# phyiscs

posted by .

Why is an antenna designed to be one-half of the wavelength of the wave it is supposed to receive?

In my book all it says is that the voltage is larger when the antenna is 1/2 the wavelength of the wave.

• phyiscs -

It is explained in detail here

http://farside.ph.utexas.edu/teaching/jk1/lectures/node82.html

The important point is that an antenna that is much shorter than the wavelength will have a radiation resistance that will be far less than the internal radiation.

Then for every joule of energy transmitted in the form of radio-waves, you'll necessarily have dissipated many more joules of energy in the form of heat.

If an antenna is of the order of a wavelength, then the radiation resistance will be quite large. To use an antenna for transmission, the impedance of the antenna needs to be the same as that of the transmitter (to be precise, it has to be the complex conjugate), because otherwise the output of the transmitter will be partally reflected back into the transmitter, potentially damaging the transmitter.

Now, since the impedance of a half wavelength antenna is of the same order as that of typical transmitters and coax cables, this makes them easy to use.

But in practice, an antenna tuner will be used to match the impedance of the antenna to that of the transmitter, so it is not necessary to have an antenna that is exactly the "right length".

• phyiscs -

Typo in first sentece:

I meant to say:

The important point is that an antenna that is much shorter than the wavelength will have a radiation resistance that will be far less than the internal ohmic resistance.

## Similar Questions

1. ### Physics

A string on a musical instrument (1 m long) carries travelling waves at 100 m/s. What are the three lowest notes that this string can play (keeping the string at its full length)?
2. ### Physics

At one time, television sets used "rabbit-ears" antennas. Such an antenna consists of a pair of metal rods. The length of each rod can be adjusted to be one-sixth of a wavelength of an electromagnetic wave whose frequency is 65.0 MHz. …
3. ### physics

Television sets sometimes use a "rabbit-ears" antenna. A rabbit-ears antenna consists of a pair of metal rods. The length of each rod can be adjusted to be one-fifth of a wavelength of an electromagnetic wave whose frequency is 60.0 …

Please let me know if my answers are not correct/not thorough enough. 1) Q:What happens when the number of loops in the armature of a generator is increased?
5. ### geometry

a cable attach to the top of a tv antenna is 100 ft. if the cable is fasten to the ground 28 ft from the base of the antenna how tall is the antenna?
6. ### geometry

a cable attach to the top of a tv antenna is 100ft long if the cable is fasten to the ground 28 ft from the base of the antenna how tall is the antenna?
7. ### trigonometry

A television antenna is on the roof of a building, the angle of elevation of the top and the button of the antenna are 51 degrees and 42 degrees respectively.How tall is the antenna?
8. ### pre- cal

An antenna is on a hill that makes a 5 degree angle with the horizon. The antenna is placed so that it is perpendicular to the horizon. A guy wire extends from the top of the antenna to a point 50 ft downhill from the base of the antenna. …
9. ### trigo

A television antenna is on the roof of a building. From a point on the ground 36.0 ft from the building the angle of elevation of the top and bottom of the antenna are 51° 23' & 42°10', respectively. How tall is the antenna?
10. ### Physics

Two sound waves are traveling through a container of unknown gas. Wave A has a wavelength of 1.2 m. Wave B has a wavelength of 3.6 m. The frequency of wave B must be __________ the frequency of wave A. a. one-ninth b. one-third c. …

More Similar Questions