Antenna dimention vs wave length

[oufa]

please anyone tell me the relation betweeen wave length of em wave and antenna dimension

 

[f95toli]

There is no "general" relation. Size and wavelength are correlated meaning you generally speaking need large antennas for long wavelengths (meaning the size will be of the order of a wavelength); but there are all sorts of "tricks" one can use to shrink antennas and make them smaller than this.
One can also increase the efficiency (or adjust other parameters) by using arrays of antennas meaning many antennas are much larger than one would expect than if size just scaled with wavelength,

 

[vk6kro]

A minimum length for an efficient antenna is a half wavelength.
This is the dipole antenna.

Even quite short pieces of wire will radiate a signal and receive strong signals, but when the antenna is cut carefully to slightly less than a half wavelength long, the antenna becomes resonant and can be fed in the middle to work well as a transmitting or receiving antenna.

Note that making it longer will make it less efficient. This is a resonance effect.

See the following:
http://en.wikipedia.org/wiki/Dipole_antenna

 

[dlgoff]

With the proper matching circuit you can load up just about anything.
http://www.eham.net/articles/10721"

 

[f95toli]

A minimum length for an efficient antenna is a half wavelength.
This is the dipole antenna.

Although that is sort of technically correct, it is important to remember that for certain types of antennas (such as the common patch antenna) it is the wavelength with dielectric loading that matters; this makes it possible to shrink that antenna to a size significantly smaller than the free-space wavelength (even with a common substrate like sapphire this makes the antenna about a factor of 3 smaller).

 

[vk6kro]

You can tune up random bits of wire and add loading coils to dipoles to get them shorter, but you pay a big price in efficiency if you do that.

Firstly, bandwidth suffers. If you move the transmission frequency by a small amount, you have to retune.

And efficiency drops rapidly. On 1.8 MHz, a Ham Radio band, efficiencies of less than 1% are common because a dipole for that frequency should be about 273 ft long and 100 ft high and very few people have room for such an antenna.

So, pieces of wire are "tuned up" to accept some sort of a signal, but the losses due to connection resistance to ground, and component losses in the tuning apparatus become very large compared to the radiation resistance of the piece of wire.

If you really have to reduce the size of a dipole, you can do it by adding inductors in series with both side of it at some cost in efficiency up until the length of the dipole gets below 0.35 wavelength or 70% of its normal length. As it gets shorter than that, there are severe losses in efficiency.