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Tuning an LC circuit to a particular EM wave.

  1. Mar 24, 2013 #1
    Hello Everyone,

    I am making an LRC circuit by having a Coil attached to a Capacitor attached to a Light-bulb which is attached to the Coil. Provided that the radio signal (of 2000 Mhz) i'm trying to "collect" is strong enough and the coil/capacitor is properly tuned I should see the light-bulb light up right?

    Secondly in order to tune it I follow this equation?
    2000 Mhz = 1/(2∏sqrt(LC)).

    Thirdly should my coil be vertical to my radio source, horizontal, facing it, in the same direction etc?

    Finally if there is something better than a light-bulb to detect current flow in such system please let me know.

    Thanks for your help :D
  2. jcsd
  3. Mar 25, 2013 #2
    1. Yes that will work provided you get enough current to turn on the light bulb. This depends entirely on the properties of the light bulb. You may need an amplifier.

    2. Yes.

    4. A resistor driving an oscilloscope would be more sensitive. Are the RF waves you are measuring modulated? If they are AM waves you could consider rectifying them.
  4. Mar 26, 2013 #3
    No they are not or if so very slightly.

    Thanks for your help and if you want me to I can pm you the results of my experiments later when I do it as i believe it will be very interesting.
  5. Mar 26, 2013 #4


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    You won't usually be able to use an oscilloscope to view 2000 MHz signals, as most oscilloscopes don't work at that frequency.

    You can just rectify the received signal (modulated or not) and view the resultant DC voltage on a meter. The sensitive 200 mV range of a digital multimeter would be ideal for this.

    There are many designs for "field strength meters". Just Google it.

    If you really mean 2000 MHz, you will need special tuned circuits and diodes. There are microwave oven leakage detectors available that would probably work OK at 2000 MHz.
  6. Mar 27, 2013 #5


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    I have to ask what source of 2GHz signals and receiving antenna that you are planning to use. If you plan to light up a filament lamp this way, you could possibly need to be using a hazardous level of signal. A small (indicator) LED would probably be a better proposition - or a sensitive multimeter, after a rectifier diode.
    It might be an idea to estimate the actual power levels you will be dealing with before you launch into actually building something. Your selective circuit is unlikely to have a very high Q factor. Have you considered the practicalities of making such a circuit for that frequency? You should look at images of successful examples. because a lot of practical, extremely high frequency, RF resonant circuits tend not to look like you might imagine.
  7. Mar 27, 2013 #6
    Throwing in some hypothetical numbers (assuming 2GHz):

    Let's say we have a typical 23dBi parabolic transmitting antenna with a dish diameter of 0.87m representing a typical aperture efficiency of 0.6.

    Let's say receiving antenna is a coil hanging from a circuit, probably poorly tuned and lossy, with a generous gain of 0dBi.

    Let's say distance between antennas is right at Fraunhofer distance from the parabolic (10m).

    Friis equation gives 35dB path loss.

    Let's say we find a tiny light bulb that only requires 1W to light up.

    We need 3.1KW transmitted from the dish at 2GHz. This would raise serious regulatory and safety issues.

    @AkInfinity: can you enlighten us further on your idea? Is it a near field approach?
    Last edited: Mar 27, 2013
  8. Mar 27, 2013 #7
    @ emi and everyone else; Yeah man thanks for your insight here's what i'm doing.

    I was trying to make a circuit with a resonant frequency to that of light. So that i could take it outside (or shine a laser through it) and generate power; thus show that light is an em wave (which after much experimental research i am not too sure it is; although i know we have all been taught that way).

    However I have found my inductor and capacitor to need of extremely small values, both in the 10^-18 range to have the circuits resonant frequency be that of light.

    I have not found any inductors or capacitors that small but if any of you know where to find some please let me know.

    As of now it may not be possible since those values are so small...
  9. Mar 27, 2013 #8
    Done. http://en.wikipedia.org/wiki/Faraday_effect

    Visible light is no different from any other EM wave... the only difference between X-rays and visible light is the wavelength. We can tune lasers from X-ray to ultraviolet to visible to infrared to microwave (called Masers). That's also pretty good evidence, wouldn't you say?
  10. Mar 27, 2013 #9


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    The reason you can't find them is that they don't exist. There are situations where we deal with capacitances and inductances in that range, but then it is usually in nanoscale science; and then often when dealing with the various stray effects. Even the inductance of the "leg" of an ordinary component will be many order of magnitudes higher than that (there is a reason why we work with SMD components and lithographic circuit elements at microwave frequencies).

    It is not possible to make LC resonators (with "discrete" L and C) in the GHz range with good Q values (say higher than 50) if you are working with normal materials at room temperature, the resistive losses at those frequencies are simply too large.

    (for the record, I work with LC resonators that work in this range, but they are superconducting an operated at cryogenic temperatures)

    That said, you can always make a high-Q 2 GHz resonator lambda/2 resonator out of say a copper tube, or maybe a tin-can. These would be 3D geometric resonators which is why Q can be high.
    In the lab I have a 6 Ghz resonator that used to be contained for dried milk, I've just stuck a piece of coax with an SMA connector at one end and a small loop at the other through the lid. Q is a few thousand depending on how I adjust the coupling.
  11. Mar 27, 2013 #10
    @ carl; that is too what i believed before, however upon careful scientific and philosophical research I have found that faraday rotation, nor the tuning/mapping of em waves by hertz is proof that light is indeed an em wave. However I respect that you believe light is a an em wave based on those effects (which may be very well true but i am not 100 percent convinced) :D

    @f95 Yes, after running the numbers I found that it would be a very complex experiment dealing with minute values and thus of course more vulnerable to small effects and noise. Wow great that you work with LC resonators in that range that is a lot of encouragement for me thanks! Could you give me somewhat detailed instructions or refer me to somewhere where i can find how to make this 3D geometric resonator for specific Ghz waves?
  12. Mar 27, 2013 #11
    Just curious where the 2000MHz come from then? Did you mean 2000THz (ultraviolet)?
  13. Mar 27, 2013 #12


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    You do realise how out-field this stuff all is?
    Google Metamaterials if you want an idea of just how hard it is to build the equivalent of 'electric circuits' at optical frequencies. But you seem to have some doubt about the essential similarity between EM waves and light so why are you bothering?
  14. Mar 28, 2013 #13
    @ emi, it was just a sample number to ensure the theory was right that's all, since 2k mhz is a typical doable signal.

    @ sophi I have looked at metamaterials including several lectures by the professor who makes them with microwaves and got his "invisibility cloak" his latest lecture is on youtube and just came out this month (i recommend everyone at leasts skims it). Yes I agree that this is out-field; but as a scientist, through reproducible experiments can we find the truth; which is why I am doing this.

    Apparently f95 works with similar stuff so there is hope to my experiment :D
  15. Mar 28, 2013 #14


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    There are LOTS of people who work with electronics around 2 GHz, e.g. everyone who develops mobile phones and WIFI equipment (2.4 GHz). Not to mention the people who designed the CPU your computer runs at (which is likely to run at a frequency around 2 GHz).

    Making a 2 GHz resonator is not hard -just find a textbook on basic microwave engineering- and making say a LED light up is not -in principle- very difficult either. However, the problem is of course as has already pointed out that you would need potentiall dangerous levels of microwaves.

    Why not try one of the many microwave owen experiments you can find describe on the web instead? There are some safe and interesting ones, say
  16. Mar 28, 2013 #15
    @f95 Yes I misread, I do not want a LC circuit to resonate to 2 Ghz but rather the 400 000 gigahertz range. So f95, is there a way to make a 3d geometric resonator for 400, 000 giga hz?
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