I am looking for a true omnidirectional antenna design for 2.4 gig(within a few dBs). I mean 360 degrees in all axis. Is there such an animal?
Ron
? All the various vertical antennas, including the little ones that are factory stock for WiFi w-routers, are 360 degree omni-directional. Perhaps you mean an antenna that is omni-directional for both horizontal (as I just said) and vertical. This kind of antenna can be called an isotropic radiator - it has a spherical radiation pattern. The common dipole vertical antennas have a fat doughnut pattern, decreasing to a squished flat doughnut if the antenna’s gain spec is high (like 12dBi gain is the highest omni I’ve used - just 7 degree vertical beamwidth.
So If I understand your question - the answer is no.
FYI: Antenna gain is often stated in “dBi” - this is the gain (in dB) relative to this ideal spherical isotropic radiator. Such an impossible to build antenna would have 0dBi gain in any x, y, or z direction.
You may also see gain as “dBd”. This is gain relative to a dipole, where a dipole is often thought of as about 2dBi gain. So if an antenna is spec’d at 3dBd gain, it’s also 5dBi.
For WiFi, you can merely use the two-antennas on most w-routers - orient one parallel to the floor and the other perpendicular to the floor. Together, their patterns are similar to an isotropic radiator. Common 11g radios do switched diversity - where the radio chooses to use the antenna with the best signal from the client device. While that antenna is used, the other antenna is ignored. High end 11n radios may use 2+ antennas simultaneously (this is called MIMO) - but there must be two radios. And client devices like PCMCIA cards don’t lend themselves to physically giving the necessary antenna spacing - at least a half wavelength.
There’ll be a quiz Friday.
That post was crammed full of info. Ever consider writing a tutorial for sfe?!
I didnt know that about the dBi and dBd, thats pretty cool.
Shucks about the isotropic radiator, and about the wireless routers that use only 1 antenna of the 2, thats a good trick on their part, lol, fooled me.
-Nate
Now, onto sizing. I have come across a resource, RF toolbox, through my contacts in the ham radio club of the high school I attended. It is RF toolbox, and it calculates values for length and the such based on frequency. I find it much easier than using the normal equations, as it is able to calculate many types of antennae. It may be found here:
http://www.blackcatsystems.com/download/rftoolbox.html
It will bother you to pay for the software, but you can use it without paying. Simply wait a while until it says ‘not yet’
If you don’t want to use the program, the equations are:
L = 142.6m / f or
L = 468ft / f
where f is in megahertz, and the measurement unit is indicated in the numerator. These numbers correspond to a half wavelength of the radio wave, which is the total length of the dipole.
Onto more useful information. Antennae, for most applications, must be tuned for the desired frequency of operation. The antenna sizing application will get you close, but you have to do some tuning yourself.
The impedance of the antenna should be 73 ohms, for a dipole antenna, at the desired frequency of operation. This factors both reactive and real impedance. You can measure the standing wave ratio to determine if the antenna is the correct length. You should have a ratio of about 1.
If the ratio is not around 1, your antenna is either too short or too long. Luckily, due to the fact that this is an AC circuit, we can correct for this without having to destrpy the antenna by shortening it or lengthening it. Simply add inductance if the antenna is too short or capacitance if it is too long into the antenna. However, shortening an long antenna is easier than adding capacitance.
*edit:
I looked up some information on the decibel system. It is a model for power gains and losses due to amplifies etc. I found this equation
Gain [dB] = 10log(Pout/Pin)
I don’t know how helpful this was to you, but I found it interesting.