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Cantenna Calculation Problem

Last response: in Wireless Networking
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April 2, 2011 3:37:59 PM

Hi Every Body

I am going to build a cantenna for my wireless adapter ( Alfa AWUS036H)
so read some article about that

1- http://www.oreillynet.com/cs/weblog/view/wlg/448
2- http://flakey.info/antenna/waveguide/
3- http://www.netscum.com/~clapp/wireless.html

but today i find a article about how to make Omni Antenna
that the calculation makes me confused

http://wireless.gumph.org/articles/homemadeomni.html


all of the website that explain how to make cantenna use this type of calculation :

Quote:
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"


but in the web site that explain how to make omni antenna add Velocity factor of coax cable in the calculation :

Quote:
Each sector of the antenna needs to be a 1/2 wavelength long multiplied by the velocity factor of the cable. The velocity factor of RG-213 is 0.66 . If you decide to use different cable (such as LMR-400) then you need to get the velocity factor of that cable (which will be different), and recalculate all the dimensions.

V * C 0.66 * 299792458
1/2 wavelength = -------- = ------------------------- = 0.0405m = 40.5mm
2 * F 2 * 2441000000

V = Velocity Factor of RG213 = 0.66
C = speed of light = 299792458
F = Frequency of Signal = 2441000000 (middle of 2.4ghz range)

The 1/4 wave element is not adjusted by the Velocity factor, as it is in the open, so works out at just 31mm long giving a total antenna length of 355mm + fly-lead. (Thanks to Oscar for correcting me of this.)


I have Question about that why we not add Velocity factor in the Cantenna calculation ?
( I see there is lot of different when I add it this factor to Cantenna Calculation ) :o 


Thanks In Advance

Best solution

April 4, 2011 5:46:13 AM
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lightmoon said:
Hi Every Body

I am going to build a cantenna for my wireless adapter ( Alfa AWUS036H)
so read some article about that

<snip>

but today i find a article about how to make Omni Antenna
that the calculation makes me confused

<snip>

but in the web site that explain how to make omni antenna add Velocity factor of coax cable in the calculation :

<snip>

I have Question about that why we not add Velocity factor in the Cantenna calculation ?
( I see there is lot of different when I add it this factor to Cantenna Calculation ) :o 


Thanks In Advance


You do not need to be concerned with the coax velocity factor in the cantenna as it has only one radiator, (single element), operating in air, and it's at the end of the coax.



The collector assembly is also operating in air...



The Collinear antenna is several 1/2 wavelength sections that are stacked one on the other. How fast the signal passes through the coax sections becomes very important because the speed at which a signal travels within a coax cable is not the same as an electromagnetic wave traveling in free space. Instead it is affected by the dielectric that is used within the coax cable, and this has the effect of slowing the signal down. The 1/2 wave sections are shortened slightly to compensate for the slower speed within the coax, so it has less distance to travel and therefore will "keep up" (stay in phase) with the magnetic wave generated. If you use LMR400 or LMR200 coax, the velocity is .85 making your sections a bit closer to actual 1/2 wave size if you prefer.

The collinear antenna uses coax as the radiator, thus the speed the signal travels through the antenna (coax sections) became important.

If you are going to make this antenna, it is very important to decouple the antenna from the feed coax or the feed itself will become one more element in the antenna of a grossly incorrect length, and of course the gain will suffer as a result. Get three toroid cores (glued together) that fit over the coax feed line near the connection to the antenna, but don't set them in place permanently on the line... yet. Once the antenna is constructed and mounted, slide the cores up and down the feed coax near the antenna starting at about 1/4 wave away and slowly increase this distance, up to 1/2 wavelength, checking signal strength often until you see it peak and then start to fall off again. Tape or glue the toroid cores in place where you get best results. If you had a 2.5Ghz SWR meter, this process would be quite fast and more accurate, but who has one of those?

Last things on collinear antennas... Make sure it is truly vertical, use very rigid plastic support pipe to keep it straight, use plastic pipe that is not going to affect your antenna performance by absorption. Cut a small piece of the plastic pipe off and put it in a microwave oven (high power setting) with a half-cup of water (to supply load to oven) for about 3 minutes. If the pipe heats up, use a different brand. "Non-metallic" grey plastic conduit is very good for these antennas.

Good luck, both of these antennas have their own benefits; cantenna for singe-point, and collinear for multi-point communications.

April 6, 2011 6:05:12 PM

Best answer selected by lightmoon.
!