This article is due in part to a recent experience I had with a new client. Needless to say, he had some erroneous notions about how Radio Frequency (RF) propagation worked.
I'll admit, it's hard to visualize how electromagnetic radiation acts when it exhibits both wave-like and particle-like properties, and propagates in three dimensions. Still, understanding the fundamentals is simpler than most people think.
As funny as it sounds, all it takes is a balloon. Many years ago, while studying for my first amateur radio license, one of my mentors handed me a balloon, telling me to inflate it. Ignoring the quizzical look of a sixteen year old, he then proceeded to explain RF propagation.Isotropic radiator
Understanding RF radiation begins with the concept of an isotropic radiator. The Sun and the results of the Big Bang Theory are both real-world examples of isotropic radiators.
When it comes to antennas, an isotropic radiator is theoretical. So, imagine a RF source emitting electromagnetic radiation in three dimensions with equal intensity and 100 percent efficient. At first, I didn't get it. My mentor told me the spherical shape of the balloon represents an isotropic radiator, with the balloon's surface being where RF radiation stops. OK, that works.dBi
One more theoretical construct needs to be understood. That is Decibels (isotropic) or dBi. It is the gain in radiation intensity of an antenna when compared to the isotropic radiator. Here is the tricky part. Think of my balloon again and imagine that the air inside is RF energy. There is a set amount, so how does an antenna achieve gain? It does by concentrating the RF energy. Looking at some real-world antennas will help explain.The ubiquitous dipole antenna
The 15 cm long vertical element you see on most Wi-Fi equipment is actually a dipole antenna. It consists of two elements and is popular because of its omnidirectional radiation pattern. It has approximately two dBi gain over the isotropic radiator. Let's see why that is.
The dipole schematic (courtesy of Wikipedia) hints at how the gain is achieved. Let's take my balloon once again and squeeze it in the center as shown in the image. The air/energy inside the balloon is forced into a different shape. This shape is representative of the dipole antenna radiation pattern.
Notice the balloon is longer, that would be considered gain in directions perpendicular to the axis of the antenna. The amount of energy is the same, it's just being redirected. The following antenna radiation diagram is of a typical dipole.
Looking at antenna specifications, you may have noticed that similar antenna styles have different dBi ratings. Ever wonder how that works?
Back to the balloon; it's still simulating a dipole antenna, but let's squeeze the balloon between two pieces of cardboard. Notice how it gets longer, that's additional gain. Did you also notice there is less vertical coverage? The next radiation diagram shows what's going on.
This is how my client started getting into trouble. His Wi-Fi router was in the center of the building and offices along the outer walls were not connecting. He bought two antennas similar to the one shown here and was pleased as the offices with weak reception were now connecting.
Guess what, computers on the second floor right above the router lost access to the network. That's because dipole antennas achieve gain by squishing the radiation pattern along the axis of the antenna. They are still omnidirectional, but only in the space perpendicular to the axis of the antenna as shown in the radiation diagrams. So, does he need more gain?Directional antennas
As you can guess, to increase gain, direction antennas further restrict the radiation pattern. In fact, the pattern is no longer omnidirectional along either axis of the antenna. Notice in the radiation diagram below, there is only one chart. That's because the pattern is the same for elevation and azimuth.
I bet you're wondering if I was going to get back to my client. He thought he still needed more power. So on the recommendation of a salesperson, he bought two panel (directional) antennas. Their logic was the building would be fully covered by pointing the antennas in opposite directions.
Can you figure out what problems this caused? Now additional people on the second floor were complaining of lost connections. We know that's because directional antennas radiate less in the vertical plane when compared to dipole antennas. But, why were people on the first floor complaining of slow connections?MIMO and Multipath
I was remiss in not mentioning the client's network was built with 802.11n equipment. I'll bet you can see where this is going. By using directional antennas, my client lost two of the best features of 802.11n, MIMO and multipath propagation.
802.11n leverages something called multipath interference. It does this by using multiple signal streams and conditioning the disparate feeds into a stronger more reliable signal at the receiver. That's why some of my client's employees whose computers had RF Non-Line of Sight to the Wi-Fi router were able to make a connection. But, that ability goes away when directional antennas are used.Final thoughts
Setting up Wi-Fi networks so they meet expectations can be challenging. Knowing a bit about RF propagation and how antennas work should be one of the first steps, not an after thought.
For a well-written explanation of basic antenna principals and radiation diagrams, check out Dr. Trevor Marshall's article Antennas enhance WLAN security. I also want to thank the fellow hams at Force 12, Inc for allowing me to use their balloon pictures.
Michael Kassner is currently a systems manager for an international company. Together with his son, he runs MKassner Net, a small IT publication consultancy.