why quarter wavelength/half wavelength?
I just joint to edaboard and this is my first question in here. I am naive in RF and Microwave subject. I will appreciate if someone explain to me why λ/2 or λ/4. Sorry I know it is too basic!
Why quarter and half wavelength for what?
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Amr
I found in this field, like an antenna, they mention these two alot. why is that for?
Normally it would relate to the design of an antenna.
Not suprisingly the most efficient physical sizes for reception are based on the wavelength.
Hence you might see phrases such as "quarter dipole antenna" etc in some sales literature.
For very high frequencies the wavelenght is obviously very small allowing
for a small antenna for maximum performance.
For very low frequencise (VLF) (such as used for submarine communication) very
long wavelengths can require very large antennas.
Its a design consideration for the frequency in use and the physical constraints
of the product which will work best. The trick is always to keep the mathematical
relationship betwen the antenna length and wavelength.
jack
http://en.wikipedia.org/wiki/Wavelength
For High Frequencies:
If you have a Transmission Line(T.L.) of λ/4 wave you know that at the
beginning of the T.L. when the wave is at a Max the end of the λ/4 T.L.
will be at a Null. 90° Phase shift of the signal.
λ/2 is 180° of the signal
Also you should learn about the "smiths chart" and see how a λ/2 & λ/4 T.L.
affects a Circuit
Cheers
In addition to that, simply, when you have a TL. If you measure an impedance on somewhere in TL, you are certain that, you will measure the same impadance after going half wavelength on the TL.
quarter wavelength is useful if you have try to match two TL. because the output impedance is related with the characteristic impedance and the loaded impedance of the first TL (depending of your view)
I am sorry, if I am confusing =)
It is a basic knowledge in antenna field.
Please have a look some books about theory of antenna.
If you take a look at some the theory and equations for basic transmission lines you will notice that at a quarter, one eighth and half a wavelenght points, interesting transformations happen.
For example, a short can become an open (and viceversa)
You can transform some reactive elements to real/complement elements.
You can transfer you load as it were connected directly to your generator.
this is just a sample, grab a book and enjoy the reading or go to microwaves101 website.