Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Squared pulses and antennas

  1. Jan 25, 2006 #1
    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. jcsd
  3. Jan 25, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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 [itex] \frac 1 4 \lambda [/itex] (wavelength). That means that the length of the antenna is [itex] \frac 1 4 [/itex] the wavelength of the base frequency. Recall that [itex] c = \lambda f [/itex] 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.
  4. Jan 26, 2006 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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 !
  5. Feb 21, 2006 #4
    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?
  6. Feb 21, 2006 #5


    User Avatar

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?