antenna-theory.com

[tansy]

Hello everyone.

I found myself in place where I wanted to test some new stuff and build simple wifi antenna. Simple first, then a bit more sophisticated but KISS first. After studying the subject I found that the simplest option is half wave dipole made pretty much from coaxial cable itself. Tested that and it was working ok, but not spectacularly. Then found this
pigtail-pipe kind antenna in some old wifi card and also on inet. It's made of pipe put on coax and inner wire left as second arm of dipole.

Now there is a question, or few:
1. either my sample antenna and the same kind of on internet have 25-26mm long arms of dipole - what's the reason for that?
Now, I found that making this antenna 0.48 wave length (L) makes its impedance 70 ohm but it's still 30mm long. I also found about shortening factor linked with thickness of dipole arms (something like here, but chart is bit incomplete). In my example pipe is 5mm in diameter which makes "slender factor" S=L/2d {S - slender, L - lambda, d - arm diameter} ~0.46L (0.92 on above chart), which is still 28.75mm.
Does anyone know how it works and why? There is a reason, why they use 26mm long arms (some people on inet even say 25mm) or maybe they're wrong?
2. In my card's antenna "hot" arm is 29mm long, which makes it asymmetrical. Is there a reason for that too or it should be 26mm as well as the other arm?
3. What does it change, for good or bad, that one arm is thin? Or it makes no difference?

[admin]

The target length for a very thin wifi dipole antenna is about 0.49lambda, or 61mm. This means each arm is about 30mm. Two things reduce the correct length:

- The fatter you make the arms, the wider the bandwidth, lower the impedance, and lower resonant frequency (all good things. In antennas, more volume = more bandwidth, no expense)
- plastic/dielectrics surrounding the antenna have a permittivity > 1. This means there is a slight decrease in the speed of light, which means the wavelength is slightly shorter, which means the overall antenna size is slightly shorter than a perfectly thin wire with no plastic around it.

There is no "hot side" of your dipole. Both arms contribute equally to radiation. The current/voltage on the dipole is oscillating at 2.4GHz, it doens't know which side you think is hot and which is the other side, they are interchangable.

There is no large effect to one arm being asymetrical to the opposite side. The radiation pattern will tilt a bit, but not a concern.