Inside the Comtelco 7.5dBi Antenna

by Michael Renzmann


Introduction

Here is another article initiated by the work of outcast_one, a user in the forum of netstumbler.com. He asked me to put his pictures into an article as the one about the Lucent Range Extender antenna (which can be found here). It took me a long time (about 4 to 6 weeks) to do so, as I was very busy. I'm sorry for that.


Some information on the antenna

This text completely was written by outcast_one, as I don't have access to this antenna. He wrote:
This antenna is directional, seems to have about a 60 degree beam pattern
(horizontal and vertical).
Gain is advertized at 7.5 dbi (about 5.5db). Frequency range is 2400 to
2500 MHZ.  It has a 12 inch pigtail with a female N connector on it.  I have
tried using it for Netstumbling by mounting it to the rear view mirror
(facing forward through the windshield) of my truck.  It seems to have as
good reception to the sides as compared to the range extender antenna (reant
antenna is mounted on the roof).  It will pick up access points from the
front about 10 to 20 percent farther away than with the range extender.  Of
course, the reception from the rear is poor.  Not too bad considering the
antenna is mounted inside the truck.  It should be easy to build from copper
or brass sheet from a hardware store. A short cut length of pvc pipe could
be used to get the proper spacing on the radiator/ reflector.  Hot glue or
silicone can be used to assemble the pieces.

For those of you who never heard of this antenna before (as I did), here is a link to the page describing the antenna on the manufacturers website:
http://www.comtelco.net/cgi-bin/w3-msql/cc-item.html?part_num=B25247PW

If anyone starts experimenting with a "self-made" version of this antenna type, let me know about it please. I would like to give additional information on this here. outcast_one made a small schematic that could help you:


Click on the picture to zoom in (opens new browser window).


The pictures

These pictures show what the Comtelco antenna looks like inside. You can see the circular reflector disc and the brass, which is above the reflector. As with the Orinoco Range Extender Antenna, there is really no magic inside. Just some copper plates in the right dimension... one just has to know what to do with them :-)
 

fig. 1: antenna with cover
 

fig. 2: top view on brass/reflector
 

fig. 3: connection of pigtail and brass
 

fig. 4: side view, brass over reflector
 

fig. 5: another view from top and side
   
 
Note: Click on the pictures to zoom in (opening a new browser window).


How it works

I'm no antenna pro, as you already might have found out. Everything that would come to my mind about the antenna has already been mentioned by outcast_one. It is a directional antenna, and this type seems to be quite common for microwave setups like wireless lans. I know of two other antennas that use a similar design, and there are some guides out there in the net describing how to build such an antenna.

If you have additional information you would like to tell, please send them to me by mail. I would be happy if anyone of you could contribute some more details. See below for mail address.


User comments

lincomatic posted the following two articles to the netstumbler.com forum:
[1]
thanks for the design :)
making this thing is a joke. i fashioned the metal bits out of a Fisherman's Friend
lid and a piece of scrap metal from my garage.
i don't have an N socket for my pigtail yet...will try to get one this weekend. to
make it discreet, i plan to house the thing inside a cheap plastic food container.
after it's done and i've tested it against my "cantenna," i'll post pics and the
test results.

btw, one dimension you left out is the distance to the mounting screw for the top
piece, but i've just estimated it from the photos.

[2]
ok, finally got the antenna assembled. initial quick tests show performance a little
better than my hunts beans cantenna what i like about it is that it seems a bit less
directional, so it's more forgiving about precise aiming. also the compact design
makes it easier take with me, and the plastic food container casing makes it more
discreet...pointing that cantenna at someone lookes kind of sinister...

i'll post some pix later this weekend when i have time.

He posted his experiences (including some pictures) to the following website: http://www.geocities.com/lincomatic/wifipatchantenna.html
As one can see: wireless lan antennas don't have to be expensive :-)
 

outcast_one answered to lincomatic's postings and gave some additional infos:
I have this antenna attatched to one of my access points, mounted inside the peak of
my attic- pointing straight down. It easily gives whole house coverage through 2
floors, right down to the basement. As you stated, it does have a fairly wide
beamwidth. I can easily pick up this access point more than 3 blocks away (when
there are no trees in the way) even though it is pointing straight down.

As with all antenna testing, get some distance away fron the AP. You will get much
better results if you are 1 block away from the AP compared to 10 feet away inside
the house. The AP's signal bouncing all over the house will give strange results
sometimes. Yow will not get a fair comparison of gain as well as directivity
(beamwidth). It is easy to build, and can be a good antenna to compare gain with
other homemade ones.

As for the dimension for the mounting screw, it is not critical. I think most people
would simply use a piece of plastic pipe and glue to attatch the radiaitor to the
reflector. Just make it look like the pictures, and it will work fine.


The small print

This page has been set up by Michael Renzmann (mrenzmann AT web DOT de). Feel free to contact me any time if you have questions or suggestions regarding the content of this page. Please be kind, I am no native english speaker :-)

Many thanks go to outcast_one for contributing the pictures and the text. Keep up that work, I would be happy to see more antenna internals in the future.

The idea for the design of this page has been shamelessly stolen from the excellent article "Wireless LAN und Linux", written by Marcel Holtmann. His article can be found here. It has been written in german language, describing how to set up wireless lan under Linux (explaining it for SuSE Linux).


Copyright 2002 by Michael Renzmann / planet-wireless.de published: 2002-05-10
last edited: 2002-05-12