Antennas

By Joshua Erdman
Digital Foundation, inc.

Between all those wireless protocols and transmitter specifications (sensitivity, watts, network capabilities, etc) it is already difficult enough when choosing a transmitter, but you still need to pick out the right antenna. You have come to the right place.

What Antennas Do

Antennas do not give your transmitter more power. They are directional devices that channel the energy that your transmitter produces into a directional pattern; at the same time, using that same directional pattern, they also can recieve a better signal from other transmitters. The best example I can think of is the plastic cone megaphones that cheerleaders use. These work great to project your voice in the direction in which you point the megaphone, these megaphones also work great for hearing things by capturing more sound.

Polarity

Polarity is difficult to explain. The best way to understand it is to consider two rod antennas. If you were to stand a rod antenna up vertically and transmit a signal, this signal would be considered vertically polarized. The frequency wave is radiating outward from this antenna similar to a wave in a pool after you drop in a pebble. For the recieving antenna to best pick up this signal, the recieving antenna should also be vertically polarized. That way the radiating signal (the wave) will traverse the whole length of the antenna. Just by changing the polarity of one of the antennas will drastically effect the quality and strength of the signal recieved.

Typically almost all antenna types have a polarity. If you are setting up a point-to-point wireless backbone and are running into intereference, simply change the polarity of both antennas. Provided that there are no other neighboring networks using your new polarity, your interference should be virtually gone.

Fresnel Zone

The Fresnel Zone is pretty much like a football shaped bubble between your antennas. Keep in mind is that anything in your Fresnel Zone (even if it is a tree that you can totally see through) could cause major interference. Notice I said football shaped, so a tree that is not directly in between the antennas but even slightly to the side is still in the Fresel Zone.

The Fresnel zone is important to consider because most wireless applications are transmitting in the microwave band. Just like the microwave in your kitchen, it is water that responds to microwaves. So water (or anything containing water) usually has an effect when in your Fresnel Zone - including trees, clouds, and rain.

Antenna Types

With wireless networking you have 4 basic antenna types:

Parabolic - usually dish shaped out of a grid of metal
Rod - looks like a traditional antenna, varies in length from approximately 1' to 4'
Sectorized - approximately 3' in height, may have small pnels that run along its height
Panel - the Antenna elements are hiding inside a 1" thick plastic panel

Parabolic Antennas

The antennas fit perfect with the plastic megaphone analogy, they focus the signal in the direction they are pointed. The higher dB parabloic antennas focus in a tighter 'beam'. Like a telescope that you use for star gazing, they also focus the signals recieved.

Application: Point-to-Point - usually for a wireless backbone between 2 networks (or buildings). These antennas provide a tight beam making it more impervious to interference and also makes it more difficult for people to stumble upon this wireless network and attemt to sniff the traffic data.

Rod Antennas

Radiate 360 degrees around. So if your antenna rod is mounted virtically (this is vertically polarized) you are transmitting in 360 degrees to the horizon. The transmission is focused along the horizontal. The shape can be explaned better if you took 2 cones and put the tips together and imagine the rod sanding upright in the middle. See the pitiful picture below.

     rod
      |

    \ | /
     \|/    - radiation spread
     /|\
    / | \

The longer the rod, the tighter the 360 degree beam; which sends a stonger signal to the horizon but much less to recievers that might be vertically below or above the antenna.

With a rod antenna you are stuck with radiating all 360 degrees. But what if you need more bandwidth for this zone. With a sectorized antenna you can cut that 360 degrees into quarters and have one transmitter and sectorized antenna for North, South, East, and West.

Sectorized Antennas

Very similar to a rod antenna but it allows the installer to vary the horizonal radiating angle.

Panel Antennas

These are great for radiating an area but keeping the antenna as stealth as possible. Panel antennas have a difficult to explain radiation pattern. The signal is more focused in front of and behind the flat surface more than it is on the sides. You can imagine it like the shape of the number 8 with the panel being located in the middle where the 2 circles of the number 8 intersect.

Application: Rod, Sectorized and Panel antennas are typically used for Point-to-Multipoint and Distributed wireless networks. Typically in a small office environment you would use a rod antenna (the cheap access points from D-link, Linksys and NetGear already have very small rod antennas built in).

Article last reviewed: 06/08/2004

Created by: Digital Foundation, inc.

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