1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about electromagnetic waves

  1. Feb 5, 2010 #1
    There is one thing that’s bugging me.
    I read about electromagnetic spectrum. It says that light is, by its nature, electromagnetic wave just like radio signals, microwaves, X and gamma rays, infrared waves etc. The only difference between these waves is their frequency, right?
    Now imagine this: I have a radio transmitter and I am increasing its frequency. Does this mean that the antenna of my transmitter will begin to emit visible light when the frequency of my signal enters the frequency range of the visible light?
     
  2. jcsd
  3. Feb 5, 2010 #2

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Yup! In fact, that is what is done in synchrotron light sources where they pass electron bunches through a series of undulators. While this is not exactly an antenna, the principle is the same - electrons being jiggle up and down, just like an antenna. How fast they jiggled up and down determines the frequency of the EM wave being produced. By varying the spacing in between the undulator components, one can vary the frequency, and they could get everything from IR to hard x-ray using such a technique.

    Now, whether it is feasible to generate visible light with an ordinary antenna is a different matter.

    Zz.
     
  4. Feb 5, 2010 #3
    Your radio will not enter that range simply put. The transmitter has a range of operation, if you look at it there is probably a magnet with coils wrapped around it, that will determine its frequency range. But it has no capability to reach light frequency. You can tinker with your equipment and probably get up to microwave stuff though. Thats one thing I do know pretty good is RF.
     
  5. Feb 5, 2010 #4
    OK, thank you for your answer. Now I have another question.
    What determines strength of radio or television transmitter? I know that electromagnetic transmission has a frequency and strength. Maximum reach of the signal depends on its strength and signal strength decreases with square of the distance, right? I know a few things about frequency now I would like to know what factors determine strength of transmitter (reach of its signal).
     
  6. Feb 5, 2010 #5

    ZapperZ

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    I'll give a rather naive answer to this one: the amplitude of the oscillation. So to a first approximation, the height of the antenna. But this really is simplifying it too much, since other factors are also involved. But if we simply go by wave mechanics, one can equate that with the amplitude of the wave.

    Zz.
     
  7. Feb 5, 2010 #6

    jtbell

    User Avatar

    Staff: Mentor

    That depends on how much current you have in the antenna, which correlates with the amount of power you feed into the antenna. A full-power TV transmitter in the USA might put out 30 to 35 kW. Its antenna is designed to broadcast most of that power close to a horizontal plane (there isn't much profit in broadcasting the Simpsons straight upwards into space :smile:), so we usually talk about the effective radiated power (ERP) which is the power than an isotropic antenna would have to radiate, in order to get the same signal strength towards the horizontal. In the USA the maximum ERP for a TV station is 1000 kW. The ratio between the ERP and the actual power is the gain of the antenna.
     
  8. Feb 5, 2010 #7
    It will have a silver cube looking thing in it, that is an amplifier. The strength will be measured in DBV, or simply decibles. Also you have to have pretty good interferance filters, those will be air coils. The rating of the amplifier itself should be in db's though. There is formulas for each frequency too. The lower frequencies have a longer range and less signal loss. If you are building your own transmitter you want to keep in low frequency... unless you want to get a license for it through the FCC. If you want to look at equipment specifically email me andrew@libertybb.com I have equipment and stuff you could check out.
     
  9. Feb 5, 2010 #8
    YOu guys are talking way too detailed here. This dood probly is looking at a transmitter or making one. Its far simpler than you guys are making it sound here. You get a few pieces together, solder them up, and you have a transmitter.
     
  10. Feb 5, 2010 #9

    Pythagorean

    User Avatar
    Gold Member

    Actually, he seemed genuinely curious about the nature of electromagnetic waves.
     
  11. Feb 5, 2010 #10
    RF isn't optical transmission. You are broadcasting an RF signal that your tuner is decoding and turning into optical signals. RF doesn't transmit visible light. You guys know all the formulas but as far as the hardware and what you need to look at to really determine your broadcast power is more in your amplifier. I can turn on transmitter in my home right now with no antenna setup or fine tuning and blur out channel three for a couple miles lol.

    But eh looking closer it does seem hes more interested in the principle than in actually hooking something up maybe your right. I'm jumping the gun here lol, I don't know the formulas but I have a handbook for that. Sorry guys thought I was seeing a question I could really be all over!!

    Optical transmission I can do too, but really its closer to infared than it is to visible light as far as the data streams. And thats through fiber optics and lasers, thats true optical transmission.

    Thats the first form of electronics I got into, my family has two 1980's vintage cable systems in rural USA. WE have some old broadcast equipment from the local channel we used to operate, its setup for channel 2 but its an agile unit I can set it from 2-6 and its even using the low frequency used for CATV reverse like what your cable modem broadcasts to your ISP.
     
  12. Feb 5, 2010 #11

    Pythagorean

    User Avatar
    Gold Member

    Optics is a general word for electromagnetic radiation. It's often used for visible light, but it's not really restricted to it. I had already edited my post to avoid this discussion but you were too fast :P
     
  13. Feb 5, 2010 #12
    there I go again rushing in guns blazing !

    See in the broadcast world we don't say "electromagnetic waves" we use radio frequency, 99% of everything you see on TV is RF. Your cable internet is RF, they call stuff digital but its a garbled analog RF signal that is compressed to be decoded at the recievers. Its all analog RF but for the places where they have fiber optics running into your house.. but even that is converted to analog at your sets, phones, and your pcs.

    You would be surprised how complicated it is to make a simple RF amplifier and keep it dedicated to a specific frequency... there is noise everywhere trying to get into your equipment and trying to get out of your equipment. Its a wonder the hardware performs as well as it does, there are some very strange things that can happen when you are troubleshooting these things. You can end up listening to airlines, or the airlines maybe listening to your cable.. you can even setup your cable tv sytems to broadcast FM radio stations through the cable tv lines NP. You can scramble, descramble, hack, burn, decode, and sniff out all kinds of things in the RF band.

    Its the most amazing thing in the world to me... you can't stop the signal.

    Its going to suck when ET picks up our signals and finally comes and kills us all because they caught a bad days of our lives episode.
     
  14. Feb 5, 2010 #13

    Pythagorean

    User Avatar
    Gold Member

    I'm guessing because of their relatively long wavelength, RF frequencies are less likely to interact with objects on the way to their destination, so they're perfect for transmitting over (relatively) long distances. Thus, all the institutions that make use of them are overpopulating the bandwidth. Reflections off of the upper atmospheric probably cause a lot more stray RF's from further away, too.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook