Electromagnetic Wave Polarization Demo

Discussion in 'The Projects Forum' started by AgentSmith, Sep 27, 2010.

  1. AgentSmith

    Thread Starter New Member

    Sep 27, 2010
    First off hello, I'm glad to have found such an active community here. I’m hoping that some of you may be able to assist me in not only this demo design but furthering my knowledge in the field.

    I'm friends with some of the guys in the physics demo department(University), and they've been looking into a way to demonstrate electromagnetic wave polarization for their Physics II lectures. I thought about this issue and posed the idea of using a rotating antenna to pick up radio waves, then playing it through a speaker, the idea being that since the waves are linearly polarized it would be possible to hear the difference between when the two antennas were in and out of phase. They tried this previously by using one of those Ipod car transmitters and a dipole antenna but to no avail. They decided they’d like a visual demo better, instead of an audible one.

    My current plan is to build/acquire a small AM transmitter(with transmitting antenna), and a dipole antenna with a LED connecting both poles (this mounted on a wooden stick so it’s rotatable). My hope is that when the antennas are in phase and within range the LED will light up. I’m looking for a reasonable solution that’s fairly cheap. I’m thinking a 1 or 2 watt LED will be visible enough for those in the back of the lecture hall to clearly see. I’m doing this on my own free time in an effort to help broaden the physics departments in-class demos.

    I’m starting my degree in Electrical Engineering so I have a basic understanding of electronics, potential hazards, and a little understanding of electromagnetic theory. I’m not exactly new but I’ll admit currently I posses fairly limited knowledge on this subject. I’m basically looking for input from people who have more experience. Am I going in the right direction? What sorts of equations should I be looking at? (Power loss, gain ratios, etc)

    Comments, suggestions, criticism are greatly appreciated.
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    What you have suggested is good. It will work. BUT make sure you don't use a transmitter of too high power as it will defeat the purpose. If you can build one that has an adjustable output power all the better or receiver gain that is adjustable (probably easier). Set the antennas at a good distance apart in the same polarised plane. ie. both vertical or both horizontal. Adjust the output of the transmitter/receiver gain till you just have the LEDs fully lit. Rotating one of the antennas SHOULD see the brightness dim.

    A better way of doing this would be to display the received signal on a CRO and project it onto a screen. You could also show the effect of frequency change on the wavelength of the antenna.

    Buy the way dipole antennas are probably the best for this demo.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If it's just EM wave polarization you want to demonstrate, use a shorter wavelength. Optical polarizers work very well to demonstrate the phenomenon.
  4. AgentSmith

    Thread Starter New Member

    Sep 27, 2010
    I certainly appreciate your input,
    I realize it would be significantly easier to acquire polarized sheets and rotate them parallel/perpendicular to each other. In fact they have a couple of those sheets that they normally use, I mentioned the polarized sheets remembering a demonstration I saw long ago using polarized sheets to view various objects including an LCD which by nature was already polarized. They're basically looking for other demonstrations to supplement that.

    Any other ideas or comments?

    Again, I appreciate all the input/criticism I can get,
  5. Wendy


    Mar 24, 2008
    Just use two conventional dipole antenna, both with short wires, each rotatable 180°. I do not believe tuning is critical. Then get two 25 in one project kits, wire one as a transmitter, the other as a reciever (or just build to simple circuit boards). They will need a metal or wire mesh enclosure to prevent RF leaking. Adjust the output power of the TX to the minimum power that you can still a clear signal. This should do it.

    The reason I mentioned the 25 in one project kits is the students can see all of it, and if they want to reproduce it it is relatively cheap.

    You might be able to demo a polarized filter by using a material such as plexiglas with grounded wires as strips (½ a mesh). I'm thinking of an RF version of polarized glass.
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
    Another help would be to greatly increase the frequency used.
  7. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    Try finding a couple of UHF CB radios. You must be able to find a few junk ones that still have usable RF stages. De-tune the transmitter so you reduce the output power and there you have it. If you can't do this for under about $40 I would be amazed. You can make your own dipole antennas by just stripping back some coax cable to the 1/4λ distance an bening the centre conductor and the shield so they form the dipole. Glue or tape this to a stick or piece of dowel.

    Another thing you could show the students is what happens when you turn away from the other antenna. This will show how much radiation comes from the ends of the antenna. If you do this right you will be able to demonstrate how you have a doughnut radiation pattern on a dipole.