Penang wrote:

...
Specifically, my interest is in the "broadcast" type of antenna ...
trying to find a way to "extend" the range of a typical wifi
basestation using antennas alone (maybe with the help of MIMO
antennas), without "range repeaters" or whatnots.
...
Thank you all !

Much research is going on and designs/parameters advanced. These two
papers represent the direction this is taking:

http://ceta.mit.edu/pierl/pierl01/10.07111810.pdf
http://www.ece.uci.edu/~ayanoglu/MIMO_MRAA_IEEEAWPL.pdf

However, the DLM (Distributed Load Monopole) antenna by Robert Vincent
is an excellent design for such applications.

However, since size is not a major consideration in my external wifi
antennas, a conventional 1/2 or 5/8 wave end fed design is what I have
implemented, with a gamma feed ...

I use a biquad with a parabolic reflector (from a direct TV sat.
antenna) for directional antenna implementations ...

I am following the KISS method (or, Keep It Simple Stupid! -- NO insult
intended ...)

Regards,
JS

On Thu, 25 Sep 2008 11:00:20 -0500, "Mike Lucas"
wrote:
"Jeff Liebermann" wrote:
No. Brain damage is a known side effect and complication resulting
from trying to digest the entire field of antenna design all at once.
Just reading some of the posting in this newsgroup should adequately
demonstrate the extent of the damage. I suggest you approach antenna
design in the same manner as eating a loaf of bread. It's done one
slice at a time. If you try to shove the entire loaf down your throat
at once, you'll choke.

Reading one or two posts from Art will evoke chortles and guffaws, but
when taken in large, sustained doses, will make your head hurt.

I was trying to be diplomatic and subtle. Apparently, I failed.

On another mailing list, I once had someone telling me that I'm wrong
because 90% of the universe is dark matter or dark energy. Since
these are total unknowns, 90% of everything is therefore unknown.
Therefore, 90% of my analysis must also be wrong. I rather like the
logic.

However, I was serious about taking it slowly on learning about
antennas. To really understand how they work and what they, it takes
far too many diverse diciplines (Maxwell's Eqn, modeling, propagation,
topography, material science, construction techniques, impedance
matching, surface geometry, etc) to absorb it all at once. Also, some
hands on experience is always useful.

It's like trying to recover an intelligent signal out of gaussian
white noise. Despite the best efforts of all involved, the noise
usually wins.

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

Jeff Liebermann wrote:
It's like trying to recover an intelligent signal out of gaussian
white noise. Despite the best efforts of all involved, the noise
usually wins.

If I listen to white noise when I'm inebriated, I can hear
Willie Nelson singing, "You Were Always On My Mind". :-)
--
73, Cecil http://www.w5dxp.com
"According to the general theory of relativity,
space without ether is unthinkable." Albert Einstein

If by searching the web you haven't found a good wifi antenna you should
look again as there are lots that will work. If you're wanting to go into
business against the major carriers as your equals or competitors, then go
back to school for a little longer.....................

Penang writes:

What's your suggestion / opinion on how to construct an excellent
(pair) of MIMO antenna for wifi (802.11 b/g/n) ?

Not MIMO, but many weird looking, but mostly sound, ideas he

http://usbwifi.orconhosting.net.nz/

"Make 2.4GHz parabolic mesh dishes from cheap but sturdy Chinese
cookware scoops & a USB WiFi adaptor"

By placing the wi-fi adaptor in the antenna, you don't lose radio power in
cables and connectors. You have to be careful about waterproofing,
though.

Jon

On Sep 25, 7:43 pm, Bob Bob wrote:
Starting with one or two basics;

- Keep the losses down
- Channel as much of the radiation as possible in the direction you need it

Losses in this sense more come from coax runs and connectors. Keep the
run short, dont use joiners and if possible mount the WiFi box close to
the antenna. You have to also allow for obstructive losses like trees
and buildings. The other path loss is from errant reflections. Moounting
the antenna high up in the clear resolves many of these issues.

Radiating in the right direction isnt rocket science in theory. The
average "omnidirectional" antenna is actually "all directions" only
parallel to the ground. ie you dont waste radiation skyward or warming
the dirt! In terms of distance covered though an omni of this kind wont
be as good as employing an antenna that "beams" in one direction.
Consider this as a kind of flashlight with reflector and you'll get the
right idea. In terms of time/construction/distance investment you'll do
a lot better with this kind of antenna than an omni, elthough you will
need to point them in the right direction. Which one you choose depends
on its intended use.

You have probably figured out that the antenna gain figure (in dB) is
the measure that is used to determine how much better one antenna is
over another. It is a log comparison of the desired radiation direction
"strength" vs that of an antenna that radiates eqaully in all
directions. An omni however will almost always have a lower gain than a
"beam" type antenna. 9 and 12 dB are some fairly high omni gains but
"beam" gains can go 15-30dB. The higher the number the greater the range
in the desired direction.

So my view.. If you intend one site being fixed and the other movable
then you'll at least need an omni at the base site. If you only want a
point to point link then a "beam" variety at each end is the way to go.
I wouldnt go for "best" but engineer the path/strength requirements to
determine the antenna gain needed. If I was making the antennas I'd
probably use end fire helixes as they seem to be most forgiving in
construction errors vs best gain.

You might want to tell us your end application, distance to cover,
movement of sites and so on so we can suggest something a little more
finite.

First of all, thanks to all for the many wonderful suggestions !!

It's for a community-type wifi network, and I'm cracking my head
trying to figure out a way to make the basestations' broadcast cover a
wider area.

I am reading all your suggestions right now and am trying as hard as
possible to digest them all.

Thanks again !!



Cheers Bob

Penang wrote:

What's your suggestion / opinion on how to construct an excellent
(pair) of MIMO antenna for wifi (802.11 b/g/n) ?

Thank you all !

On Sep 25, 9:52 pm, Jeff Liebermann wrote:
On Wed, 24 Sep 2008 22:39:23 -0700 (PDT), Penang
wrote:

Specifically, my interest is in the "broadcast" type of antenna ...
trying to find a way to "extend" the range of a typical wifi
basestation using antennas alone (maybe with the help of MIMO
antennas), without "range repeaters" or whatnots.

Broadcast implies one way transmissions. Wi-Fi is bi-directional.

The common terms are "range extender" or "repeater". These normally
refer to store and forward transmitter/receivers use to extend range
by retransmitting the data. This is roughly how a MESH network
operates. There are some severe limits and compromises to using such
repeaters, none of which have anything to do with the antenna. Ask if
you need details.

My brain is now fully clogged, and I'm having difficulties digesting
all the stuffs that I've gotten.

One must suffer before enlightenment. You're doing fine.

Is there a place (a forum, a website, a tutorial, and whatnots) where
one can learn about antennas, without having one's brain clogged up,
like mine right now?

No. Brain damage is a known side effect and complication resulting
from trying to digest the entire field of antenna design all at once.
Just reading some of the posting in this newsgroup should adequately
demonstrate the extent of the damage. I suggest you approach antenna
design in the same manner as eating a loaf of bread. It's done one
slice at a time. If you try to shove the entire loaf down your throat
at once, you'll choke.

What's your suggestion / opinion on how to construct an excellent
(pair) of MIMO antenna for wifi (802.11 b/g/n) ?

I suggest you give up immediately and read about how MIMO works.
External MIMO antennas are a problem.

There are two basic types of MIMO. One is called "beam forming",
which uses an elaborate and flat array of internal antennas to
customize the antenna pattern. Major lobes are pointed in the
direction of users. Nulls are pointed in the direction of
interference. The key point is that antennas are internal and
therefore external antennas have no place to plug in.

The other type is called "spatial diversity". It uses multiple
antennas and multiple receivers to combine reflective signal paths. In
theory, each path is independent and carry an independent data stream.
Combining these paths results in enhanced speeds. Note that this form
of MIMO is all about speed, not range.

It is possible to attach external antennas to a spatial diversity type
MIMO system. However, there are limitations. You can't just install
3 directional antennas pointed in 3 different directions. Pointed in
opposing directions, the antennas will simply not pickup any
reflections. If there are any long path reflections, the delays will
probably be excessive and the data deemed useless. Attaching a single
antenna to a MIMO spatial diversity type router will function, but
only at 802.11g speeds. Without the time delayed reflections, there's
no MIMO speeds.

Anyway, I don't have any suggestions on how to construct an excellent
antenna. You haven't specified what you're trying to accomplish, what
you have to work with, what is your operating environment, and what
problem you're trying to solve. Different types of antennas are
designed to solve different problems. Without a clue as to the
problem and limitations, it's rather difficult to select a suitable
antenna.

Hint: Forget about using "N" or MIMO with external antennas. Unless
you're trying to move video, 802.11g speeds are good enough.

Since I'm toying with a community-type of wifi network, there are
walls, and foliage, and topologies (hills and valleys) that are
effecting the way the waves work. That is why I am trying to read as
much as possible, while experimenting with an array of weird looking
antennas and putting them in various spots to try out.

The longest (world record?) for a wifi transmission is somewhere at
the 147KM, someone in South America did that. I ain't gonna try to
break it, but would be more than happy -- in fact jumping in hoops !!
-- if I can find a way to make antennas that can successfully spread
the signal. 1 or even 2 miles away.

I'd be reading up the excellent points you and so many others have
given me here, and will try to tinker a bit here and there even more.

Thanks again !!



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

On Fri, 26 Sep 2008 02:45:59 -0700 (PDT), Penang
wrote:

Since I'm toying with a community-type of wifi network, there are
walls, and foliage, and topologies (hills and valleys) that are
effecting the way the waves work.

2.4GHz wireless does not go through hills. If you plan to have
coverage in the valleys, you'll need to have a radio somewhere in the
valley. NLOS (non line of sight) radio can be made to work, but
cannot be made to stay working. It may work for a while. Then,
something is moved, and it stops working. If you want to fill in
holes, look into wireless mesh networks.
http://www.open-mesh.com
http://meraki.com

I suggest you look into sector antennas, specifically AMOS or Franklin
antennas. These are quite suitable for covering a wide flat area.
Depending on construction, the horizontal beamwidth is anywhere
between 90 and 150 degrees, with a vertical beamwidth of approximately
10 degrees. The idea is to send most of the RF toward the horizon,
and not into the air or into the ground. Gain varies from 8dBi to
perhaps 15dBi depending on type and construction.

Articles on the AMOS antenna design at:
http://www.qsl.net/yu1aw/invertamos.pdf
http://www.qsl.net/yu1aw/amos_article.pdf

AMOS and Franklin type antennas:
http://pe2er.nl/wifisector/
http://yu1aw.ba-karlsruhe.de/ANT.htm
http://yu1aw.ba-karlsruhe.de/vhf_ant.htm
http://www.qsl.net/yu1aw/vhf_ant.htm
http://www.brest-wireless.net/gallery/AntenneAmos
http://www.brest-wireless.net/wiki/materiel:amos

You'll find that there's much more to running a WISP (wireless
internet service provider) than antennas and radios. If you plan to
grow the business, you'll find that it's exactly the same as a wire
line ISP, except you have a basically unreliable method of delivery.
One leaky microwave oven will trash connectivity for a very wide area.
Like a wire line ISP, you'll need a backhaul, billing, support,
installation, service, and troubleshooting. The usual oversight is
that wire line ISP's don't make much money from connectivity. They
make their money with secondary services such as web hosting. If
customers bring their own equipment, you'll need some way to
troubleshoot *THEIR* problems and possibly charge for fixing *THEIR*
computer. You could supply the radio equipment to the customers which
will put you in the equipment leasing business. Plenty to think about
but the one that usually causes problems for startup WISP's is "who is
going to answer the phone when the customer calls to report an outage
at 2AM?" Start reading about the WISP business he
http://www.bbwexchange.com/wireless_internet_access/
http://www.bbwexchange.com/howto/

Good luck.

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

Okay well the answer is relatively simple.

"Height is might". Antenna system should be as high as possible, clear
of path obstructions and provide gain in the direction of interest. For
equal coverage in all directions this generally means a vertical
collinear design. The idea is to have a very small angle vertical
pattern as close to horizontal as possible. (ie radiates most of its
power to the horizon)

In my experience, end fed (ie at the bottom) collinear systems
(especially home made ones) tend to have current imbalances in the
elements such that the lower ones get slightly more than the upper ones.
the result is that the pattern skews undesirably upwards at a slight
angle. You also get less gain per unit length using systems that feed
from one element to the next rather than (say) a number of dipoles fed
directly in parallel. There is an optimum spacing between elements for
best gain that is difficult to get in element to element systems.

Of course 2.4GHz antennas are harder to size accurately and test
equipment can really drain the hip pocket!

If I was doing it for a community system I wouldnt bother doing antenna
construction as such. I'd probably go and buy maybe 4 of a well known
base design of moderate gain and stack them for gain. These may end up
being in panels rather than simple verticals. Keep in mind though what
you are trying to achieve and best bang for your buck. Doubling your
antenna size gives best case an extra 3dB (often less) of gain. In a
free space model the extra distance covered is double the distance for
every 6 dB. This initially sounds good but once you are no longer line
of sight, that gain is nothing compared to the losses. I dont know the
2.4GHz numbers off hand but the range numbers might be something like
5km, then 5.1km with an extra 6dB. Of course those users that have a
signal already will get a slightly stronger one and thus may get a
higher data rate but there will be a point where more gain and a
different system design is better. Using multiple channels and WAP
repeaters or bridges/links although likely bringing the bandwidth down
will give you greater coverage albeit sometimes at the tradeoff of speed.

I dont have any modeling stuff for 2.4GHz networks as such. You tend to
need a lot of obstructive rather than terrain data information. You can
however take some photos and hand enter some near field terrain info to
look like buildings. Have a look at "Radiomobile" as a prediction tool.
As far as I remember it is GPL software that you will find other users
have used for WiFi networks. You can see the effects of gain and height
very easily and it will give you a lot of go/nogo help for particular
areas you want to supply access from.

Apologies for the length!

Bob

Penang wrote:


It's for a community-type wifi network, and I'm cracking my head
trying to figure out a way to make the basestations' broadcast cover a
wider area.

I am reading all your suggestions right now and am trying as hard as
possible to digest them all.

On Fri, 26 Sep 2008 06:30:11 -0500, Bob Bob
wrote:

Okay well the answer is relatively simple.

Nothing in antenna design is simple. Mind if I disagree with a few
details?

"Height is might". Antenna system should be as high as possible, clear
of path obstructions and provide gain in the direction of interest.

I beg to differ, having established one (failed) WISP a few years ago.
The higher you place the antenna, the more interference from other
2.4GHz users you'll receive. The license free bands are really
polluted. Once you get high enough to get adequate coverage of the
client radios, any additional height is counterproductive as it only
increases interference pickup.

Clear of path obstructions is correct, but difficult to accomplish.
The most common screwup is insufficient Fresnel Zone clearance,
especially when it hits the ground. For example, let's pretend our
WISP is running out if his house to a similar client radio, both with
rooftop antennas. That would be about 7 meters off the ground.

Quiz: At 2.4GHz how far away can these they be before an 80% Fresnel
zone hits the ground?

Answer:
http://www.terabeam.com/support/calculations/fresnel-zone.php
Plugging in various ranges until I get a 7 meter Fresnel Zone radius,
I get about 2.5km range. Any further and the antenna at one (or both)
ends will need to be elevated.

For
equal coverage in all directions this generally means a vertical
collinear design. The idea is to have a very small angle vertical
pattern as close to horizontal as possible. (ie radiates most of its
power to the horizon)

I again beg to differ. Vertical collinear antennas suck for WISP
service. Lots of problem. On omnis with sufficiently high gain, the
extremely narrow vertical radiation pattern, and the apparently
incurable uptllted pattern, results in sending the RF to various
useless directions. There are antennas with a few degrees of
downtilt, but my testing shows that they still have uptilt when
mounted on a tower outrigger.

In my never humble opinion, the proper antenna for WISP service is a
90 or 120 degree sector antenna. These offer a solution to the uptilt
problem in that they can be pointed downward without having most of
your RF go into the sky on the backside. See photo at:
http://www.qsl.net/yu1aw/3InvAmosa7.JPG
These are 3 antennas, connected to 3 different access points. However,
the 3 antennas could just as easily be run by a single access point,
by using a combiner/splitter, yielding plenty of gain with the
advantage of downtilt.

In my experience, end fed (ie at the bottom) collinear systems
(especially home made ones) tend to have current imbalances in the
elements such that the lower ones get slightly more than the upper ones.
the result is that the pattern skews undesirably upwards at a slight
angle. You also get less gain per unit length using systems that feed
from one element to the next rather than (say) a number of dipoles fed
directly in parallel. There is an optimum spacing between elements for
best gain that is difficult to get in element to element systems.

Yep. However, it's not an "imbalance" that causes the problem. End
fed vertical collinear antennas belch half of the RF from the first
1/2 wave dipole near the connector. 1/4th or half of what's left
comes out next half wave section. 1/8 of the RF from the next section
and so on ad infinitum. what that means is that there's not much RF
left by the time the you get to the tip of the antenna. It's for this
reason that I like to mount my end fed omnis upside down on the tower
outrigger. This is often vetoed by the tower aesthetics committee but
when I get my way, it works very nicely.

Incidentally, the uptilt problem is mostly solved by some commercial
omni antennas with a center fed collinear dipole arrangement inside
the fiberglass tube. This is common at VHF/UHF frequencies but I
haven't seen it at 2.4GHz.

Of course 2.4GHz antennas are harder to size accurately and test
equipment can really drain the hip pocket!

Bah. The stuff costs money but there's no need to buy state of the
art hardware. A good RF sweep generator, direction coupler, diode
detector, general purpose oscilloscope, marker generator, and a mess
of coax cables, adapters, and terminators, are usually sufficient:
http://802.11junk.com/jeffl/pics/home/slides/BL-shop5.html
The HP8620A sweep generator is fine (but my 2-4GHz plug needs work).
There are two Wiltron 610D sweep generators behind the pile. I don't
recall what I paid for all the sweepers, but it wasn't huge.

The real key to sweeping the antenna is in the directional coupler or
VSWR bridge. Good ones are expensive and easy to destroy. So, build
your own for just 2.4Ghz:
http://pe2er.nl/wifiswr/

If I was doing it for a community system I wouldnt bother doing antenna
construction as such.

Maybe. Commercial sector antennas are somewhat expensive.
http://www.superpass.com/2400-2483M.html
It's a marginal proposition to build your own for just the base
stations. However, if I were supplying client radios for the
customers, I would certainly consider rolling my own to save a few
dollars per install.

I dont know the
2.4GHz numbers off hand but the range numbers might be something like
5km, then 5.1km with an extra 6dB. Of course those users that have a
signal already will get a slightly stronger one and thus may get a
higher data rate but there will be a point where more gain and a
different system design is better. Using multiple channels and WAP
repeaters or bridges/links although likely bringing the bandwidth down
will give you greater coverage albeit sometimes at the tradeoff of speed.

Using repeaters (range extenders) or mesh networks also has the
disadvantage of hogging air time. With a single channel store and
forward repeater, it takes twice the air time to move a single packet
between link endpoints. That's not a big deal in an isolated client,
but with a heavily used and high access point, it can easily be
considered a form of interference. Wireless is a shared medium. If
there are packets flying through the air because of repeaters, they
will reduce the available air time rather rapidly. Such repeaters
also have a lousy delivery success rate, resulting in retransmissions,
which reduce the available air time even more.

Speed is also an issue, but for a non-obvious reason. In such a
shared environment, you want each transmission to occupy as little air
time as possible. That means you have to send your data as fast as
possible. Signal strength and maximum speed are directly related (all
else being equal). The last thing you need is a customer stuck at the
minimum 1Mbit/sec and monopolizing a disproportionate amount of air
time. I setup mine to do 802.11g OFDM modes only and to ignore all
802.11b speeds. That gives me a 6Mbit/sec minimum OFDM speed, which
uses approx 6 times less air time than 1Mbit/sec. It also changed
having all the beacons and broadcasts from 1Mbit/sec to 6Mbits/sec.
When possible, I like to use fixed data rates, typically 12Mbits/sec
or more.

I dont have any modeling stuff for 2.4GHz networks as such. You tend to
need a lot of obstructive rather than terrain data information. You can
however take some photos and hand enter some near field terrain info to
look like buildings. Have a look at "Radiomobile" as a prediction tool.
As far as I remember it is GPL software that you will find other users
have used for WiFi networks. You can see the effects of gain and height
very easily and it will give you a lot of go/nogo help for particular
areas you want to supply access from.

Yep. Great program. I use it often:
http://www.cplus.org/rmw/english1.html
However, it's only as good as the topographic data. Most commonly
available (for the US) are the SRTM v2 data at 1 arc second (30
meters). That's good enough for doing general topography (mountains
and valleys) but not good enough for doing urban canyons and small
neighborhood WLAN's. Data for the rest of the world varies by
country, but most of it seems to be 10 times less resolution.

Apologies for the length!

I won't apologize for my length. I'm partial to details and
explanations over the all too common one-line quips and cute remarks
devoid of any useful content.

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