Version: 0.3
Dated: Feb. 2003
Author: Ekkehard Plicht, Wimo Antennen und Elektronik GmbH
This overview explains the radio part of WiFi/WLAN and what to consider when connecting external antennas.
"Radio waves and love go strange ways" - is a well known adage in the radio world. Since more and more data users get in touch with radio based wireless LANs, here is some help and practical tips for those people who want to understand more.
The "strange ways" are valid, especially at the very high frequencies which are used with wireless LANs (2.4 and 5 GHz). In this frequency range, the propagation of radio waves change even with small changes in antenna position or with some obstacles in the way. Obstacles can be anything, from a metallic heating radiator, to a potted plant (well wetted) to the PC chassis itself. Especially metallic objects are a big obstacle, and so is anything large or thicker than a few cm thickness, even when not conducting (Walls etc.). The result is generally that connection is ok with a few meters around the access point, but after that nothing works.
To understand the strong influences of nearby objects, one has to look at the wavelength of the frequencies used: it's approx. 13cm for frequencies at 2.4GHz and 6cm for such at 5GHz. In general that means that nearly any conducting object in that dimension or larger can be an obstacle which changes the behavious of the antenna.
But also somehwat smaller objects can be problematic: Water molecules tend to react strongly at radio frequencies at 2.4GHz, this is why this frequency band is used in microwave ovens. In the case for WLAN that means that any precipitation like fog, rain, snow etc. can be a problem and disrupt communications, but also other objects containing water: human bodies, plants, trees and so on. So maybe you have to ask your significant other to move a little to complete that urgent download...
Another harmful (harmful for the communication) factor can be reflections. Radio waves are reflected by some surfaces nearly like a mirror reflects light (which is (simplified) just another form of radio waves, but at much shorter wavelengths). In such situations wave peaks and crests can overlap in a way that they cancel each other. This process is called interference. Maybe you know it from the FM radio in your car. You stop at a traffic light, the reception of the station you are listening to is very bad. Just be releasing the brake and rolling a few cm the reception suddenly improves - you have moved out of an interference zone.
But reflections can be beneficial as well. In some case where the intended link does not have a line of sight it might be possible that a nearby building can act as a passive reflector. Large buildings with a smooth surface are most suitable, and of course the line of sight (LOS) must be free from each end of the link to that building.
Another annoying phenomenon with reflections is the rotation of the polarization plane. Due to reflection the polarization is changed from (maybe) vertical to something undefined, arriving at the other vertical antenna. Just tilting the other antenna a little bit might be helpful.
To overcome problems with reflections on point-to-point links one can use antennas which provide not linear (vertical, horizontal) but circular polarization. The Helix antenna is such a device, as well as some radiators used for parabolic dishes. SO ifyou suspect an unwanted rotation of the polarization plane, try antennas with circular polarization.
Some materials are practically impenetrable for radio waves. Simplified: the better an object conducts electrical current the worse it is for radio waves to popagate through this object. This is a big problem, especially indoors: walls and ceilings are a very good attenuator for radio waves. A connection might work over 20m in an older brick building, but not at all in a building where a lot of steel reinforced concrete was used.
In such cases it is usually best to place an antenna in the staircase to allow the radio waves to propagate between the various floors. A small directional antenna can be helpful as well, but this requires individual testing. It is very hard to predict whether a link will work or not.
To summarize: Due to the very short wavelength a lot of environmental factors are of great influence to the propagation of radio waves. What works at one places may not work at another, superficially similiar place. A precise, 100% reliable prediction about which antenna will work in your environment is impossible, at least without a more detailed analysis of the local situation.
Antennas come basically in two variations: with or without directional radiation. An omni directional antenna can be imagined like a candle, a directional antenna like a car headlamp or like the narrow beam of a flashlight.
The gain of an antenna is simply the factor of how much the energy fed to the antenna is bundled in a certain direction.
The usual antennas, supplied with PCI boards or accesspoints are generally omni directional antennas. The energy is radiated more or less evenly in all directions around the antenna (antenna in an upright position). Normally these antennas do not have a specific gain compared to a standard antenna, because they are nothing more than this standard antenna: a dipole. Some omni directional antennas do have a lot of gain, this is achieved by reducing the amount of radiation up and down from the antenna, bundling the energy in a donut shaped form around the antenna rod (see drawings).
For larger areas (indoor or outdoor) like hotel foyers, yards etc. omni directional antennas can be a good solution if they can be placed in the middle of the area. These antennas are usually mounted on a wall or mast, and are connected to the accesspoint with a cable.
Simple omni directional antennas radiate the energy in practically evenly all directions, an ideal antenna would have a spherical radiation shape. The more gain an omni directional antenna has, the less energy is radiated up and down but more to the side of the vertical antenna rod. This can lead to larger coverage perpendicular to the antenna, but less reach up and down from the antenna.
Ideal omni antenna with spherical radiation pattern |
Real world omni antenna with flattened pattern |
Real world omni antenna with more gain and even more flattened radiation pattern |
Directional antennas bundle the energy in a certain direction. In a perfect world all energy would be directed in the intended direction, but, alas, the world isn't perfect, so a small amount of energy is also so radiated to the side and even to the back.
The gain of a directional antenna is simply achieved by bundling as much as possible energy in that direction. So there is a catch: either high gain (and longer reach) in a small area, or less gain in a wider area. Possible gain values are from 5 to more than 20dBi. ANother value to look at is the so called "half power beam width" (HPBW), that is the angle (in degrees) at which the signal has falled to the half of the value at the maximum. Usually that angle is considered the area in which good coverage is achieveable.
The problem with strong directional antennas is to align the antenna properly to the other side. Especially with dishes with small HPBW this can be a non-trivial task. The best approach is usally to take a look at a high resolution map, taking the beam direction, use a compass to align the antenna roughly in that direction and then optimize by slightly moving the antenna while the signal strength is monitored on the other side of the link. A voice conenction (two way radios, cell phones) is of course required for this procedure. Allow at least half a day for setting up and aligned long distance link antennas.
Directional antennas are available for indoor and outdoor use (latter are weatherproof).
Sector antennas are a specialized form of directional antennas with a somewhat larger HPBW. This is useful to cover larger areas from the side, e.g. yards, marketplaces, large halls etc. The usually high gain of these antennas is achieved by sharply bundling the vertical axis of radiation, a HPBW of 10 to 20 degrees is common with these antennas.
Rule of thumb: the thinner the cable the higher the attennuation. Although the high flexibility of thin cables is desired, their high attenuation is not. Cable attenuation is measured in dB/100m. The higher the attenuation, the more energy is lost on the cable. Attenuation works both ways, for transmitted and received signals.
There is only one way to get lower attenuation: use thicker cables, this is dictated by the physics of this world. But the problem is that good, thick cables can be easily handled on fixed antennas mounted on a mast, but are very rigid and in most cases cannot be used to attach to a small notebook or other wireless device. Say good bye to mobility.
To overcome this problem to a certain extent so called "pigtails" are used. Pigtails are short, flexible, thin cable runs, usually 20 to 30cm long, which connect between the large cable and a wireless device. This introduces more attenuation, but that's the only way. Or attach your wireless device to a cable with the rigidity of a water hose...
Another issue is that the small connectors, usually found on wireless devices (APs, routers, PCI cards) are not available for large (thick) cables. So the pigtails also adapts between large and small connectors. Common cable types are found below under "practical tips".
Many WiFi/WLAN manufacturers use very uncommon connectors on their equipment. The reason for this is the US FCC, which told the manufacturers to use non common connectors to stop people from using high gain antennas, the ultimate goal was to protect the frequency spectrum from illegal emmissions...
So at first you have to find out which connector your WiFi/WLAN euqipment uses. Meanwhile most of the types used in wireless gear is known, in case of doubt just ask us. If you have a very good, hi-res macro picture of the connector we are certain to identify nearly all possible connectors.
With some connector types the use of adaptors is useful, but as a general rule try to avoid adaptors is possible. They introduce more even attenuation. For a professional attached connector calculate with 0.2 dB attenuation, each adaptor about 0.4dB. If an adaptor is not available for the system in use, we are able to assemble a pigtail for your need.
Not all connectors are available for all cable types. The connectors most common to WiFi/WLAN gear are usually available only for relatively thin cables.
Coaxial connectors are also differentiated by the method of how they are attached to the cable. The inner conductor is usually soldered, the outher conductor (shield) also soldered or crimped. For crimp connectors special tools are required, which WiMo offers as well. Crimping is considered the better method, the yield of working connections is much higher than with soldering (when used by an experienced person).
When attaching a connector to a cable proceed with utmost care. A wrongly attached connector can introduce more attenuation than any cable and destroy all thework you had with the antenna and laying the cable. If you have never done this before, buy one or two spare connectors and practice. Or use our cable service, we assemble any cable you can think of (nearly), at reasonable cost.
Common connector types are listed below (practical tips).
Goal: to achieve better coverage indoors.
| Outer conductor | Inner conductor | |
|---|---|---|
| Normal plug: | Nut with inner thread | Male pin |
| Normal jack: | Outer thread | Female jack |
| Reverse plug: | Nut with inner thread | Female jack |
| Reverse jack: | Outer thread | Male pin |
 SMA jack, showing normal and reverse polarity (ca. 200kb) |
 SMA plug, showing normal and reverse polarity (ca. 200kb) |
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