Antenna theory for wireless Networking
A short tutorial by Jinxy
1/ With the proliferation of wireless networks I believe an understanding of radio waves and antenna theory is becoming important, after all a wireless network is nothing more than a group of computers and ancillary equipment working together over radio waves. Exactly as a radio network is a group of radios working together over the airways. They both have the same goal, sharing information between pieces of equipment, via data or voice.
2/ Why should this be of interest to me you might ask. Well I think that if you no a little about how radio waves are propagated then you can make the most of your wireless equipment ensuring that your network runs smoothly you can also cut down on expense by not having to bye equipment that is not necessary.
3/ What is a radio wave? Well radio waves are an electro-magnetic field that radiates between an antenna and an electrical earth or ground plane. To explain this you need to look at a basic electrical circuit, if you take a direct current power supply such as a battery and using some wire make a circuit between the positive and negative terminals you will have created an electrical circuit and electricity will flow though the circuit. Now if you make a break in the circuit and place two peaces of metal strip in the circuit one connected to one side off the circuit and the other connected to the other side of the circuit leaving a small gap. What will happen is that electrons will build up on one side and because of the lack of electrons o the other side eventually they will jump across the gap between, therefore causing the circuit to continue. If you now change the DC power supply to a AC power supply the same thing will occur, electrons jumping back and forth across the gap. In effect you have created a fully charged capacitor.
Now if you move the metal strips apart at one end the electrons will still jump back and forth. Move the ends of the strips still further apart until they are end to end and they will still jump back and forth they will also radiate outwards like the ripples from a stone dropped into water. In effect you have created a basic radio transmitter. The metal strips acting as the antenna. You should note at this point that there is two parts to antennas the antenna its self and the ground plane or electrical earth. In your home radio you can see the antenna quite easily as it sticks out of the top of the box you may not have realised that it has a ground plane but it will be there, most often the chassis is used, a car radio uses the body work. With a handheld transmitter it is the casing and also the person holding it. The earth also has ground plane this is usually at the water table. And some radio equipment uses this ground plane, you may have seen a radio with an earth spike stuck in the ground this is to increase the size of the radio’s electrical earth.
You can thus describe a radio transmitter as an alternating current generator working at a specific frequency. This is a very simplified explanation but should be enough to allow the reader a basic understanding how radio waves are formed.
4/ Now we no how radio waves are formed let’s look at the ways in which they travel. This can be classified into three groups:
Direct waves. The radio wave travels directly through the air in a straight line between the transmitting radio and the receiving radio, the antennas for these waves are normally within line of sight and mounted above the ground on masts.
Ground waves. These waves travel along the ground following the earth’s contours. Wireless networking equipment uses these waves although some direct waves may be radiated also.
Sky waves. These waves are transmitted upwards towards space and are reflected back to earth by the ionosphere. These waves are transmitted be antennas that are positioned horizontally parallel to the ground.
For wireless networking we only need to concern ourselves with ground and direct waves. So I will leave sky wave out of this tutorial. All antennas transmit ground and direct waves to a greater or lesser extent depending on how high or low they are above the ground.
5/ When talking about radio waves you will often come across the word attenuation. Attenuation is the weakening of a radio wave and there are various factors that can cause this.
a. Ground. Porous surfaces tend to absorb radio waves whilst more dense surfaces tend to reflect them.
b. Obstacles. If an object is in the path of a radio wave it can block the signal or reflect it in an undesireable manner.
c. Atmospherics. Sunspots or flares can interfere with radio waves bad weather can also adversely affect radio waves.
d. Distance. Radio waves tend to get weaker over a distance and get weaker the further they travel.
6/ Some more facts about radio waves, a common misconception is that radio waves travel at the speed of sound. This is not true they travel at the speed of light. They also travel in straight lines although they can be bent under some circumstances for example to go over a hill. They can also be reflected. Another word that is used with radio waves is black spot. This is an area were radio waves cannot be received and there are a few reasons this can occur. One is if an antenna is situated where a radio wave cannot penetrate, a Faraday cage would be an extreme example of this. Another is if a transmitted wave has taken two separate routs and meets at the receiving antenna at the same time, this can occur when an antenna receives a direct wave and a wave that has been reflected off an object. A point about antennas here, all antennas working in a network must be polarised in the same plane. That is to say if one antenna is positioned vertically all antennas in that network must also be positioned vertically.
7/ How can we use this information to optimise our wireless computer networks? Well each piece of equipment that belongs to the network will have an antenna attached to it wireless cards etc, so firstly make sure that all the antennas are polarised in the same plane vertically or horizontally. Try to position the equipment at the same height above the floor and get line of sight this is especially important over longer distances. Line of sight is not always possible but try to look for the path that the wave will take try to avoid having the wave go through large dense objects. Look for ways to reflect the wave if it is possible to avoid dense objects. Remember the further a wave has to travel the weaker it will get, the more furniture walls etc the signal has to go through the weaker it will get. Think about what else can interfere with the wave, electrical power cables. A lot of walls especially stud walls can have a lot of electrical cables in them if a wave will not go through a wall it may have cables in it so look for a place where the signal can pass through. Other electric equipment can interfere with radio waves so be aware of things like power drills washing machines etc anything that gives off electrical noise. Be aware of black spots, if you cannot get a signal, move the equipment. Very slight position changes can make a huge difference to the strength of a signal even a few inches can mean the difference between receiving a signal and not at all. Basically it can take a lot of experimenting with the position of pieces of equipment to get it right but if you think about the way a radio wave is transmitted and the path it will take you should be able to get your network to work over a decent range reliably. A point here about wireless network card, all the cards I have seen are pci so they slot into a vacant pci slot at the back of the pc case, if you think about what I have said above you should come to the conclusion that this is not the ideal place for them as the antenna is basically blocked from transmitting in one direction by the casing of the pc. Something for you to consider when setting up your network. If you have you pc box on the floor and you are having problems raise it and position it so that the antenna is pointing in the direction of the other equipment.
8/ I’ll touch very briefly on the types of antenna that there is and how they propagate radio waves at this point. Basically there are two types of antenna. Directional, these are antennas that transmit waves mostly in one direction and omni directional, these are antennas that transmit in all directions. An example of a directional antenna is a parabolic antenna, these are the type of antenna that the telecom companies use to transmit signals between their networks they look like big dustbin lids and are very directional to the point that they have to be aligned with each other very accurately and they must have line of sight. They transmit a very narrow beam and almost all the signal is direct wave, very little ground wave is transmitted and they broadcast at the top end of the radio spectrum super high frequency and microwaves. Examples of omni directional antennas are the dipole type of antenna usually these are of the pole type of design they are also found on mobile phones and wireless network equipment, these type of antenna can be very long. If you look at the antenna on your mobile phone it will appear to be quite short however the antenna is coiled like a spring to save space.
Purely for interest at this point, the length of an antenna is critical to its effectiveness. Each frequency a radio broadcasts on requires a different antenna length. To find the length of antenna needed for a given frequency you have to divide the frequency in use by the speed of light. You can make the maths easier by using the following. Frequency in use divided by 234 = antenna length in feet. End fed antennas, these are antennas that plug straight into a radio without the use of coaxual cable nead a tuning unit to optimise them, a tuning unit acts similar to a reastat they consist of a coil with a contact that moves along the length of the coil,
this has the effect of lengthening or shortining the antenna as required. Antennas that connect via a coaxual cable, such as a television ariel plug straight into a modulater/demodulater and do not nead to be tuned. A point her you may notice that i refered to the Television Ariel and not antenna. This is because i was taught to differentuate between an antenna (used to transmit and recieve) and an ariel (used to recieve only).
Well I think that is enough for now, turned out to be not so short. This was written of the cuff from knowledge that I learned along time ago, so if I have made any errors please feel free to point them out to me and I will edit this post. I hope some of you find what I have written informative this is my first tutorial so please be gentle with me.:D
Jinxy
