Hello. In this video we’re going to discuss antennas. Some Mist access point models have the option for external antenna connections instead of the integrated omni antenna, and this allows us to connect a variety of antennas to the access points when we want to focus the RF energy in a particular direction.
Now, the number one rule of using antennas is to cover what you want while not covering what you don’t want. Let’s consider this light post analogy below. The first light post just has a light in the center of a clear glass bowl. In this example, the light goes everywhere, including up into the sky, which produces a lot of unnecessary light pollution, and a lot of the light’s energy is wasted in places where we’re not trying to light. The second example has a reflective half-dome on the light, focusing all the light down onto the street where it is wanted, providing a large coverage area. And the final lamppost also has a reflective dome, but this time with only a small opening at the bottom, keeping the light focused just on the area directly below the lamppost. So when considering which lampposts to deploy down my street, which one would I choose? Well, the first one is probably not the right solution because of all the light pollution, and a lot of the light’s energy is wasted. And while the third option works well when close to the lamppost, the cost of light in a full street would probably be prohibitive. So option two would be the most logical choice, providing light where we want it and not where we don’t. And you see, we need to consider the same thing when we’re choosing a wifi antenna.
Now, one of the things to understand about antennas is that they add gain to our RF signals. They do this because they focus the energy in one particular direction. Now, if you could make an antenna which radiated power equally in all directions in a perfect sphere, you would’ve created something which we call the isotropic radiator, and it would add no gain at all to the RF signal. But in reality, it’s not possible to build an antenna which has this perfect theoretical radiation pattern. You see, all antennas actually focus the RF signal in some direction.
Even when we consider an omnidirectional antennas, they do not radiate power equally in all directions in a perfect sphere. The coverage pattern is more like a donut. Now, imagine a ring donut with an omni antenna sticking up through the hole in the middle of the donut. That’s what the coverage pattern is like of an omnidirectional antenna. And then we get directional antennas, which focus the RF energy in one particular direction away from the antenna. Now, when you were a child, I wondered if you ever did the experiment with a magnifying glass and a piece of paper on a hot summer’s day, when you use a magnifying glass to focus the sun’s energy onto one spot on the paper, causing the paper to burn. All the sun’s energy that hits the surface of the magnifying glass is focused onto one very small spot on the paper, and that causes enough energy to set the paper on fire. And it’s the same principle is true of antennas. They focus the RF energy they receive from the transmitter in a particular direction, and the process of doing it adds gain to the signal.
Now, when we want to visualize the coverage patterns of antennas, most antenna data sheets will provide what we call a polar coordination chart to show us the antenna pattern. The diagram shows how directional the antenna is and also shows us a presence of any side or back lobes. There are two main types of charts we see for antennas. One is called the elevation. An elevation chart shows a side-on view of the antenna RF radiation pattern. And the other is an Azimuth chart, which is the RF radiation pattern when viewed from above the antenna.If we look at the ones on this slide here, they’re showing the radiation patterns from an omnidirectional antenna, and you can see that the elevation pattern is like a cross section of our donut shape, whereas the Azimuth pattern shows a top-down view, where we can see the full 360 degree coverage of this omnidirectional antenna.
Now, we get all sorts of different types of omnidirectional antennas. The typical one that I’m sure most people are used to seeing is what we would call our dipole antenna or rubber duck antenna, looking very similar to the one I’m holding here. This is typically the sort of antenna you might see on access points if they don’t have integrated antennas. But we get different types of on the omnidirectional antennas. We get ones that look quite different.
This is another omnidirectional antenna. It’s a ceiling mount omni, and it’s got what we would call a downtilt omni antenna in it. What that means is the donut is focused down into the room, so it’s got a reflector behind it, which focuses that donut down into the room. When we look at the integrated antennas that are built into Mist access points, they are also downtilt omnis. They’re designed to be ceiling-mounted with the donut being pushed down into the room.
We also get high gain omnidirectional antennas as well. A high gain omnidirectional antenna, the higher the gain, the more squashed our donut is going to be, as you can see in the antenna charts on this slide. High gain omnidirectional antennas also tend to be bigger than lower gain antennas. Here’s a slightly bigger and taller five gig omnidirectional antenna, which has a higher gain to it.
Now, it’s important to understand that as we change antennas, we’re changing the coverage pattern. So by just putting a higher gain omni, while you’re adding more gain to the signal, you’re also changing where the coverage goes. A friend of mine once wanted to get more signal in his house from his home wifi router, and while he was in an electronic store, he found something which they called a range extender antenna, and it was a high gain omnidirectional antenna. And it was a fairly long one. It was probably about this sort of length. So he brought his range extending antenna, and he took it home, and took off the little omni antenna that was connected to his home wifi router, and added on this big, tall, high gain omni.
Now, the reason he wanted to do this is he lived in an old Victorian house, and in the UK, old Victorian houses tend to be long and thin. So you’ve got like a living room, a sitting room, a dining room, a kitchen. And then right behind his kitchen, he had a utility room, and that utility room had become an office office. And the problem was his wifi router came into the living room right at the front of his house, and he couldn’t get the signal all the way through his house into the utility room. So when he swapped for this high gain antenna, it squashed the donut and caused more signal to go left and right, and it gave him enough signal to now pick it up in his utility room, exactly what he wanted.
The only problem was directly above the living room was his bedroom, and he all of a sudden didn’t have any signal in his bedroom anymore. That’s because we’re squashing that donut pattern, and therefore we get less… We might get more coverage and more power going out from the antenna, but directly above and below, you have less coverage, because its donut is more squashed. So just remember that as we change antennas, we’re also going to change what we’re covering and where we’re getting coverage.
Let’s next look at some directional antennas. Now, directional antennas tend to focus the RF energy from an access point in one particular direction from the antenna both horizontally and vertically. A common type of directional antenna using wifi systems is the patch antenna, and patch antennas come different shapes and sizes. Generally the size of the antenna tells you how much gain it has. So a small patch antenna, which we have here… This is only a four dB gain antenna. And then we’ve got a slightly bigger one here, which has up to six dB gain. And again, as we go bigger still, a bigger patch antenna, we’ve got one which is 8.5. And it’s all to do with how many active elements it has, and the more it has, the higher the gain of the signal.
A certain type of patch antenna I just want to show you, which has been specifically designed for warehouse environments. It’s sometimes referred to as a battleax antenna due to what its coverage patterns look like, and that’s because it has a very wide antenna pattern in one orientation, 120 degrees, but only 15 degrees in the other. So this is an example of that battleax or warehouse antenna from AccelTech. It’s designed to be mounted in high racking areas pointing straight down, and it’s got 180 degrees, so it can cover the whole aisle while not spilling into the other aisles with its thin 15 degree coverage area. So this is one of those from AccelTech. Ventev also do an antenna very similar. It’s a little bit bigger, but this is again, the warehouse antenna, similar sort of concept. Mount it above a high rack area, and it will cover an entire warehouse aisle. Now, when we get to our warehouse design videos, I will be showing you the coverage from these antennas and how you might use one for designing high rack areas.
Next, we come to what I would class as highly directional antennas. These tend to have very narrow beam widths in both the horizontal and vertical, and they’re mainly going to be used for either wifi point-to-point links or if you have matched links which need to go over a long distance. Now, highly directional antennas come in many different shapes, sizes, and flavors. For example, one common type of directional antenna is a Yagi antenna that looks a little bit like this. Yagi antennas have an active element at the back and have these little directors that go along a metal bar. The longer the Yagi antenna, the higher the gain. It’s basically the more directors it has, the more gain it adds.
We also get what we call dish antennas. If you’ve got a very long link where you want to go a considerable distance, you might put a dish antenna at both sides of the link. This dish antenna here is 24 dB gain, with the active element in the center, and the dish which reflects a signal onto the active element. The bigger the dish, the higher gain the antenna is. I also have a high gain two foot square antenna here. Let me lift this one up. You can see this here. This is actually a 28 dB gain directional antenna with a 4.5 degree both horizontal and vertical beam width.
Well, I hope that that gives you a flavor for some of the different types of antennas you might use with a Mist access point. Thank you for watching, and goodbye.