Choosing the Right Antenna

As you learned, the strength of your wireless signal is a critical feature of your network. It’s what separates the more expensive equipment from the less expensive equipment.

Requires the use of more powerful equipment and advanced protocols, contributes to your network’s effective range, and determines what lengths you have to go to in order to get the network coverage you require.

In this tutorial, we will look more closely at some of the equipment that you can use to improve the coverage of your network and maximize the potential of the devices that you have deployed—particularly antennas.

Not surprisingly the last thing people consider when purchasing a NIC card or access point is the antenna(s) that come with them. It’s natural to assume that the vendor has supplied you with the best antenna, one that optimizes your device’s performance. This is almost never true.

The fact is that most devices sold into the home and small business market are sold as commodities and the vendors are always trying to shave off a dollar here and a dollar there. Antennas are one place people don’t tend to notice the compromise.

You can greatly improve your range by correctly positioning the antenna, and by replacing the antenna that came with your device with one that provides the characteristics you need. Antennas come in different shapes and sizes and are generally either omnidirectional (radiating in all directions) or directional.

With a directional antenna, you can have your network signal jump from building to building on a campus hundreds of yards away, forming a link in an MLAN (a so-called Metropolitan LAN). For a ranch owner, a directional antenna can be used to control sprinklers and irrigation equipment a thousand yards or more away.

There are other means of improving your range, including adding repeaters and other devices. Repeaters extend your range by amplifying the signal. Other means for extending your wireless network provide strong point-to-point connections with amplification at both ends.

Antenna Principles

Antennas receive transmissions and focus their energy to improve the signal. An antenna to which power is applied also emits an energy transmission and has a profile characteristic of the particular design used in its construction.

The efficiency of an antenna is called its gain, with a rating in decibels (dB) The gain is defined as the ratio of the output signal strength of an amplifier to the input strength of the signal, and it is more usually given in terms of the number of decibels that a hypothetical isotropic radiator (equal in all directions) would have, in units of dBi.

An isotropic radiator is one that radiates signal equally in all directions. Most antennas are made from metallic material, from items as simple as a coffee can to antennas made of exotic rare earth metal alloys.

Antennas offer what is called reciprocal gain—that is, amplification is for both transmission and reception. For two points in a connection, adding an antenna at either end will improve the signal for both devices.

For an antenna that radiates a signal isotropically (with the signal strength the same in all directions), the gain would be 1 or 0dB. Any antenna that has a gain of 2 or 3dB would double the signal strength. Each additional gain of 3dB doubles the signal strength again.

To improve the quality of a transmitter, you would use a higher gain antenna. Be aware, though, that because you are using radio bands there is a legal limit to the strength of the signal you can create.

For the United States the limit is 1000mW, in Japan it is 10mW/MHz, and for Europe it is 100mW (EIRP). EIRP stands for Equivalent Isotropic Radiate Power. The gain of an antenna is often different depending upon whether it is sending or receiving.

So just be sure that whatever the minimum gain is, you can create and maintain a stable communications link over your desired equipment. You can measure the gain in a particular antenna by measuring the signal strength of a connection between two known antenna types.

Carefully position each antenna on both ends so that the maximum signal strength is achieved. After waiting a while to test that the signal level is stable and doesn’t fluctuate, replace one of the antennas with the one you wish to test. There are a number of ways to test signal strength.

Whatever method you use, your measurement of the difference in signal strength will give you your antenna’s gain. For example, if you measure an original signal strength of 48dBi, and then replace a directional antenna with a Yagi antenna, your measurement jumps to 58dBi.

You know you removed a 6dBi antenna, so the measured strength of your new antenna is 16dBi (58–42). It’s particularly important when selecting an antenna to have some idea of how the antenna behaves in a directional sense. A measure of an antenna’s directional properties is the so-called Front to Back (F/B) ratio.

As you might expect the F/B ratio takes the measure of the strength of the beam at its center point and compares it to the amount of energy that is radiated along that line in the opposite direction.

Omnidirectional antennas have F/B ratios approaching 1.0, and for highly directional Yagi antennas the ratio can be on the order of 5 or 6 to 1. Some manufacturers publish their antenna’s radiation pattern—that is, the directions that a signal can cover and the amount of drop-off in signal strength.

That’s particularly valuable when selecting an antenna because it can help you optimize your placement of an antenna. When considering a radiation pattern be aware that you really need to see two profiles.

The vertical slice of the radiation pattern, also called the elevation cut, is generally very different from the horizontal slice of the radiation pattern, called the azimuth.

To really understand an antenna, you need to see this three-dimensional dispersion, but most of the time people tend to simply measure the transmissions in their WLANs when they do the site survey by considering just two-dimensional slices, one for each floor of a building.

When you increase the power of a signal coming from an antenna, you narrow either the elevation plane or the azimuth plane coverage, or both. Furthermore, a radiated signal can be circular, or it can be primarily horizontal or vertical. An antenna doesn’t work alone.

When considering an antenna, you must also consider the nature of the connection, that is, the interaction of both a sending and receiving antenna. The way the orientation of an antenna is measured is as a function of its polarization.

Antennas combine a magnetic field and an electric field that are perpendicular to each other. Some antennas are horizontally polarized, which means that the antenna is supposed to be positioned with its electric field placed parallel with the ground.

A vertically polarized antenna, by contrast, should have its electric field positioned perpendicular to the ground. The important thing to remember is that both antennas in a connection need to be polarized in the same direction.

You’ll also see the beamwidth of an antenna described. This term is expressed as an angle, and it measures the drop-off of a transmission in each of the two main axes. The angle measures the points in the transmission beam where the signal has dropped in half from the maximum signal found on the centerline.

The narrower the beamwidth, the more powerful and longer range the antenna is. However, very narrow beamwidth antennas are also more difficult to adjust to create the best pointto- point link. Antennas are also constructed so that they are tuned for a specific frequency, or they are tunable.

An antenna designed for an 802.11b signal running at 900 MHz won’t work well for a higher frequency 802.11b/g signal running at 2.4 GHz, although the reverse is commonly true. You’ll see many antennas for the 802.11b/g market.

When an antenna is matched to a specific radio device, it is described as having a low standing wave ratio or SWR. The SWR is a measure of the amount of energy that is emitted from the antenna versus the amount of energy that is reflected from the radio back to the antenna.

You can measure the energy using a SWR meter or with a reflectometer. When there is a significant mismatch and a high SWR, you get poor reception. You can increase the power level to improve performance, but there is a point where you risk damaging your equipment.

A better means is to tune the antenna by adjusting its length, the kind and size of its elements, or the reflection path length within the antenna. If this is possible, consult your antenna’s manual or the manufacturer’s Web site to see what they recommend.

some point when you encounter a dead spot, it only takes a few feet of movement to get the signal back. You observe the same effect listening to a radio in your car; rolling forward a car length is enough to get the signal to return.

Multipathing also has the effect of interfering with a signal so that you can encounter a strong signal which is not decipherable. In that case too, moving the antenna’s location or changing the antenna type can eliminate problem and clear up the reception.

Small movements of wireless transmitters and receivers can offer such an improvement in eliminating dead spots that if you have a dead spot in another room, try simply moving your devices and their antennas around a little.

For example, if one bedroom is a dead spot and you have a repeater in another bedroom down a hall, move that repeater to a position where the signal can have direct lineof- sight down the hall to the other room.

When you deploy an antenna outdoors, the loss in the signal over distances is described by a standard function called the free space path loss equation. It’s predictable and varies as a square of the distance; see for example at

Indoors signal loss is much more complex and dependant upon the material that you’re transmitting the signal through. There are a rough list of loss in dB for several materials, but here’s one experiment that you can try that graphically illustrates loss of transmission.

If you have an aquarium in your home, move your access point in back of it and measure the signal that is successfully transmitted through the tank. Water is an amazingly good signal absorber, so just the one obstruction is enough to dramatically reduce your signal.