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hiyan

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i wantto ask why we take Antenna length lemda/2 and lemda/4?




2nd in the end of antenna why resiatnace high?


3rd length of transmission line is depends on the wavelength of the signal being transmitted why?




4th how the standing waves are genrated in transmission line?



i hope all members who know guide me and explain me briefly.thanks
 
When a signal is sent along an antenna, its basically like a transmission line with an open end (infinite resistance). Its similar to a rope, where you wiggle one end and watch the wave travelling down it. Now imagine wiggling the rope constantly and as the wave hits the end of the rope, it bounces back and meets the next wave. This is called wave reflection and happens when the wave encounters a mis-matched impedance, and reflects back in the opposite direction. If the rope is a half wavelength (y/2) or quarter wavelength (y/4) then a standing wave is set up and you'll have maxima and minima due to the interference of the waves. This is covered really well in the ARRL handbook by the way. The interference pattern for the current and voltage will have different maxima and minima, and you'll be able to work out from these, why the resistance is a particular value at any place along the line.

The aerial doesnt have to always be a multiple of the wavelength, but if it isnt you'll have a mismatch and you wont be able to radiate the signal efficiently. I find that quarter wavelength aerials on my transmitters work just as well as half wavelength.

Megamox :D
 
The speed of electromagnetic radiation is 300 meters/second. The speed in a wire is a little less but for most purposes 300 m/s is close enuf. The wave length is the distance the wave moves during one cycle. When the wave comes to an open circuit it is reflected back toward the source and creates a standing wave.
 
that must have been typo, he must have meant 300000km/sec.
speed of sound in the air close to sea level is some 330-340m/s
and if the air pressure is a bit higher, one could yell to neighbour that his phone call is on it's way...
 
i know what u mention but u all left one confusion why we take lemda/2 and lemda/4 why we not take lemda /3 or lemda /5?

kindly answer my questions
 
As already suggested, ARRL and RSGB handbooks are good place to look. Some simple statements might help you along:

A. An antenna is most efficient when it is resonant at the frequency of interest. A dipole is resonant when it is center fed and is approximately 1/2 wavelength long. When it is resonant is behaves as if it were a resistor in that it takes all the power delivered by the transmission line IF the line and antenna are matched. The precise value of impedance or resistance is dependent on many things.

B. In a center fed dipole that is a 1/2 wavelength or multiple of 1/2 wavelength long the current is high at the feedpoint and the voltage is low. At the end the current is low and the voltage is highest. This really isn't an answer just more information.

C. If the antenna feedpoint impedance appears like a pure resistance and matches the transmission line then the length of the transmission line will not affect the apparent impedance at the trasmitter end of the transmission line. In any other case the length of the transmission line between 0 wavelengths and 1/2 wavelength will affect the apparent impedance at the transmitter end of the line. Note that at 1/2 wavelength or multiples thereof the impedance at the feedpoint will also be the impedance at the transmitter end.

D. If the antenna doesn't absorb all of the power delivered by the transmission line then the power has to go somewhere so it is reflected back toward the generator. The interaction between the forward and reflected power is reinforcement or cancellation. The result is the standing wave.

Note that I've done some simplification here and also that I may have made some errors however I'd hope that others will offer improvements or corrections so that you and others have the best information.
 
doesnt the lamda/2 length have something to do with the directivity of the antenna as well?
Using something other than lamda/2 will give you a bunch of side lobes and a funny looking radation pattern.
 
The length of the antenna does play into directivity but is not the only factor. For a dipole the height above ground, in terms of wavelengths, is actually more of a governing factor. Note that at 80 meters a dipole antenna would have to be extremely high off the ground in order for the ground effects to be reduced. At higher frequencies it is less of a problem.

In a vertical configuration a VHF and even an HF antenna can see improved performance by being longer than one might otherwise make it for resonance. What this does is adjust the radiation angle to put more energy in a direction that is useful.

At 80 meters a dipole that is 20 ft off the ground has a radiation pattern that would favor receiving stations that are overhead - unlikely to ever be the case.

A vertical VHF antenna for 2 meters might be 5/8 wavelength long and have appropriate matching sections to bring the angle of radiation closer to the horizontal - useful for ground stations. For overhead work (satellites, aircraft) a 1/4 wave might be more suitable.

ARRL Handbook and website has some good info on the relationship of height above ground and directivity.
 
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