The discussion revolves around the potential for capacitors to function as antennas, particularly in the context of their electric fields when alternating current (AC) is applied. Participants explore whether capacitors can radiate electromagnetic waves and if they can be classified as dipole antennas, examining the underlying mechanisms and conditions necessary for radiation.
Participants express varying opinions on the conditions under which capacitors can radiate, with some agreeing that capacitors alone do not radiate while others propose scenarios where they might. The discussion remains unresolved regarding the classification of capacitors as antennas and the mechanisms involved.
There are limitations related to the assumptions about the physical configurations of capacitors and antennas, the definitions of radiation resistance, and the specific conditions required for effective radiation. The discussion also touches on the complexity of radiation mechanisms in different types of antennas.
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.DianaN said:
Patch antennas work with spacings in the millimeter range. It uses a different radiation mechanism however.tech99 said:
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.DianaN said:
(This comment not intended for DianaN, who asked for a simple explanation).ZVdP said:
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.tech99 said:
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.DianaN said:
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 ideatech99 said:
) 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!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.DianaN said:
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.Victor S said: