Antenna - 19dbi homemade?

Hi folks,

I'm looking to become part of a community "mesh", mostly as a matter of
interest and partly to open a free AP in my neighbourhood. I've
contacted the guy who runs the mesh and have discovered I'm 1.7km from
one of his nodes.

I have clear LOS from one corner of my house; checked this earlier with
a pair of binoculars.

He recommends I use a 19dbi for a reliable connection. Can anyone
suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
along the lines of a wok/strainer antenna but I'm not sure.

Unfortunately, a commercial antenna is out of my budget for now.

Thanks,
--
- dcf

David Fairbrother wrote:
> He recommends I use a 19dbi for a reliable connection. Can anyone
> suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
> along the lines of a wok/strainer antenna but I'm not sure.

By the time you spend your time, a 15 dB gain panel antenna for $19 is
worth it.

On Nov 13, 4:09 pm, DTC <m...@nothingtoseehere.zzx> wrote:
> David Fairbrother wrote:
> > He recommends I use a 19dbi for a reliable connection. Can anyone
> > suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
> > along the lines of a wok/strainer antenna but I'm not sure.
>
> By the time you spend your time, a 15 dB gain panel antenna for $19 is
> worth it.

Has anyone bothered to verify the gain of these cheap wifi antennas?
Can you really get 15dbi for under $20?

miso@sushi.com wrote:
> On Nov 13, 4:09 pm, DTC <m...@nothingtoseehere.zzx> wrote:
>> David Fairbrother wrote:
>>> He recommends I use a 19dbi for a reliable connection. Can anyone
>>> suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
>>> along the lines of a wok/strainer antenna but I'm not sure.
>> By the time you spend your time, a 15 dB gain panel antenna for $19 is
>> worth it.
>
> Has anyone bothered to verify the gain of these cheap wifi antennas?
> Can you really get 15dbi for under $20?
>
>

I have yet to even find one of those available to me (in Australia).
If you'd like to provide a link that would be good.

What's the typical max. gain on the woktennas?

--
- dcf

David Fairbrother wrote:
> miso@sushi.com wrote:
>> On Nov 13, 4:09 pm, DTC <m...@nothingtoseehere.zzx> wrote:
>>> David Fairbrother wrote:
>>>> He recommends I use a 19dbi for a reliable connection. Can anyone
>>>> suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
>>>> along the lines of a wok/strainer antenna but I'm not sure.
>>> By the time you spend your time, a 15 dB gain panel antenna for $19 is
>>> worth it.
>>
>> Has anyone bothered to verify the gain of these cheap wifi antennas?
>> Can you really get 15dbi for under $20?
>>
>>
>
> I have yet to even find one of those available to me (in Australia).

Cheaply, that is. I can get them for $70AUD+

--
- dcf

David Fairbrother <schoolsarge@schoolsarge.org> hath wroth:

>I'm looking to become part of a community "mesh", mostly as a matter of
>interest and partly to open a free AP in my neighbourhood. I've
>contacted the guy who runs the mesh and have discovered I'm 1.7km from
>one of his nodes.

Ummm.... that's kinda far.

>I have clear LOS from one corner of my house; checked this earlier with
>a pair of binoculars.

You need more clearance than optical line of sight. See:
<http://www.terabeam.com/support/calculations/fresnel-zone.php>
At 1.5km, you need at least 6 meters clearance around the line of
sight line at midpoint. Do you have that? Note that this implies
that both ends of the link are at least 6 meters off the ground. If
not, the Fresnel Zone will hit the ground at midspan.

>He recommends I use a 19dbi for a reliable connection. Can anyone
>suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
>along the lines of a wok/strainer antenna but I'm not sure.

Sigh. First, the way I understand it, mesh networks kinda look like a
spider web, where connections arrive from all angles of the compass.
What you're building is a point to point link. That's fine, but what
happens when someone wants to connect to *YOUR* node, and finds that
your antenna is pointed at a distant connection. Here's an example of
one mesh network layout:
<http://pdos.csail.mit.edu/roofnet/doku.php?id=map>
Where would you fit in with a highly directional antenna?

>Unfortunately, a commercial antenna is out of my budget for now.

19dBi dish antennas are about $40 plus shipping.
<http://www.fab-corp.com/product.php?productid=2431&cat=251&page=1>
That's about the price of the wok, strainer, and coax connectors.

--
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

miso@sushi.com hath wroth:

>On Nov 13, 4:09 pm, DTC <m...@nothingtoseehere.zzx> wrote:
>> David Fairbrother wrote:
>> > He recommends I use a 19dbi for a reliable connection. Can anyone
>> > suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
>> > along the lines of a wok/strainer antenna but I'm not sure.
>>
>> By the time you spend your time, a 15 dB gain panel antenna for $19 is
>> worth it.

>Has anyone bothered to verify the gain of these cheap wifi antennas?
>Can you really get 15dbi for under $20?

Yeah, sorta. I did this about 3 years ago.

I coudn't get an absolute gain measurement because my antique test
equipment just wasn't accurate enough. However, I could run a
comparison with other antennas of (allegedly) known gain. My
reference antenna is a simple Maxrad 24009PTNF PCB loop antenna:
<http://802.11junk.com/jeffl/antennas/Maxrad%209dBi/index.html>
I've tested this antenna with a tolerable outdoor test range and found
it to have an average gain of 8.3dBi at mid band (ch6).

Using the Maxrad as a reference, I compared receive levels on a
spectrum analyzer from various local sources. I installed a fairly
well calibrated attenuator in line with the higher gain antenna and
used my HP 140T spectrum analyzer to match signal levels.

I tested PacWireless 19 and 24dBi antennas and found the midband gain
to be 18 and 22dBi respectively. Not great, but close enough for my
sloppy methods. I tried an Antenna Spec 8.4dBi omni, which was about
8dBi at midband. I also tested my various home made biquads, which
did not look anything like my 4NEC2 models. One loser varied in gain
from 6 to 10dBi across the band. The one's that worked (after
tweaking) were about 8dBi at midband. Sorry, but I didn't have a
cantenna, wok, or salad bowl available.


--
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

Jeff Liebermann wrote:
> David Fairbrother <schoolsarge@schoolsarge.org> hath wroth:
>
>> I'm looking to become part of a community "mesh", mostly as a matter of
>> interest and partly to open a free AP in my neighbourhood. I've
>> contacted the guy who runs the mesh and have discovered I'm 1.7km from
>> one of his nodes.
>
> Ummm.... that's kinda far.

It's not that far..similar point-to-point links of 6km have been
established in this area. I am also able to pickup a signal from his
node without any kind of amplifier (via a builtin laptop wireless of all
things) though it doesn't stick around for long.

>
>> I have clear LOS from one corner of my house; checked this earlier with
>> a pair of binoculars.
>
> You need more clearance than optical line of sight. See:
> <http://www.terabeam.com/support/calculations/fresnel-zone.php>
> At 1.5km, you need at least 6 meters clearance around the line of
> sight line at midpoint. Do you have that? Note that this implies
> that both ends of the link are at least 6 meters off the ground. If
> not, the Fresnel Zone will hit the ground at midspan.

Actually I do, but I see your point.

>
>> He recommends I use a 19dbi for a reliable connection. Can anyone
>> suggest a homemade antenna that will (or may) reach 19dbi? I'm thinking
>> along the lines of a wok/strainer antenna but I'm not sure.
>
> Sigh. First, the way I understand it, mesh networks kinda look like a
> spider web, where connections arrive from all angles of the compass.
> What you're building is a point to point link. That's fine, but what
> happens when someone wants to connect to *YOUR* node, and finds that
> your antenna is pointed at a distant connection. Here's an example of
> one mesh network layout:
> <http://pdos.csail.mit.edu/roofnet/doku.php?id=map>
> Where would you fit in with a highly directional antenna?

A second AP with omni antennae?

>
>> Unfortunately, a commercial antenna is out of my budget for now.
>
> 19dBi dish antennas are about $40 plus shipping.
> <http://www.fab-corp.com/product.php?productid=2431&cat=251&page=1>
> That's about the price of the wok, strainer, and coax connectors.

Cheers, will look into it. Yeah, a professional antenna is a damn sight
easier to setup than a homebrew, and as you say (in the long run) cheaper.


--
- dcf

David Fairbrother <schoolsarge@schoolsarge.org> hath wroth:

>Jeff Liebermann wrote:
>> David Fairbrother <schoolsarge@schoolsarge.org> hath wroth:
>>
>>> I'm looking to become part of a community "mesh", mostly as a matter of
>>> interest and partly to open a free AP in my neighbourhood. I've
>>> contacted the guy who runs the mesh and have discovered I'm 1.7km from
>>> one of his nodes.
>>
>> Ummm.... that's kinda far.

>It's not that far..similar point-to-point links of 6km have been
>established in this area. I am also able to pickup a signal from his
>node without any kind of amplifier (via a builtin laptop wireless of all
>things) though it doesn't stick around for long.

Picking up the signal and maintaining a full time connection are quite
different. For example, if your friend with the node is using an
amplifier, his transmit range will be much farther than his receive
range. You could hear him, but may not be able to talk to him. If
you can talk to him, you may not be able to stay connected.

The laptop test is a good one as it indicates that you have a chance.
The internal antenna laptop antennas usually have fairly low gain (0
to 2dBi), so a 19dBi antenna or better will certainly be a major
improvement. It's also possible that you won't need a 19dBi antenna
and can get away less gain.

The unknown here is interference. A high gain antenna has a big
advantage, which could turn into a big disadvantage. The narrow
horizontal beamwidth means that any interference to the side and back
of your antenna will be greatly reduced. The bad part is that any
interference along the line of sight whil be greatly increased.

>> Sigh. First, the way I understand it, mesh networks kinda look like a
>> spider web, where connections arrive from all angles of the compass.
>> What you're building is a point to point link. That's fine, but what
>> happens when someone wants to connect to *YOUR* node, and finds that
>> your antenna is pointed at a distant connection. Here's an example of
>> one mesh network layout:
>> <http://pdos.csail.mit.edu/roofnet/doku.php?id=map>
>> Where would you fit in with a highly directional antenna?

>A second AP with omni antennae?

Maybe. You can't just plug in a 2nd different antenna into the 2nd
port on a typical routers diversity switch. That doesn't work. The
explanation is a bit messy. See:
<http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml>
especially the Golf Course case study, where they have an
omnidirectional antenna in one port, and a highly directional yagi in
the other. The switching problem is for real.

A 2nd AP might be possible, and would certainly not have the diversity
switch problem, but might be an issue with whatever mesh networking
protocol and firmware you're using.

You could use a Wilkinson splitter/combiner. That will divide your
transmit power equally between the two antennas. However, the receive
power will not be split making this a fairly useful solution. The
only catch is about 0.5dB additional loss through the splitter.
<http://www.hyperlinktech.com/web/signal_splitters_2400_2way.php>
You can easily build one of these yourself:
<http://yu1aw.ba-karlsruhe.de/spliter-n.gif>

May I suggest a compromise. Build a sector antenna. These have the
advantage that they are very easy to build (if you can solder well).
I've built a few and they are quite good. See:
http://pe2er.nl/wifisector/
http://yu1aw.ba-karlsruhe.de/vhf_ant.htm
http://www.brest-wireless.net/gallery/AntenneAmos
http://www.brest-wireless.net/wiki/materiel:amos
These antennas have a reasonably high gain, very narrow vertical
beamwidth, but also have a very wide horizontal beamwidth. Two or
three of these combined can make an excellent omni antenna, with the
added bonus of facilitating down-tilt. You can start with one, and
add sectors as required.

>Cheers, will look into it. Yeah, a professional antenna is a damn sight
>easier to setup than a homebrew, and as you say (in the long run) cheaper.

I like building my own antenna, but I have some of the (antique) test
equipment necessary to do the job. Building antennas is easy.
Testing, tuning, and optimizing them, is not.
<http://802.11junk.com/jeffl/pics/home/slides/lab.html>
Low gain antennas (<12dBi) can be built without much testing and
tuning. Mistakes and creativity are easily tolerated. That's not the
case with higher gain antennas, where the bandwidth of the antenna is
roughly the same as the 2.4GHz wireless band, and a small mistake can
send the tuning out of the desired frequency range. If you must have
high gain, they buy the antenna. If you can live with lower gain,
then make your own.


--
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

Jeff Liebermann wrote:
> David Fairbrother <schoolsarge@schoolsarge.org> hath wroth:
>
>> Jeff Liebermann wrote:
>>> David Fairbrother <schoolsarge@schoolsarge.org> hath wroth:
>>>
>>>> I'm looking to become part of a community "mesh", mostly as a matter of
>>>> interest and partly to open a free AP in my neighbourhood. I've
>>>> contacted the guy who runs the mesh and have discovered I'm 1.7km from
>>>> one of his nodes.
>>> Ummm.... that's kinda far.
>
>> It's not that far..similar point-to-point links of 6km have been
>> established in this area. I am also able to pickup a signal from his
>> node without any kind of amplifier (via a builtin laptop wireless of all
>> things) though it doesn't stick around for long.
>
> Picking up the signal and maintaining a full time connection are quite
> different. For example, if your friend with the node is using an
> amplifier, his transmit range will be much farther than his receive
> range. You could hear him, but may not be able to talk to him. If
> you can talk to him, you may not be able to stay connected.

Yeah, I suspected that might be the case.
I thought it was a little odd that I could see the network but not connect.

>
> The laptop test is a good one as it indicates that you have a chance.
> The internal antenna laptop antennas usually have fairly low gain (0
> to 2dBi), so a 19dBi antenna or better will certainly be a major
> improvement. It's also possible that you won't need a 19dBi antenna
> and can get away less gain.

That's what I was hoping to hear :)

I'm going to have a play around with a few homebuilds and see what I
come up with. Then if the results aren't satisfactory I'll bite the
bullet and buy a proper one.

After all, this is intended as a learning experience.

>
> The unknown here is interference. A high gain antenna has a big
> advantage, which could turn into a big disadvantage. The narrow
> horizontal beamwidth means that any interference to the side and back
> of your antenna will be greatly reduced. The bad part is that any
> interference along the line of sight whil be greatly increased.
>
>>> Sigh. First, the way I understand it, mesh networks kinda look like a
>>> spider web, where connections arrive from all angles of the compass.
>>> What you're building is a point to point link. That's fine, but what
>>> happens when someone wants to connect to *YOUR* node, and finds that
>>> your antenna is pointed at a distant connection. Here's an example of
>>> one mesh network layout:
>>> <http://pdos.csail.mit.edu/roofnet/doku.php?id=map>
>>> Where would you fit in with a highly directional antenna?
>
>> A second AP with omni antennae?
>
> Maybe. You can't just plug in a 2nd different antenna into the 2nd
> port on a typical routers diversity switch. That doesn't work. The
> explanation is a bit messy. See:
> <http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml>
> especially the Golf Course case study, where they have an
> omnidirectional antenna in one port, and a highly directional yagi in
> the other. The switching problem is for real.
>
> A 2nd AP might be possible, and would certainly not have the diversity
> switch problem, but might be an issue with whatever mesh networking
> protocol and firmware you're using.
>
> You could use a Wilkinson splitter/combiner. That will divide your
> transmit power equally between the two antennas. However, the receive
> power will not be split making this a fairly useful solution. The
> only catch is about 0.5dB additional loss through the splitter.
> <http://www.hyperlinktech.com/web/signal_splitters_2400_2way.php>
> You can easily build one of these yourself:
> <http://yu1aw.ba-karlsruhe.de/spliter-n.gif>
>
> May I suggest a compromise. Build a sector antenna. These have the
> advantage that they are very easy to build (if you can solder well).
> I've built a few and they are quite good. See:
> http://pe2er.nl/wifisector/
> http://yu1aw.ba-karlsruhe.de/vhf_ant.htm
> http://www.brest-wireless.net/gallery/AntenneAmos
> http://www.brest-wireless.net/wiki/materiel:amos
> These antennas have a reasonably high gain, very narrow vertical
> beamwidth, but also have a very wide horizontal beamwidth. Two or
> three of these combined can make an excellent omni antenna, with the
> added bonus of facilitating down-tilt. You can start with one, and
> add sectors as required.

I was worried about the AP issue myself, but I reasoned I'd cross that
bridge when I came to it. I understand the issue a good deal more too,
and I'm going to put some more thought into it. Thanks :)

>
>> Cheers, will look into it. Yeah, a professional antenna is a damn sight
>> easier to setup than a homebrew, and as you say (in the long run) cheaper.
>
> I like building my own antenna, but I have some of the (antique) test
> equipment necessary to do the job. Building antennas is easy.
> Testing, tuning, and optimizing them, is not.
> <http://802.11junk.com/jeffl/pics/home/slides/lab.html>
> Low gain antennas (<12dBi) can be built without much testing and
> tuning. Mistakes and creativity are easily tolerated. That's not the
> case with higher gain antennas, where the bandwidth of the antenna is
> roughly the same as the 2.4GHz wireless band, and a small mistake can
> send the tuning out of the desired frequency range. If you must have
> high gain, they buy the antenna. If you can live with lower gain,
> then make your own.
>

Thanks for the advice Jeff. Your replies are always well detailed and
easy to understand, and it's greatly appreciated. You've certainly given
me a few more things to consider and plan for - things I had previously
overlooked.

Cheers.

--
- dcf

On Thu, 15 Nov 2007 08:37:57 GMT, David Fairbrother
<schoolsarge@schoolsarge.org> wrote:

>Jeff Liebermann wrote:
>>>> Here's an example of
>>>> one mesh network layout:
>>>> <http://pdos.csail.mit.edu/roofnet/doku.php?id=map>
>>>> Where would you fit in with a highly directional antenna?

Please note that I have a really bad attitude about mesh networks. I
generally don't like them. I'll spare you my usual rant (mostly
because I'm tired and wanna get some snooze).

See:
<http://pdos.csail.mit.edu/roofnet/doku.php?id=interesting>
for some interesting test results and statistics from the MIT Roofnet
project. Not the large packet loss (delivery probability) and very
slow average thruput. That's typical.

Here's another mesh network status map:
<http://www.netequality.org/map.php?id=hacienda>
If you tinker with the web page, some nifty statistics appear.

This one is a bit tricky to use:
<http://layereight.de/freifunkmap.php>
Click on the various names on the upper right of the map so that they
switch to bold face type. The mesh will eventually appear, along with
path statistics. The percentages shows are the packet delivery
reliability. Lots of links show really high packet loss.

Anyway, get some numbers from your mesh network admin.

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