So antennas antennas are probably the most important part of making a wireless connection, definitely for long distance links. An antenna is a passive element that brings the signal from a wire into free space and back again that's important to be aware of it's not an amplifier it doesn't actively amplify a signal it only squeezes the signal into a given direction and the more it does that the stronger we say the antenna is but it's only strong for a particular direction it doesn't add overall power to the signal it directs the signal in one direction and it's therefore weaker in other directions in that sense it's not an amplifier it's only focusing power here's a principal view of what an antenna might look like here it's a two-wire element in that wire charges electrons can be moving they can react to the field that hits them they will try to dance along you could say they will move in in the same frequency as the electromagnetic wave the radio wave that's hitting it and if the length if the dimensions are well fitting then this will be in good resonance and be able to absorb or emit this frequency in a very efficient manner that in principle is what an antenna does as a side comment you can also see here radio waves electromagnetic waves have something which we largely neglect here they have a direction they have a polarization this one here oscillates in the horizontal plane not in a vertical and we we could go into much detail here we won't have the time for it but keep in mind when working with antennas polarization direction is important the general antenna properties what we're typically most interested in is the directivity like how does it focus power into one direction how narrow is the beam we're talking of the beam width is it symmetrical to all directions or only in one direction do we have something that goes backwards talking about the front to back ratios so directivity directionality is the first thing we're interested in i mentioned polarization obviously we have to think of the frequency and the space in frequency around that center frequency the bandwidth of this antenna the physical sizes like can we even deploy such an antenna and lastly impedance which is the resistance of an oscillating wave we know a resistor for dc direct current likewise alternating currents ac's oscillating fields moving in a wire a connector antenna they have a certain impedance and that needs to match if it doesn't match between the elements you get reflections between the different elements so impedance has to match something you can look up in the data sheets of connectors antennas and so forth if it doesn't it's like sticking two water pipes together one with the small radius the other with the big one and then if you pump water through that from the big one into the small one well it doesn't travel nicely you get reflections that's what we want to avoid therefore we match impedance the radiation patterns and we'll be seeing quite a few of these are probably the most important part of what we're looking at it's describing the direction that the antenna is focusing the power in we typically show these as polar diagrams like showing the angles both looking from above onto a horizontal plane and from the side looking at the elevation the up and down that's the typical two views that you see if we're looking at antenna radiation patterns so and those can be very very different what's happening in the horizontal plane and in a vertical direction can be and will be very diffic different from one another and to complicate things further it could be different and will be different for different polarizations so radiation patterns get quite complicated if you look into detail most of the time we don't need them that detail but in certain cases yes we have to look into all the details the beam width i mentioned earlier quickly on how do we actually measure this how do we define this we say it's that direction where the power has fallen from 0 db that's the strongest that's the direction it goes into to minus 3 half the power so that's how define beam width it's the direction where the edge of the beam we still have half the power compared to the absolute maximum.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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