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Is a capacitor an antenna?

  1. Jul 29, 2016 #1
    I was looking up capacitors and the E-field that is produced by a capacitor and I couldn't find much literature about how the E-field changes when AC is flowing through a capacitor. Is this similar to the propagation of an antenna? Do capacitors radiate? Could a capacitor be considered a dipole antenna? Any answers and pointers to books would be much appreciated.
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  3. Jul 29, 2016 #2
    If you look at a picture of the first antenna, used by Heinrich Hertz, you will see it looks like a capacitor. It has two plates separated by about a metre and a wire linking them together. A generator of very high frequency AC is inserted at the middle of the wire. The separation of the plates and the wire are crucial in order to obtain radiation. The generator causes charges to be accelerated back and forth along the wire, as the capacitor plates charge first one way then the other. It is this acceleration of the charges which causes radiation. If there were just two plates closely spaced, this would not occur, so that we can say that a capacitor by itself does not radiate.
  4. Jul 29, 2016 #3
    Alright, so it's the insulator that is the problem. If the two plates would be connected through a resistor, it would radiate like an antenna, right?
    Last edited: Jul 29, 2016
  5. Jul 29, 2016 #4
    Patch antennas work with spacings in the millimeter range. It uses a different radiation mechanism however.
  6. Jul 29, 2016 #5


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    A “capacity hat” is often used on vertical dipole antennas to reduce the required height for resonance. The inductance of the vertical is tuned to be in resonance with the capacitance between the hat and ground. The current that drives the vertical conductor radiates the magnetic field component.
  7. Jul 29, 2016 #6
    The resistor is physically small, so radiation will be small. To obtain efficient radiation we usually need a structure that is approaching half a wavelength in length. In this way the structure can be resonant at the wavelength of interest, allowing the charges to be given large acceleration over a long length.
  8. Jul 29, 2016 #7
    (This comment not intended for DianaN, who asked for a simple explanation).
    Not sure I agree that the radiation mechanism is totally different for a patch antenna, which is sometimes regarded as a slot. So far as I can see with such antennas, the radiation still arises because charges in the metal structure are accelerated. This occurs when the current in the sheet is forced to deviate around the slot. It is rather like an object placed in an airstream.
    If it is alternatively proposed that radiation occurs due to fields within the slot, it may be observed that radiation does not occur if the surrounding sheet is cut down to be just a wire. Further, radiation still occurs equally well if a slot is minutely narrow.
  9. Jul 30, 2016 #8
    Further comment: I would agree, however, that radiation can also arise from the acceleration of electrons in a dielectric, though not from a vacuum and "not much" from air. There are, of course, dielectric antennas such as polyrods. For a patch antenna, a relatively thick dielectric substrate would contribute to the radiation.
  10. Jul 30, 2016 #9


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    No need for a "resistor". There is no resistor connected between the ends of a dipole or in series. (But of course, any source of RF will have its own internal series resistance). Current will flow round the open loop, consisting of the source and the capacity between the plates (completing the loop for RF). But there is also the effect of the free space around the circuit. For a half wavelength dipole, the capacitance and the inductance of the wire just about cancel out so you would expect the input impedance to be zero BUT, because energy is being radiated, you will measure a resistance (called the radiation resistance) of about 70Ω. This is not an 'added component', it's there because of the radiation and the transmitter 'sees' that resistance as its load. It only has that value at the dipole tuned frequency. As the dipole gets shorter and shorter, the radiation resistance goes down and down and the reactance increases and is capacitive. If you turn the wires into plates, you get a capacitor with an extremely low radiation resistance. The radiation resistance is very very low so you cannot feed such a radiator efficiently (the transmitter output resistance will dissipate most of its own power). Small antennae are not what we want unless there is no option.
  11. Jul 30, 2016 #10


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    It's a bit confusing to introduce patch antennae here - definitely third year work, I think. However, it is worth making the point that a slot, cut in a large sheet of metal behaves very much like a wire dipole; you just have to feed it in a different way. For every wire arrangement, there is, at least in principle, an equivalent slot / patch antenna. (Don't ask me about specific designs - I would have no idea :wink:) Phones and cars are fairly new technology and the change from sticky up whips has been a good thing - even if only to outwit the hooligans in car parks!
  12. Jul 21, 2017 #11
    I would look into Steinmetz' Book "Electric Waves, Discharges, and Impulses", concerning dielectricity.

    Also, look into how power is transformed through antennas and transformers. There is a similarity within discrete capacitor energy transformations, but the difference is in its convergent vector. This is older science and may only be relevant to theoretical study, but I think it will help understand things from the original source.
  13. Jul 22, 2017 #12


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    Pretty much anything you think of can be considered as an antenna. Whenever you have alternating currents and PDs EM wave are radiated. It's just a matter of degree.
    (An old Aerial Engineer I knew in the BBC used to say you could match one of their HF transmitters into a dead sheep, if you wanted to.)

    Ye Gods. That is certainly a Rave from the Grave (1911). I would hesitate to take that as my source of EM knowledge. There is not even a mention of "Radiation Resistance" in the Index. RR is a pretty fundamental concept when dealing with radiating structures. It's not even very new. Plenty of Text books on Antennae that were published before WW2 use it. Well established books by Kraus and by Jordan will give an easier (up to a point) approach.
  14. Jul 22, 2017 #13

    To gather as many sources for understanding fundamental energy forces is a good thing, and Steinmetz certainly produced good fruit.

    I find that many people get confused by the definition of permittivity, because it almost seems to be an inverted description. When you read about why it was called permittivity in the first place, it makes a lot more sense, and that was the point for the answer.

    I would gather that most people do not know that since Einstein, the definition had changed.
  15. Jul 22, 2017 #14


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    It"s interesting as a piece of history but isn't it a bit heavy going when it has since been developed in a more usable way and now antennae are all commonly recognised? Someone pre WW1 would not have made much sense of the idea of transmitting dipole arrays but everyone knows a bit about them.
    "fundamental energy forces". What are they?
  16. Jul 22, 2017 #15
    I didn't mean to have anyone over think "fundamental energy sources", I was using it in its broad sense.

    The difference of the definition of permittivity is that it represented a materials "ability" to form a dielectric field, and not its ability to reject an electric field.

    Where the differences lay according to Faraday through Steinmetz, that the magnetic field had a divergent, circular field force, as opposed to the dielectric convergent, radial field force.

    To understand the original meaning of the forces may be helpful in understanding how they are understood today. And this is directly applicable to the original question in the thread.
  17. Jul 22, 2017 #16


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    I think Maxwell sorted it out OK, didn't he?
  18. Jul 22, 2017 #17
    I forget to add Maxwell's name to the list. He certainly used the same understanding in his equations, it's just that "permittivity" was changed later. Steinmetz is employing Maxwell's equations. The math still works out the same, there is just a different understanding from modern views. The history prevents the "42" syndrom IMHO.
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