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 )

Thanks In Advance