Antennas

Almost everyone uses at least one antenna each day. In fact, the majority of people use antennas for many conveniences in their daily life, whether they realize it or not. Devices such as keyless entry systems, freeway toll passes, satellite TV systems, pagers, cell phones, and wireless networks all require antennas. Very few people who use these antennas can explain how and why they work. Let’s take a brief look at antenna technology, and how antennas relate to our radio frequency networks.

Antennas are merely an extension of a radio transmitter or receiver. As a signal is generated, it is passed from the radio to the antenna to be sent out over the air and received by another antenna, then passed to another radio. The signal that is generated and later transmitted is measured in Hertz (Hz); not the car rental company, but rather a measurement unit of cycles per second. This is better defined as the amount of time it takes a radio wave to complete a full cycle. Imagine that you have a Slinky (a coiled metal spring) on a smooth surface with one end attached to the floor. If you start to move the other end from side to side, you will begin to create waves. These waves represent the radio frequency (RF) energy being sent out over the air. By moving your hand side to side at a slow pace, thus creating longer waves, you are creating a low frequency. If you speed up the movement from side to side, making the waves shorter but more frequent, you are generating a higher frequency. Lower frequencies generally have the ability to travel farther distances, but are more subject to high latency that limits data flow. A higher frequency has a lower (better) latency, but it is limited in distance and penetration of objects such as buildings and other obstructions.

For example, consider your local FM radio station. If they broadcast their signal on frequency 103.5MHz, this translates to 103,500,000 cycles per second. Their signal can be heard all over your city, even inside buildings and houses, with very little interruption. Meanwhile, an AM radio station two states away is broadcasting on 1320KHz, which translates to 1,320,000 cycles per second. With the correct antenna placed outside, you can receive their signal from a longer distance, but with the added difficulty of needing to adjust your antenna.

As you can see, antennas are fundamental components to the transmission of radio frequencies. In many situations, a lower power signal transmitted using a good antenna can arrive at its destination with more accuracy than a high-powered signal transmitted using a poor antenna. Antennas are rated by the amount of gain that they provide. Gain is the increase in power you get by using a directional antenna.

If an antenna’s gain is just specified as dB, check with the manufacturer to see whether the rating is dBi or dBd. If they cannot tell you, or simply do not know, save your money and go someplace else.

A dipole antenna has 2.14dB gain over a 0-dBi isotropic antenna. So if an antenna gain is given in dBd and not dBi, add 2.15 to it to get the dBi value.

As stated above, most antennas are sold with gain measured in dBi, but this is not the only factor to consider when evaluating overall performance. For example, the power input to the antenna plays a major part. Most 802.11b wireless cards transmit 32mW of power. Looking at the conversion chart in Table 1.1, you can see that 32mW (the Pwr column stands or “Power”) is equal to 15dBm. The dBm is calculated by the following:

dBm = 10 log (32mW/1) 

For instance, if you know that a typical card is transmitting 15dBm and you want to use, say, a 3-dBi antenna, you can use the following equation to calculate the Effective Isotropic Radiated Power (EIRP):

15dBm + 3dBi = 18dBm (64mW) EIRP 

The Federal Communication Commission (FCC) currently limits mobile 802.11 stations to 1W or 30dBm EIRP. Fixed stations are given a slight exception to the rule, and are allowed to exceed the 1W limitation. When calculating for fixed stations, they are required to subtract 1dB for every 3dB over 6dBi of antenna gain. The following example demonstrates this for a Linksys WAP11 and a 24-dBi antenna:

20dBm + 24dBi = 44dBm or 25W 
(44dbM – ((24dBi – 6dB)/3)) = EIRP 
(44dBm – (18dBi / 3)) = EIRP 
(44dBm – 6dBi) = EIRP 
EIRP – 38dBm or 6.4W 

In addition to antenna gain and transmitter power, you should also consider the difference in sizes of antennas. Depending on the frequency and type of antenna, there will be a variety of sizes to choose from. The size of the antenna is directly related to the frequency for which it is used. For example, consider a CB radio installed in a car that operates between 26.965MHz (channel 1) and 27.405MHz (channel 40). If you want to have a full wavelength antenna for channel 1, it would need to be 36.491 feet long. This is calculated as follows:

L(in feet) = 984/f(in MHz) 
L = 984/26.965MHz 
L = 36.491 feet 

Now compare that CB antenna to a full wavelength antenna used by a police officer to communicate with his dispatcher on 460.175MHz.

L(in feet) = 984/f(in MHz) 
L = 984 / 460.175 MHz 
L = 2.142 feet 

As you can see, there is a difference of about 34.349 feet between the two antennas. Fortunately for us, wireless 802.11b networks operate in the 2.4GHz or 2400MHz range, thus making the antennas very small.

There are two primary types of antennas that are used on wireless networks— omni-directional and directional. Omni-directional antennas can receive and transmit from all sides (360 degrees). These are useful when covering a large room, or for providing general coverage. Contrary to popular belief, a true omni-directional antenna is not capable of having any gain. Most antennas sold as omni-directional do not send the radio frequency in all directions. The design of the antenna will null the signal on the Y-axis, and concentrate the power across the X-axis.

Directional antennas take the RF energy and concentrate it in a specific direction. This can be compared to a naked light bulb versus a flashlight. The light bulb would be similar to the omni-directional antenna, as it gives off light in all directions equally. In contrast, the flashlight (similar to the directional antenna) focuses the light bulb with the help of a reflector, and concentrates it in a single direction. Directional antennas are helpful when you are creating point-to-point wireless links, or when you are trying to reduce the RF signal “bleed” in a specific location.