Squared pulses and antennas

1. Jan 25, 2006

dervast

Hi. Yesterday i have had a strange question.. why cant we use squared pulses in antennas? What kind of pulse can we use in antennas?
Thx a lot

2. Jan 25, 2006

Integral

Staff Emeritus
A perfect square pulse is nearly impossible for ANY electronics circuit. Computer engineers would give precious body parts for a perfect square wave.

The reason lies in Fourier Analysis. From that you learn that a square wave is composed of the sum of all odd harmonics of the base frequency. Therefore it requires an infinite bandwidth to create a perfect square wave. Unfortunately all physical circuits have a somewhat less then infinite bandwidth. So perfect square waves are not possible.

Antenna's are tuned to specific wavelengths, working best with a narrow band of frequency's and the harmonics. For instance it is common to find antennas that are $\frac 1 4 \lambda$ (wavelength). That means that the length of the antenna is $\frac 1 4$ the wavelength of the base frequency. Recall that $c = \lambda f$ so frequency uniquely determines wavelength and visa versa.

By combining the output of several antenna there are amazing things that can be done with the power distribution of the transmitted signal. That is not my bailiwick, perhaps some of the EEs can give an in depth explanation.

3. Jan 26, 2006

vanesch

Staff Emeritus
Referring to Integral's reply, you CAN of course apply pulses that are more or less squared to an antenna, and then you get a lot of troubles with your local control agency for the use of the electromagnetic spectrum, because you're polluting !

4. Feb 21, 2006

dervast

Thx a lot vanesch i have found your answer really intrested.. I am wondering what do u mean by syaing that some square pulses will pollute ? Can u explain me this plz?

5. Feb 21, 2006

Staff: Mentor

The electromagnetic spectrum is a shared resource. That's why there are regulations about what frequencies are used for what, and how much power can be used with what kinds of modulations in different windows in the EM spectrum.

Are you familiar with the frequency content of a square wave? What kind of harmonic energy is present? If you drive a square wave of frequency f into an antenna, what other frequencies will be launched off the antenna? The antenna will have a finite bandwidth, of course, but you will still likely be transmitting out of your intended band, and causing interference with other bands.