluminum can't be soldered and the
> typical "conductive" epoxy is worthless at RF frequencies. So, I use
> aluminum foil duct tape to finish the seams.

What thickness of copper flashing would you recommend?
Same samples: http://users.skynet.be/chricat/horn/...avascript.html

Also, I'm still confused about the proper dimensions as there are so
many cantennas that claim to work. Or, like you said, are they
forgiving enough that it isn't really critical?

sillyputty <karmictaragem@2die4.com> hath wroth:

>On Aug 25, 7:58 am, Jeff Liebermann <je...@cruzio.com> wrote:
>
>> If you're really looking for the ultimate can antenna, then I suggest
>> you visit the hardware store, not the grocery store. Purchase a small
>> roll of aluminum or copperflashing and roll your own can. Copper can
>> easily be soldering into shape. Aluminum can't be soldered and the
>> typical "conductive" epoxy is worthless at RF frequencies. So, I use
>> aluminum foil duct tape to finish the seams.
>
>What thickness of copper flashing would you recommend?
>Same samples: http://users.skynet.be/chricat/horn/...avascript.html

Any thickness. It has no effect on the performance. However, with
todays spectacular price increases for copper, methinks you should
make your initial tests using aluminum foil and cardboard. Only the
INSIDE of the horn needs to be accurate. Use aluminum foil coated
duct tape to hold the seams together. Once you have something that's
worth building, either build a better one out of aluminum roof
flashing or dive into your pocket book and solder one using copper
roof flashing.

A problem you're going to need to deal with when using copper is that
it oxidizes rather badly. Gain is affected by surface conductivity
and a copper horn, covered with green crud, is not going to work very
well. I have my own silver plating system, but it's a dangerous mess
and I'm not going recommend it. Last time I built a few prototypes
out of copper, I found the local plating shop and had them silver
plate the antenna. Be sure to remove or carefully mask the connectors
before plating.

If you don't want to deal with silver plating, it is possible to paint
the antenna. I've had some luck with clear Krylon (acrylic) but
suspect that almost any other paint will work. I've done little
painting because I also tend to modify the antenna after plating and
need to maintain a solderable surface.

One trick you should consider when building a rectangular horn is to
make the back end of the horn, near the feed, adjustable in position.
Besides the length of the driven element, the exact location of this
"reflector" is the only other critical part of horn construction. You
could make the position of the feed adjustable, but it's much easier
to make the position of the reflector adjustable. When a maximum gain
location is found, tack solder it into position.

>Also, I'm still confused about the proper dimensions as there are so
>many cantennas that claim to work. Or, like you said, are they
>forgiving enough that it isn't really critical?

Note: I'm not an antenna expert. In general, the requirement for
precision is proportional to the gain. At best, the common coffee can
antenna could yield about 12dBi of gain when build perfectly. Few
are, and what I've seen is more like about 8-10dBi. Good enough.
However, if you're going to be building a high gain big dish, big
horn, or some kind of array, dimensions become more critical roughly
with increasing gain.

Also, I had original (wrongly) proclaimed that can antennas and do
thyself reflectors were horrible antennas because of construction
tolerances and material surface imperfections. Aparently, that's not
the case as long as the gain is fairly low. I was accustomed to
squeezing every last fraction of a dB of gain out the system for long
range point to point, satellite, and TVRO systems, where every
fraction of a dB is precious. That's not the case with Wi-Fi, where
considerable sloppiness can be tolerated with low gain antennas.

My suggestion is to build something cheap and easy to start, and spend
some effort setting up a method of comparing the gain to a known
reference. Netstumbler is good enough. Check your gain against a
commercial antenna with known gain. Tweak, tune, cut, trim, and
hammer on the antenna. When it's close to the theoretical
calculations, you're done.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Thanks for all the good advice! :)