Saturday, July 4, 2009

A comparison between 3 different types of antenna

Now that we have a way to get the Internet connection from the neighborhood, we want to optimize the signal strength. Actually, we want to maximize the signal, but keep the noise as low as possible. We will call the ratio Signal/Noise a SNR. The Signal & Noise are measures in decibel isotropic (dbi). To simplify, we will say that a dbi is the same measure as a db, applied to an antenna.
The isotropic antenna is the "perfect" antenna that radiates in all the direction with the same magnitude, with no loss. In other words, the isotropic antenna has no preferred direction of radiation.

Radiation of a punctual isotropic antenna

Without digging too much into details, the comparison of the highest peaks of the antenna with the isotropic one gives us the Gain; we will assume that the bigger the SNR is, the best the reception will be.

Usually, an antenna has a horizontal plane pattern called an azimuth, and a vertical plane pattern, called the elevation.

Good, now we are interested for the end-user, which antenna is/are the best ?

  • Omnidirectional antenna
An omnidirectional antenna is an antenna system which radiates power uniformly in one plane with a directive pattern shape in a perpendicular plane. This pattern is often described as "donut shaped".

The image of the donut is good (this is a dipole here):

Azimuth of the donuts, the emitter is in the middle.

This shape can be achieved by a typical 1/2-wave dipole. Hum, for wireless, 1/2-wave means around a 6cm dipole.

When increasing the size of your antenna, the azimuth goes smoother. To give an example, imagine that you are living at the 3rd floor of a building. If you've a small omnidirectional antenna, people from the ground floor will be able to catch your signal. If you increase the size of your antenna, people from the ground floor won't be able to capt it anymore (and vice-versa), but it will reach more people horizontally.

Typically, this kind of antenna is cool when wardriving, but for everyday, especially if you stay at the same point (ie at home), it's not very useful. Suppose you fix it on your balcony, do you really care about the backward radiation ? We want then a more directive antenna to maximize the [forward] gain.

Finally, it is nice to notice that an omnidirectional antenna does depend on the wavelength (and obviously on the frequency), thus an antenna used for FM radio (100MHz) will be much bigger than an omnidirectional antenna used for Wifi (2.4 GHz). This also explain why the car antennas were so big few years ago, now electronic tries to replace them, but usually an inadequate antenna requires a damn lot of power... so maybe the solution for cellphone to have a longer battery life is simply to add them an antenna ? Yes, probably, but it wouldn't fit into your pocket anymore.
  • Yagi-Uda antenna

This antenna became famous with the advent of the television. This is widely used for terrestrial transmission. This is already a directional antenna, where the dipoles are working in a synergistic way. Mathematically, it is pretty difficult to understand the exact way it works, but intuitively, the signal is "driven" by the dipoles. The gain of the antenna can be approximated by

GT = 1.66 * N

Where N is the number of dipoles (source: Wikipedia). It also means that the taller is the antenna, the highest is the gain. Again, it is worth to notice that the distance of the dipoles depends on the wavelength (1/10 of the wave length) and thus, the Yagi-Uda antenna is not a universal antenna, ie an antenna working for Wifi won't work for TV for example.

Its [forward] radiation pattern looks like a pear. This antenna is a good trade off between the omnidirectional antenna and the parabolic one: it offers a good gain without having to be too precise when pointing. Moreover, we have essentially a forward radiation pattern, which is what we are looking for when being on a fixed point. Indeed, we try to minimize the backward radiation, which is usually not really desired.

The Yagi antenna is very easy to do - or to buy on eBay :P - and is an antenna of choice when you need something discrete / light, to take with when you go on vacation for example. If you are looking for a maximum gain, this is probably not the one you'd choose.

Finally, in the same family, you have the log-periodic antenna, which is a derivative of the Yagi-Uda. In the log-periodic antenna, each dipole is smaller than the previous (how much smaller do you think ?).

  • Parabolic antenna
This family of antenna is extremely well known due to its use in satellite communication. Moreover, its principle of radiation is easy to understand, because one may do analogy with optical theory. Basically, it works like a lens.

What is cool about parabolic antenna, is that they are universal: one antenna working for Wifi can work for other purpose. Moreover, the gain is directly linearly proportional to the area of your antenna, meaning that the biggest is your antenna, the more gain it will have.

As an analogy, we can see the parabolic antenna as a laser: it is difficult to point on the right place, but when it is pointed right, you get the maximum SNR. Note that the radiation pattern has a sort of unwanted "flower" radiating backward.
The higher the gain, the more peaks you'll have, the thinner will be the forward radiation pattern, the more difficult it will be to point on the desired direction.

Typically, this is the antenna I prefer, when you don't have to move it. It is very difficult to point on the emitter (they are different techniques though), but once you get it, it's just awesome the SNR you get ! Sadly, this kind of antenna has only an efficiency of around 50% :(. Moreover, it is cumbersome: an antenna of 24dbi has a 1 meter diameter... so it's not a good choice when traveling. However, producers try to make them pretty light, since they are not full like the TV parabolas, that's why they have the denomination of grid. It does not impede the signal while tremendously reducing the weight. So you might guess why not doing that for TV sats ? I let you think a bit about that.

Note that with a parabolic antenna, you might introduce the hidden node problem, hence RTS/CTS becomes a need. Moreover, when dealing with big distances, other problems may occur, but we will probably talk about this later on.

There exists many more antennas, like the cantenna (the famous pringle box), the "panel antenna" that can have an efficiency up to 90%, but these are those I had the opportunity to try. Depending on your needs, the choice of the antenna can be very important. For example, if you decide to gather information about your surrounding neighbourgs, you'll choose an omnidirectional antenna, whereas if you are trying to jam a special node, a high gain antenna will do its job.

Antennas are a wild topic, sometimes difficult to express purely mathematically and they usually require deep analysis to exactly now the radiation pattern. There is so much to say about antennas that it cannot be resumes in one post and to fully understand them, it requires a deep understanding in electromagnetism. However, I just wanted to give an insight about how they work, and where they can be used.

Finally, I opted for the parabolic antenna, because I was looking for maximum gain, since I have big distances between houses. Next, we will see how to tweak your repeater to have the best internet possible.