Wireless LAN security

One issue with corporate wireless networks in general, and WLANs in particular, involves the need for security. Many early access points could not discern whether or not a particular user had authorization to access the network. Although this problem reflects issues that have long troubled many types of wired networks (it has been possible in the past for individuals to plug computers into randomly available Ethernet jacks and get access to a local network), this did not usually pose a significant problem, since many organizations had reasonably good physical security. However, the fact that radio signals bleed outside of buildings and across property lines makes physical security largely irrelevant to Piggybackers. Such corporate issues are covered in wireless security.

Concerns

Anyone within the geographical network range of an open, unencrypted wireless network can 'sniff' or record the traffic, gain unauthorized access to internal network resources as well as to the internet, and then possibly sending spam or doing other illegal actions using the wireless network's IP address, all of which are rare for home routers but may be significant concerns for office networks.

If router security is not activated or if the owner deactivates it for convenience, it creates a free hotspot. Since most 21st century laptop PCs have wireless networking built in (cf. Intel 'Centrino' technology), they don't need a third-party adapter such as a PCMCIA Card or USB dongle. Built in wireless networking might be enabled by default, without the owner realizing it, thus broadcasting the laptop's accessibility to any computer nearby.

Modern operating systems such as Mac OS, or Microsoft Windows make it fairly easy to set up a PC as a wireless LAN 'base station' using Internet Connection Sharing, thus allowing all the PCs in the home to access the Internet via the 'base' PC. However, lack of knowledge about the security issues in setting up such systems often means that someone nearby may also use the connection. Such "piggybacking" is usually achieved without the wireless network operators knowledge; it may even be without the knowledge of the intruding user if their computer automatically selects a nearby unsecured wireless network to use as an access point.

Security options

There are three principal ways to secure a wireless network.

* For closed networks (like home users and organizations) the most common way is to configure access restrictions in the access points. Those restrictions may include encryption and checks on MAC address. Another option is to disable ESSID broadcasting, making the access point difficult for outsiders to detect. Wireless Intrusion Prevention Systems can be used to provide wireless LAN security in this network model.
* For commercial providers, hotspots, and large organizations, the preferred solution is often to have an open and unencrypted, but completely isolated wireless network. The users will at first have no access to the Internet nor to any local network resources. Commercial providers usually forward all web traffic to a captive portal which provides for payment and/or authorization. Another solution is to require the users to connect securely to a privileged network using VPN.
* Wireless networks are less secure than wired ones; in many offices intruders can easily visit and hook up their own computer to the wired network without problems, gaining access to the network, and it's also often possible for remote intruders to gain access to the network through backdoors like Back Orifice. One general solution may be end-to-end encryption, with independent authentication on all resources that shouldn't be available to the public.

Access Control at the Access Point level

One of the simplest techniques is to only allow access from known, approved MAC addresses. However, this approach gives no security against sniffing, and client devices can easily spoof MAC addresses, leading to the need for more advanced security measures.

Another very simple technique is to have a secret ESSID (id/name of the wireless network), though anyone who studies the method will be able to sniff the ESSID.

Today all (or almost all) access points incorporate Wired Equivalent Privacy (WEP) encryption and most wireless routers are sold with WEP turned on. However, security analysts have criticized WEP's inadequacies, and the U.S. FBI has demonstrated the ability to break WEP protection in only three minutes using tools available to the general public (see aircrack).

The Wi-Fi Protected Access (WPA and WPA2) security protocols were later created to address these problems. If a weak password, such as a dictionary word or short character string is used, WPA and WPA2 can be cracked. Using a long enough random password (e.g. 14 random letters) or passphrase (e.g. 5 randomly chosen words) makes pre-shared key WPA virtually uncrackable. The second generation of the WPA security protocol (WPA2) is based on the final IEEE 802.11i amendment to the 802.11 standard and is eligible for FIPS 140-2 compliance. With all those encryption schemes, any client in the network that knows the keys can read all the traffic.

Restricted access networks

Solutions include a newer system for authentication, IEEE 802.1x, that promises to enhance security on both wired and wireless networks. Wireless access points that incorporate technologies like these often also have routers built in, thus becoming wireless gateways.

End-to-End encryption

One can argue that both layer 2 and layer 3 encryption methods are not good enough for protecting valuable data like passwords and personal emails. Those technologies add encryption only to parts of the communication path, still allowing people to spy on the traffic if they have gained access to the wired network somehow. The solution may be encryption and authorization in the application layer, using technologies like SSL, SSH, GnuPG, PGP and similar.

The disadvantage with the end to end method is, it may fail to cover all traffic. With encryption on the router level or VPN, a single switch encrypts all traffic, even UDP and DNS lookups. With end-to-end encryption on the other hand, each service to be secured must have its encryption "turned on," and often every connection must also be "turned on" separately. For sending emails, every recipient must support the encryption method, and must exchange keys correctly. For Web, not all web sites offer https, and even if they do, the browser sends out IP addresses in clear text.

The most prized resource is often access to Internet. An office LAN owner seeking to restrict such access will face the non trivial enforcement task of having each user authenticate himself for the router.

Open Access Points

Today, there is almost full wireless network coverage in many urban areas - the infrastructure for the wireless community network (which some consider to be the future of the internet) is already in place. One could roam around and always be connected to Internet if the nodes were open to the public, but due to security concerns, most nodes are encrypted and the users don't know how to disable encryption. Many people consider it proper etiquette to leave access points open to the public, allowing free access to Internet. Others think the default encryption provides substantial protection at small inconvenience, against dangers of open access that they fear may be substantial even on a home DSL router.

The density of access points can even be a problem - there are a limited number of channels available, and they partly overlap. Each channel can handle multiple networks, but places with many private wireless networks (for example, apartment complexes), the limited number of Wi-Fi radio channels might cause slowness and other problems.

According to the advocates of Open Access Points, it shouldn't involve any significant risks to open up wireless networks for the public:

* The wireless network is after all confined to a small geographical area. A computer connected to the Internet and having improper configurations or other security problems can be exploited by anyone from anywhere in the world, while only clients in a small geographical range can exploit an open wireless access point. Thus the exposure is low with an open wireless access point, and the risks with having an open wireless network are small. However, one should be aware that an open wireless router will give access to the local network, often including access to file shares and printers.
* The only way to keep communication truly secure is to use end-to-end encryption. For example, when accessing an internet bank, one would almost always use strong encryption from the web browser and all the way to the bank - thus it shouldn't be risky to do banking over an unencrypted wireless network. The argument is that anyone can sniff the traffic applies to wired networks too, where system administrators and possible crackers have access to the links and can read the traffic. Also, anyone knowing the keys for an encrypted wireless network can gain access to the data being transferred over the network.
* If services like file shares, access to printers etc. are available on the local net, it is advisable to have authentication (i.e. by password) for accessing it (one should never assume that the private network is not accessible from the outside). Correctly set up, it should be safe to allow access to the local network to outsiders.
* With the most popular encryption algorithms today, a sniffer will usually be able to compute the network key in a few minutes.
* It is very common to pay a fixed monthly fee for the Internet connection, and not for the traffic - thus extra traffic will not hurt.
* Where Internet connections are plentiful and cheap, freeloaders will seldom be a prominent nuisance.

On the other hand, in some countries including Germany, persons providing an open access point may be made (partially) liable for any illegal activity conducted via this access point.

Wireless community network

Wireless community networks or wireless community projects are the organizations that attempt to take a grassroots approach to providing a viable alternative to municipal wireless networks for consumers.

Because of evolving technology and locales, there are at least four different types of solution:

* Cluster: Advocacy groups which simply encourage sharing of unmetered internet bandwidth via Wi-Fi, may also index nodes, suggest uniform SSID (for low-quality roaming), supply equipment, dns services, etc.
* Mesh: Technology groups which coordinate building a mesh network to provide Wi-Fi access to the internet
* WISP: A mesh that forwards all traffic back to consolidated link aggregation point(s) that have centralized access to the internet
* WUG: A wireless user group run by wireless enthusiasts. An open network not used for the reselling of internet. Running a combination of various off the shelf WIFI hardware running in the license free ISM bands 2.4 GHz/5.8 GHz

Certain countries regulate the selling of internet access, requiring a license to sell internet access over a wireless network. In South Africa it is regulated by ICASA. They require that WISP's apply for a VANS or ECNS/ECS license before being allowed to resell internet access over a wireless link. The cluster and mesh approaches are more common but rely primarily on the sharing of unmetered residential and business DSL and cable Internet. This sort of usage might be non-compliant with the Terms of Service (ToS) of the typical local providers that deliver their service via the consumer phone and cable duopoly. Wireless community network sometimes advocate complete freedom from censorship, and this position may be at odds with the Acceptable Use Policies of some commercial services used. Some ISPs do allow sharing or reselling of bandwidth.

History

These projects are in many senses an evolution of amateur radio, and more specifically packet radio, as well as an outgrowth of the free software community (which in itself substantially overlaps with amateur radio). The key to using standard wireless networking devices designed for short-range use for multi-kilometre Long Range Wi-Fi linkups is the use of high-gain directional antennas. Rather than purchasing commercially available units, such groups sometimes advocate homebuilt antenna construction. Examples include the cantenna, which is typically constructed from a Pringles potato chip can, and RONJA, an optical link that can be made from a smoke flue and LEDs, with circuitry and instructions released under the GFDL. As with other wireless mesh networks, three distinct generations of mesh networks are used in wireless community networks. In particular, in the 2004 timeframe, some mesh projects suffered poor performance when scaled up.

Organization

Organizationally, a wireless community network requires either a set of affordable commercial technical solutions or a critical mass of hobbyists willing to tinker to maintain operations. Mesh networks require that a high level of community participation and commitment be maintained for the network to be viable. The mesh approach currently requires uniform equipment. One market-driven aspect of the mesh approach is that users who receive a weak mesh signal can often convert it to a strong signal by obtaining and operating a repeater node, thus extending the mesh network.

Such volunteer organizations focusing in technology that is rapidly advancing sometimes have schisms and mergers. The Wi-Fi service provided by such groups is usually free and without the stigma of piggybacking (internet access). An alternative to the voluntary model is to use a co-operative structure.

Wireless network

Wireless network refers to any type of computer network that is wireless, and is commonly associated with a telecommunications network whose interconnections between nodes are implemented without the use of wires.Wireless telecommunications networks are generally implemented with some type of remote information transmission system that uses electromagnetic waves, such as radio waves, for the carrier and this implementation usually takes place at the physical level or "layer" of the network.

Types of wireless connections

Wireless PAN

Wireless Personal Area Networks (WPANs) interconnect devices within a relatively small area, generally within reach of a person. For example, Bluetooth provides a WPAN for interconnecting a headset to a laptop. ZigBee also supports WPAN applications. Wi-Fi PANs are also getting popular as vendors have started integrating Wi-Fi in variety of consumer electronic devices. Intel My WiFi and Windows 7 virtual Wi-Fi capabilities have made Wi-Fi PANs simpler and easier to set up and configure.

Wireless LAN

* Wi-Fi: Wi-Fi is increasingly used as a synonym for 802.11 WLANs, although it is technically a
certification of interoperability between 802.11 devices.
* Fixed Wireless Data: This implements point to point links between computers or networks at
two locations, often using dedicated microwave or laser beams over line of sight paths. It is
often used in cities to connect networks in two or more buildings without physically wiring the
buildings together.

Wireless MAN

Wireless Metropolitan area networks are a type of wireless network that connects several Wireless LANs.

* WiMAX is the term used to refer to wireless MANs and is covered in IEEE 802.16d/802.16e.

Wireless WAN

Wireless Wide Area Networks are wireless networks that typically cover large outdoor areas. These networks can be used to connect branch offices of business or as a public internet access system. They are usually deployed on the 2.4 GHz band. A typical system is as per the one deployed by Gaiacom Wireless Networks contains base station gateways, access points and wireless bridging relays. Other configurations are mesh systems where each access point acts as a relay also. When combined with renewable energy systems such as photo-voltaic solar panels or wind systems they can be stand alone systems.

Mobile devices networks

With the development of smart phones, cellular telephone networks routinely carry data in addition to telephone conversations:

* Global System for Mobile Communications (GSM): The GSM network is divided into three major systems: the switching system, the base station system, and the operation and support system. The cell phone connects to the base system station which then connects to the operation and support station; it then connects to the switching station where the call is transferred to where it needs to go. GSM is the most common standard and is used for a majority of cell phones.

* Personal Communications Service (PCS): PCS is a radio band that can be used by mobile phones in North America and South Asia. Sprint happened to be the first service to set up a PCS.

* D-AMPS: Digital Advanced Mobile Phone Service, an upgraded version of AMPS, is being phased out due to advancement in technology. The newer GSM networks are replacing the older system. See also mobile telecommunications.

Uses

Wireless networks have had a significant impact on the world as far back as World War II. Through the use of wireless networks, information could be sent overseas or behind enemy lines easily, efficiently and more reliably. Since then, wireless networks have continued to develop and their uses have grown significantly. Cellular phones are part of huge wireless network systems. People use these phones daily to communicate with one another. Sending information overseas is possible through wireless network systems using satellites and other signals to communicate across the world. Emergency services such as the police department utilize wireless networks to communicate important information quickly. People and businesses use wireless networks to send and share data quickly whether it be in a small office building or across the world.

Another important use for wireless networks is as an inexpensive and rapid way to be connected to the Internet in countries and regions where the telecom infrastructure is poor or there is a lack of resources, as in most developing countries.

Compatibility issues also arise when dealing with wireless networks. Different components not made by the same company may not work together, or might require extra work to fix these issues. Wireless networks are typically slower than those that are directly connected through an Ethernet cable.

A wireless network is more vulnerable, because anyone can try to break into a network broadcasting a signal. Many networks offer WEP - Wired Equivalent Privacy - security systems which have been found to be vulnerable to intrusion. Though WEP does block some intruders, the security problems have caused some businesses to stick with wired networks until security can be improved. Another type of security for wireless networks is WPA - Wi-Fi Protected Access. WPA provides more security to wireless networks than a WEP security set up. The use of firewalls will help with security breaches which can help to fix security problems in some wireless networks that are more vulnerable.

Environmental concerns and health hazard

In recent times, there have been increased concerns about the safety of wireless communications, despite little evidence of health risks so far. The president of Lakehead University refused to agree to installation of a wireless network citing a California Public Utilities Commission study which said that the possible risk of tumors and other diseases due to exposure to electromagnetic fields (EMFs) needs to be further investigated.

The History of Wireless Networking

The history of wireless networks and of wireless networking goes hand in hand. Without the discovery of technology such as the radio, wireless technology would not exist at all today. The history of wireless networking goes as far back as the 1800's with the advent of radio waves. The advent of more technology grew throughout the years and expanded to what we communicated with today.

In 1888, a Hamburg, Germany born physicist named Heinrich Rudolf Herz produced his first radio wave ever. By 1894 this radio wave production became a way of communication. Telegraph wires were used to receive the radio waves in signal form. Herz opened the way for radio, television, and radar with his discovery of electromagnetic waves. An Italian inventor named Marchese Guglielmo Marconi then expanded the radius of radio wave sending to two miles, becoming the "father of the radio." By 1899, this form of telecommunication could travel pretty far for its time. Marconi could send a signal 9 miles across the Bristol Channel. He eventually expanded the radius to 31 miles across the English Channel to France. By 1901 the communication area became immense. Marconi could send signals across the entire Atlantic Ocean.

World War II became a big stepping stone for the radio wave. The United States was the first party to use radio waves for data transmission during the war. This use of radio waves could have quite possibly won the war for the Americans. The use of radio wave data communication lead to a lot of speculation to whether the radio signals could be expanded into something bigger than it currently was. In 1971, a group of researchers under the lead of Norman Abramson, at the University of Hawaii, created the first "packet-switched" radio communications network entitled "Alohanet." Alohanet was the first wireless local area network, otherwise known as a WLAN. The first WLAN was not much, but it was a large discovery. The Alohanet WLAN was comprised of seven computers that communicated to each other. In 1972, Alohanet connected with the WLAN system Arpanet on the mainland. This length of connect was ground breaking in telecommunications between computers.

The first types of WLAN technology used an interface in which became over crowded for communication. Small appliances and industrial machinery caused interference so the technology had to be updated. The second type of WLAN technology to be released ended up being four times faster than its predecessor at 2Mbps per second. We use the third format of WLAN today, though our current WLAN system runs at the same speed as the second system released.

In 1990, the 802.11 Working Group was established to work towards a WLAN standard for all computers to communicate from. In 1997, IEEE 802.11 was accepted as the standard data communication format for wireless local area networks. The technology continues to grow today. Governments and large corporations are constantly looking out for the latest and fastest standard to work from.

The expansion of wireless networking will likely continue for decades to come.

What is the best WiFi antenna for me? ( 2 )

Home

Home antennas are always the easiest types of antennas to purchase and take the least amount of effort in choosing and installing. In most circumstances, only one antenna is needed on the remote computer. We recommend putting any external antenna on the remote computer, simply because if you install it on your router and don’t plan on setting up security, it will provide less signal strength outside of your home and your system will be less prone to hackers. If you have a multi-story home or a very large house, you may have to install antennas on every computer to get the range or bandwidth required. Every wall that you have to penetrate will decrease the signal strength of your system. For the best signal strength and signal, we recommend installing a 5dB ceiling dome antenna and either wireless desktop antennas or RL-1000 antennas on all remote computers. It is best to start with 1 antenna on a remote computer and test the signal strength and range.

Office

Office antennas are pretty straight forward. If you want to run a network system inside of your office building and don’t want to run cables all over the place, first, purchase a good wireless card, install a Ceiling Dome, Desktop or Wibberduck antenna to extend and maximize the signal to your office router. It’s that simple. However, this can get a little complex if the office is split between 2 different points or if the office is really large or on multiple stories of a building.

Mobile WiFi antennas

Why would anyone want WiFi in their car? Well, there are a lot of truck-stops and RV parks around the country now that offer wireless access. In fact, many public high speed wireless networks can be accessed directly from your car, truck, or RV. There is also something called WarDriving which is where bad people drive around neighborhoods and get their high-speed access for free.

Yagi Antennas

Yagi antennas were the design of two Japanese people, Hidetsugu Yagi and Shintaro Uda, and are sometimes referred to as Yagi-Uda antennas. They were originally designed for radio, but are now also used for 802.11 systems. These antennas are typically very directional and are used for point to point, or to extend the range of a point to multi-point system. We highly recommend using the RadioLabs 14 or 16 element weatherproof Yagi antenna if you want to install your system outside. They have excellent signal strength and in the right circumstances can communicate for miles!

Backfire antennas - The backfire is a small directional antenna with excellent gain. They look similar to a parabolic dish, but the gain isn't as high. We highly recommend Backfire antennas for point to point or point to multipoint systems because of the excellent gain and the good noise figures. We offer a backfire antenna with 15 dBi of Gain!! This is excellent considering the antenna is only 10 inches diameter. Almost invisible!!

Parabolic or dish antennas

This is where the real power is! Parabolic dish antennas put out tremendous gain but are a little hard to point and make a connection with. As the gain of an antenna increases, the antenna’s radiation pattern decreases until you have a very little window to point or aim your dish correctly. Dish antennas are almost always used for a point to point system for long haul systems. The Parabolic Dish antennas work by focusing the power to a central point and beaming the radio’s signal to a specific area, kind of like the adjustable reflector on a flashlight. These antennas are highly focused and are the perfect tool if you want to send your signal a very long distance.

Gain Considerations

The gain you will require for each individual WiFi antenna system will dependant on any direct objects in your path, the distance you must cover and the individual wifi cards. These all must be taken into consideration before choosing the proper antenna system. If our calculator is too difficult to use, please feel free to contact us for information.

Interference

As with all radio systems, interference is always a problem. If you are listening to an AM radio and you hear static, this is interference. The same thing applies to WiFi systems, however not to such a large degree. Things that cause interference with WiFi systems are Microwave ovens, certain lighting systems, other 802.11 access points or systems, microwave transmitters, even high speed processors for computers can cause interference for 802.11 systems. All these problems must be isolated before you can expect any significant range out of your system. If you need help, please don't be afraid to ask us. Afterall, WiFi is our business.

What is the best WiFi antenna for me?

You have probably arrived at this page because you are looking for a range extender antenna for your WiFi system. We have written this article to make the complex task of choosing the correct antenna for your system much easier and also to let you know how the different types of antennas perform.

The single most important thing you can do to extend the range of your 802.11 system is to install an external antenna with some good gain and directional or omni-directional qualities. WiFi is simply a radio, which is used for computer. You can think of your antenna as the “speaker system” of your WiFi card. Get a bigger antenna; your WiFi will go a lot further. However, don't install a speaker on your wifi system or your range will be horrible!

Directional Antennas

Directional antennas are used for Point-to-Point or sometimes for Multi-Point systems depending on the setup. If you are trying to go from one location (say for instance your router), to another location, this is the type of antenna we recommend. Directional antennas are Backfires, Yagi, Panel and dish type antennas.

Omni-Directional

This is the common “Base” antenna used for Point-to-Multi-Point or can be an omni-directional antenna for your car. An Omni-Directional antenna would serve as your main antenna to distribute the signal to other computers or devices (such as wireless printers, PDAs, etc) in your workgroup. You can use 2 Omni-Directional antennas for a point to point system, but this is usually not recommended because there is no real point to distributing your signal all over the place when you only want to going from point A to point B. Please refer to Directional antennas above. Typical Omni-Directional WiFi antennas consist of Vertical Omnis, Ceiling Domes, Rubber ducks, Small Desktops and Mobile vertical antennas.

Point-to-Point

Point-to-Point systems usually involve 2 different wireless points, or building to building wireless connections. But there are exceptions to every rule. If the access point is across a long valley and the owner of the system wishes to share the connection with multiple users on the other side of the valley. This would be a point to Multi-Point system but using directional antennas.

Point to Multi-Point

Point to Multi-Point systems is usually for sharing a WLAN (Wireless Local Area Network) or a high-speed internet connection inside of your home or with neighbors (oops, we didn't say that). They can also be for WAP (Wireless Access Points) such as you find at local coffee shops, truck stops, airports, RV parks and the ever expanding list of WAPs becoming available. Traveling with a notebook computer is extremely fun and can be a great business tool for the frequent traveler.

Range

The range of the signal will depend on several factors, including power output of your wireless card or router, receive strength of the wireless card or cards you are transmitting to, obstructions buildings or trees which may be in the way of your transmitting path, walls, etc. Since there are so many factors which can determine the overall range of your wireless system, it is impossible to cover it in this simple article. A rule of thumb however is to always choose an antenna which you think may be overkill. Why? Because the power output is extremely small it is necessary to have as much gain as possible. Most wireless cards have a power output of 32 milliwatts (+15dBm), which is roughly the same amount of power it takes to light a high power LED (Light Emitting Diode). LEDs are bright, but imaging trying to see one at a large distance or through a building or trees. This is why the antenna is critical for amplifying that signal so it is as strong as possible. Why is the power output so small? Because 802.11 works at the same frequency as a microwave oven (2.4 GHz) and if it put out a large amount of power…. well, enough said. Microwave popcorn! If you are somewhat technical, please visit our online wifi range calculator to learn more.

Long Range Wifi Antenna Image

One of our most frequently asked questions is how to choose the correct WiFi antenna for a particular situation. This can be either very simple or very complex depending on your particular application or what you need to do. 802.11b (11 Mbps) and 802.11g (56 Mbps) (Mbps stands for megabits per second and is a measure of bandwidth) standards provide excellent speed, but this completely depends on your signal strength and noise level of your wireless card and wireless system. This basically equates to, the larger the signal strength and the less noise your wireless card receives, the better.

There are many uses for wireless applications, either in a home, office or rural situations. Let's examine each of these applications. ==> NEXT PAGE

Basic antenna models

There are many variations of antennas. Below are a few basic models. More can be found in Category:Radio frequency antenna types.

* The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).
* The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as a reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the ground plane antenna, the whip, and the J-pole.
* The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added which are functionality similar to adding a reflector and lenses (directors) to focus a filament light bulb.
* The random wire antenna is simply a very long (at least one quarter wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies non-linearly with frequency.
* The horn is used where high gain is needed, the wavelength is short (microwave) and space is not an issue. Horns can be narrow band or wide band, depending on their shape. A horn can be built for any frequency, but horns for lower frequencies are typically impractical. Horns are also frequently used as reference antennas.
* The parabolic antenna consists of an active element at the focus of a parabolic reflector to reflect the waves into a plane wave. Like the horn it is used for high gain, microwave applications, such as satellite dishes.
* The patch antenna consists mainly of a square conductor mounted over a groundplane. Another example of a planar antenna is the tapered slot antenna (TSA), as the Vivaldi-antenna.

Practical antennas

Although any circuit can radiate if driven with a signal of high enough frequency, most practical antennas are specially designed to radiate efficiently at a particular frequency. An example of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of frequencies and is useful for its small size. A more efficient variation of this is the half-wave dipole, which radiates with high efficiency when the signal wavelength is twice the electrical length of the antenna.

One of the goals of antenna design is to minimize the reactance of the device so that it appears as a resistive load. An "antenna inherent reactance" includes not only the distributed reactance of the active antenna but also the natural reactance due to its location and surroundings (as for example, the capacity relation inherent in the position of the active antenna relative to ground). Reactance diverts energy into the reactive field, which causes unwanted currents that heat the antenna and associated wiring, thereby wasting energy without contributing to the radiated output. Reactance can be eliminated by operating the antenna at its resonant frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel its reactance as far as the source is concerned.

Once the reactance has been eliminated, what remains is a pure resistance, which is the sum of two parts: the ohmic resistance of the conductors, and the radiation resistance. Power absorbed by the ohmic resistance becomes waste heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy. The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.

Effect of ground

Antennas are typically used in an environment where other objects are present that may have an effect on their performance. Height above ground has a very significant effect on the radiation pattern of some antenna types.

At frequencies used in antennas, the ground behaves mainly as a dielectric. The conductivity of ground at these frequencies is negligible. When an electromagnetic wave arrives at the surface of an object, two waves are created: one enters the dielectric and the other is reflected. If the object is a conductor, the transmitted wave is negligible and the reflected wave has almost the same amplitude as the incident one. When the object is a dielectric, the fraction reflected depends (among others things) on the angle of incidence. When the angle of incidence is small (that is, the wave arrives almost perpendicularly) most of the energy traverses the surface and very little is reflected. When the angle of incidence is near 90° (grazing incidence) almost all the wave is reflected.

Most of the electromagnetic waves emitted by an antenna to the ground below the antenna at moderate (say < 60°) angles of incidence enter the earth and are absorbed (lost). But waves emitted to the ground at grazing angles, far from the antenna, are almost totally reflected. At grazing angles, the ground behaves as a mirror. Quality of reflection depends on the nature of the surface. When the irregularities of the surface are smaller than the wavelength reflection is good.

This means that the receptor "sees" the real antenna and, under the ground, the image of the antenna reflected by the ground. If the ground has irregularities, the image will appear fuzzy.

If the receiver is placed at some height above the ground, waves reflected by ground will travel a little longer distance to arrive to the receiver than direct waves. The distance will be the same only if the receiver is close to ground.

The situation is a bit more complex because the reflection of electromagnetic waves depends on the polarization of the incident wave. As the refractive index of the ground (average value \scriptstyle{\simeq 2}) is bigger than the refractive index of the air (\scriptstyle{\simeq 1}), the direction of the component of the electric field parallel to the ground inverses at the reflection. This is equivalent to a phase shift of \scriptstyle{\pi} radians or 180°. The vertical component of the electric field reflects without changing direction. This sign inversion of the parallel component and the non-inversion of the perpendicular component would also happen if the ground were a good electrical conductor.

This means that a receiving antenna "sees" the image antenna with the current in the same direction if the antenna is vertical or with the current inverted if the antenna is horizontal.

For emitting and receiving antenna situated near the ground (in a building or on a mast) far from each other, distances traveled by direct and reflected rays are nearly the same. There is no induced phase shift. If the emission is polarized vertically the two fields (direct and reflected) add and there is maximum of received signal. If the emission is polarized horizontally the two signals subtracts and the received signal is minimum. This is depicted in the image at right. In the case of vertical polarization, there is always a maximum at earth level (left pattern). For horizontal polarization, there is always a minimum at earth level. Note that in these drawings the ground is considered as a perfect mirror, even for low angles of incidence. In these drawings the distance between the antenna and its image is just a few wavelengths. For greater distances, the number of lobes increases.

Note that the situation is different–and more complex–if reflections in the ionosphere occur. This happens over very long distances (thousands of kilometers). There is not a direct ray but several reflected rays that add with different phase shifts.

This is the reason why almost all public address radio emissions have vertical polarization. As public users are near ground, horizontal polarized emissions would be poorly received. Observe household and automobile radio receivers. They all have vertical antennas or horizontal ferrite antennas for vertical polarized emissions. In cases where the receiving antenna must work in any position, as in mobile phones, the emitter and receivers in base stations use circular polarized electromagnetic waves.

Classical (analog) television emissions are an exception. They are almost always horizontally polarized, because the presence of buildings makes it unlikely that a good emitter antenna image will appear. However, these same buildings reflect the electromagnetic waves and can create ghost images. Using horizontal polarization, reflections are attenuated because of the low reflection of electromagnetic waves whose magnetic field is parallel to the dielectric surface near the Brewster's angle. Vertically polarized analog television has been used in some rural areas. In digital terrestrial television reflections are less obtrusive, due to the inherent robustness of digital signalling and built-in error correction.