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reflection in transmission lines

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neo_star

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i am a beginer in electronics

i read that if there is no impedance matching then you will get reflections from the load impedance. my question is how can there be reflections from a circuit

thanks
 

steveB

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Most Helpful Member
i am a beginer in electronics

i read that if there is no impedance matching then you will get reflections from the load impedance. my question is how can there be reflections from a circuit

thanks
All of the circuit theory we learn is only an approximation to the electromagnetic field equations. When the dimensions of the circuit become comparable to the wavelength associated with the operating frequency (wavelength=c/f, where c is the speed of light and f is the frequency), the limitations of circuit theory become noticable. Transmission and reflection of signals is one of the first effects to be noticable when we transition from the circuit domain to the field domain. Then electromagnetic radiation and reception become noticable as the frequency increases (or the circuit gets bigger).

If you take this to a limit and remember that light is just electromagnetic radiation, then light reflecting off of glass is the same basic effect as signals reflecting in circuits. You may own a pair of eyeglasses with an antireflection coating on it. This is an example of creating impedance matching to greatly reduce the reflection.
 
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neo_star

New Member
can any one suggest a good book to read about transmission lines
 

user_88

Member
Transmission lines, their characteristic impedance, and load matching are a big part of amateur radio. If you have access to a local Ham Club, you could probably obtain some useful information ...
 

crutschow

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Say you have a short, 10V pulse traveling down a 50Ω transmission line (line is electrically much longer than the pulse width) and the line is terminated in a mismatch of 100Ω. The pulse thus has 2W of power (stored in the inductance and capacitance of the line). When the pulse hits the resistor the pulse will dissipate 1W of power. The remaining 1W has no place to go so it is reflected back to the source. If the source is also not 50Ω then it will reflect part of the signal back down the line again. In this way a signal can bounce back and forth down the line for several reflections until the excess energy is dissipated.
 
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stevez

Active Member
Have a look at the ARRL website - they do have many magazine articles available in PDF form - many of those discuss transmission lines. Some are available to everyone - some require membership.
 
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