Dielectric Rod Antenna: Can It Radiate?

[smile]

I have a fundamental question about the antennas made by using dielectric materials that I have not found a satisfied answer online. According to Wikipedia, antennas are described as,
"In radio, an antenna is the interface between radio waves propagating through space and electric currents moving in METAL CONDUCTORS, used with a transmitter or receiver."
How come is it possible that a dielectric material can be used as a radiating antenna?

 

[sophiecentaur]

I have a fundamental question about the antennas made by using dielectric materials that I have not found a satisfied answer online. According to Wikipedia, antennas are described as,
"In radio, an antenna is the interface between radio waves propagating through space and electric currents moving in METAL CONDUCTORS, used with a transmitter or receiver."
How come is it possible that a dielectric material can be used as a radiating antenna?

The EM power for any antenna is carried ('guided') by a metal feed line. this is the same for a dielectric antenna and the energy is then launched onto a dielectric structure which shapes the phase across the wave front to form the required beam shape. Your quote from Wiki is strictly true - it just doesn't;t necessarily mention the dielectric structure that some antennae use in between the metal and space.
In many ways, beam forming with a dielectric shape works the same as a curved glass lens does with light. Light rays are deflected by different amounts through the different thicknesses of the lens and that can focus or spread the light. A dielectric lens introduces different phase shifts due to its varying thickness or, sometimes, its varying density. This can form a beam of em waves in a particular direction. For practical reasons, dielectric antennae are usually used for microwave wavelengths

 

[davenn]

How come is it possible that a dielectric material can be used as a radiating antenna?

I'm not aware of such, where did you hear/read this ?
what reference to it do you have ?

 

[davenn]

Dielectric resonator antenna
From Wikipedia, the free encyclopedia
(Redirected from Dielectric Resonator Antenna)

Jump to navigationJump to search
A dielectric resonator antenna (DRA) is a radio antenna mostly used at microwave frequencies and higher, that consists of a block of ceramic material of various shapes, the dielectric resonator, mounted on a metal surface, a ground plane. Radio waves are introduced into the inside of the resonator material from the transmitter circuit and bounce back and forth between the resonator walls, forming standing waves. The walls of the resonator are partially transparent to radio waves, allowing the radio power to radiate into space.[1]

An advantage of dielectric resonator antennas is they lack metal parts, which become lossy at high frequencies, dissipating energy. So these antennas can have lower losses and be more efficient than metal antennas at high microwave and millimeter wave frequencies.[1] Dielectric waveguide antennas are used in some compact portable wireless devices, and military millimeter-wave radar equipment. The antenna was first proposed by Robert Richtmyer in 1939.[2] In 1982, Long et al. did the first design and test of dielectric resonator antennas considering a leaky waveguide model assuming magnetic conductor model of the dielectric surface .[3]

An antenna like effect is achieved by periodic swing of electrons from its capacitive element to the ground plane which behaves like an inductor. The authors further argued that the operation of a dielectric antenna resembles the antenna conceived by Marconi, the only difference is that inductive element is replaced by the dielectric material.[4]

OK this talks about dielectric resonators that are allowed to radiate, which sort of makes them an antenna.
I'm very familiar with the mentioned dielectric resonators, there's one in virtually every satellite dish LNB/LNA (low noise block/low noise amplifier)
The ceramic resonator is inside a cavity ( as stated above) but it doesn't have a RF transparent window that allows RF radiation to radiate out from the cavity

In a dielectric resonator cavity, the size of the ceramic resonator is used to determine/fix the frequency of operation by altering the physical size of the cavity.
RF DOES NOT radiate directly off the dielectric ceramic rod/puck and I suspect it's the same in the above reference.

If you are suggesting something different, I would love to see references. I would like to see the technologyDave

 

[Tom.G]

I would love to see references. I would like to see the technology

http://www.rfwireless-world.com/Articles/Lens-Antenna-basics-and-types.html

And many others found with: https://www.google.com/search?&q=rf+dielectric+lenses

 

[davenn]

http://www.rfwireless-world.com/Articles/Lens-Antenna-basics-and-types.html

And many others found with: https://www.google.com/search?&q=rf+dielectric+lenses

but that has nothing to do with the original post or the thread title …..
the lens systems you have linked to are focussing the transmitted or received RF, they are not responsible for the initial RF radiation

Now maybe that is what the OP really intended to talk about, in which case, he/she didn't describe that very well
the thread title and OP inferred that the actual radiator was a dielectric material, you link shows that that isn't the case

Dave

 

[sophiecentaur]

If you are suggesting something different, I would love to see references. I would like to see the technology

Google Images will show you dozens and dozens of examples dielectric antennae.
It's obvious that there needs to be an interface between the feeder and the radiator and that involves appropriate matching but there's nothing different in principle between a radiator that consists of a number of parasitic metal elements and a radiator that is made of dielectric. Directivity is always achieved by tailoring the phases of the wave front that's generated to favour a particular direction and shape of beam. A Yagi antenna does this by modifying the wave front of an isolated drive element.
As with a lot of technologies, there are more suitable and less suitable applications. Convenience of production is a big plus for some dielectric designs.

 

[marcusl]

The first microwave scanning phased array antenna was built during WW2 by the MIT Rad Lab--it is the Mark 8 fire control radar antenna and it used polyrod radiating elements.

The polyrod element guided and radiated EM waves just as we now guide and radiate light through an optical fiber. You can read about them here
https://archive.org/details/bstj26-4-837

 

[sophiecentaur]

@davenn . I'm not sure where your objection to the "resonator" bit comes from. What is a simple matched dipole if it is not a resonator?

 

[davenn]

@davenn . I'm not sure where your objection to the "resonator" bit comes from. What is a simple matched dipole if it is not a resonator?

sorry, I don't understand what you are arguing against ?D

 

[sophiecentaur]

sorry, I don't understand what you are arguing against ?

Actually I wasn't sure what you were arguing against. The OP has a problem with interpreting an over simplified Wiki description and taking it too literally. The wiki statement in post#1 does not exclude a dielectric being the radiating structure.
It's quite possible that we are cross purposes. Your quoted Wiki passage makes good sense to me in how it describes the way the antenna radiates.

RF DOES NOT radiate directly off the dielectric ceramic rod/puck and I suspect it's the same in the above reference.

Doesn't the reference say that it does?
"The walls of the resonator are partially transparent to radio waves, allowing the radio power to radiate into space"
Some resonators radiate (eg a dipole) and some don't (eg a metal cavity).

 

[davenn]

Doesn't the reference say that it does?

no

the dielectric is used as a lens focussing the RF that comes of a radiator

Dielectric lens antenna

"The walls of the resonator are partially transparent to radio waves, allowing the radio power to radiate into space"

but the dielectric ISNT generating the signal … the signal is being fed into the resonator.
Or in cases where there isn't a cavity resonator, like in the image above or in something like this …..

In all of the examples, and there's lots more on the net, the dielectric ISNT doing the initial radiating, RF is being launched into the dielectric rod which
then performs beam shaping.
This includes the radar antenna that @marcusl posted aboveDave

 

[tech99]

I have a fundamental question about the antennas made by using dielectric materials that I have not found a satisfied answer online. According to Wikipedia, antennas are described as,
"In radio, an antenna is the interface between radio waves propagating through space and electric currents moving in METAL CONDUCTORS, used with a transmitter or receiver."
How come is it possible that a dielectric material can be used as a radiating antenna?

When we say "radiation" we mean the permanent loss of EM energy from the antenna. Radiation is caused when a charged particle is accelerated. Both metals and dielectrics contain electrons which can be accelerated, so both will radiate. However, a vacuum contains no charges, so for instance, a vacuum filled capacitor will not radiate.
Examples of dielectric antennas include an open optical fibre or a laser.
If we have a dielectric lens or rod antenna, it can be connected to our transmitter or receiver using a dielectric waveguide, so a transition to metallic conductors is not necessarily involved.

 

[sophiecentaur]

a vacuum filled capacitor will not radiate.

Really? That would imply that a dipole wouldn't work either, I think. The presence of air between the plates just modifies the dielectric constant a bit. Free space also has permittivity and there are still E and H vectors at the edges of the capacitor.

the dielectric ISNT doing the initial radiating,

etc.
This is an angels on a pinhead argument that we're having. You are calling the 'feed point' the 'original radiator' but I'm not sure that's really justified. The presence of the dielectric around the feed point is affecting the impedance that's seen - just as the parasitics on a Yagi modify the impedance and direct the energy. The only thing that is undisputably 'radiating' is the antenna as a whole.
Classification and terminology can waste a lot of time and we're only arguing where to draw an arbitrary dotted line to separate the feed and radiation functions.

 

[Baluncore]

An axial dielectric rod can be used to focus radiation from a launcher such as a feed horn. The rod does not launch the EM wave, it focuses, or aligns it, after it has been released from the conductive transmission line and antenna structure.
The velocity of the EM wave in the dielectric is lower than in air so the energy tends to follow the rod surface, until it runs out of rod and continues through space.

 

[sophiecentaur]

The rod does not launch the EM wave, it focuses, or aligns it,

In your model, what is the difference between the feed of a dielectric antenna and the feed of an antenna with conventional metal parasitics? Plus, are you sure that the dielectric structure (particularly the parts right next to the drive) must not be considered to be part of the 'radiator'?
In particular, in the case of a resonator antenna, does your model still hold.
I really shouldn't have got so involved in this because Does it really matter what we call the radiator? Any hard rule is bound not to fit all cases and it can only confuse the innocent.

 

[tech99]

Really? That would imply that a dipole wouldn't work either, I think. The presence of air between the plates just modifies the dielectric constant a bit. Free space also has permittivity and there are still E and H vectors at the edges of the capacitor.
etc.

My understanding is that, for the case of a dipole, the radiation occurs when the electrons in the wire are accelerated. The capacitance between the ends (for instance the plates on the ends of a Hertzian Dipole) provides a non-radiating return path for the current. If this were not the case, the radiation from the wire and the capacitance would cancel. Although Maxwell tells us that the current flowing between the plates of a capacitor creates a magnetic field, this field is in quadrature to the E-field, not in-phase with it as with a radiated wave. (I am very interested in comments on this topic).

 

[sophiecentaur]

My understanding is that, for the case of a dipole, the radiation occurs when the electrons in the wire are accelerated. The capacitance between the ends (for instance the plates on the ends of a Hertzian Dipole) provides a non-radiating return path for the current. If this were not the case, the radiation from the wire and the capacitance would cancel. Although Maxwell tells us that the current flowing between the plates of a capacitor creates a magnetic field, this field is in quadrature to the E-field, not in-phase with it as with a radiated wave. (I am very interested in comments on this topic).

The Radiation Resistance of an 'ideal' Capacitor is very low but it must have dimensions (a spacing) so it will radiate energy, albeit inefficiently.

 

[Baluncore]

In your model, what is the difference between the feed of a dielectric antenna and the feed of an antenna with conventional metal parasitics?

I think we can say that EM energy always travels as the poynting vector of an EM wave. The EM wave may be a spherical wavefront in free space, or it may be guided by a conductive transmission line or waveguide.

There is no question that an optic fibre is also a type of waveguide, with an internally reflective surface, often with a section graded in RI. It is therefore also a dielectric rod. If you cut a fibre it will radiate.

A dielectric lens flattens the spherical radiated wavefront, so as to increases the gain. A dielectric rod antenna is small in diameter but long in wavelengths. The dielectric rod is often tapered to a point, which makes it an end-fire element with gain.

You may be right, it depends on where you draw the line, metallic conduction, guided or a spherical wave.

(I am very interested in comments on this topic).

I would like to buy an argument. Consider a resonant half-wave dipole in free space. Model it as an inductive wire in the middle with capacitance between the ends. The current at the centre will be a maximum as the voltage between the ends passes zero. V and I are therefore in quadrature. But since the intrinsic impedance of free space is real, with a value of about 377 ohms, an EM wave in space has the E and H components in phase.

What gives?

The current in the element seems to generate the magnetic component. That H then creates the perpendicular E electric field which permits the wave to proceed through space. It seems the voltage on the dipole is irrelevant.

 

[davenn]

An axial dielectric rod can be used to focus radiation from a launcher such as a feed horn. The rod does not launch the EM wave, it focuses, or aligns it, after it has been released from the conductive transmission line and antenna structure.
The velocity of the EM wave in the dielectric is lower than in air so the energy tends to follow the rod surface, until it runs out of rod and continues through space.

Exactly ... and that's what I have been trying to put across on several occasions to sophi and others
Again, the dielectric rod/lens whatever its shape is ONLY beam focussing/shaping the RF coming into it from the initial normal style radiator

Thanks BaluncoreDave

 

[sophiecentaur]

the initial normal style radiator

And what would that be? It could just as easily be the end of the feeder. But the feeder is merely guiding waves from the transmitter output device. I think that trying to draw a particular line is not achieving anything. Of course dielectrics behave differently from pieces of metal but most RF electronics has both involved at nearly all stages.
I can make a metal 'lens' or a metal reflector or I can do similar jobs with dielectrics. Do the two functions get different names just because they are made ofd different things? Likewise, at some stage within any antenna, there is a feed, followed by various bits and bobs. Why should any of those bits and bobs have an official name 'radiating element'? A cast iron name is more likely to cause confusion (as with the OP) than not.

 

[tech99]

Exactly ... and that's what I have been trying to put across on several occasions to sophi and others
Again, the dielectric rod/lens whatever its shape is ONLY beam focussing/shaping the RF coming into it from the initial normal style radiator

Thanks BaluncoreDave

I think the rods and cones of the eye are dielectric antennas. The incoming radiation causes electrons in them to create a chemical reaction directly, without the need for any metal.

 

[tech99]

The Radiation Resistance of an 'ideal' Capacitor is very low but it must have dimensions (a spacing) so it will radiate energy, albeit inefficiently.

If we use the Larmor Equation to find the power radiated by a widely spaced capacitor, assuming vacuum conditions, as there are no charges between the plates, there will be zero radiation. (I agree the connecting wires will radiate).

 

[sophiecentaur]

If we use the Larmor Equation to find the power radiated by a widely spaced capacitor, assuming vacuum conditions, as there are no charges between the plates, there will be zero radiation. (I agree the connecting wires will radiate).

If you can vary the charge on the capacitor without any connecting wires then fair enough. The Larmor Equation applies to an idealised situation so it can't really apply to reality - as you acknowledge. Anyone who really believes that a Capacitor in deep space will behave differently (significantly) than in air, needs a bit of experience of RF Engineering.

 

[tech99]

If you can vary the charge on the capacitor without any connecting wires then fair enough. The Larmor Equation applies to an idealised situation so it can't really apply to reality - as you acknowledge. Anyone who really believes that a Capacitor in deep space will behave differently (significantly) than in air, needs a bit of experience of RF Engineering.

I was trying to obtain agreement that the dipole radiation comes from the wires, not from the capacitance. The electric field on the ends is not the radiated electric field. It will vary with the Q of the system, and its role is to provide the accelerating force for the electrons in the wire.
Of course, I realize that a few molecules of air make little difference, I just suggested vacuum to eliminate a point of argument. I have been an RF experimenter for 60 years incidentally.

 

[smile]

Thank you very much everyone, all the replies were very helpful for me.

 

[sophiecentaur]

I was trying to obtain agreement that the dipole radiation comes from the wires, not from the capacitance. The electric field on the ends is not the radiated electric field. It will vary with the Q of the system, and its role is to provide the accelerating force for the electrons in the wire.
Of course, I realize that a few molecules of air make little difference, I just suggested vacuum to eliminate a point of argument. I have been an RF experimenter for 60 years incidentally.

What you are describing would be an end loaded dipole and the presence of the 'plates' would modify the impedance, increase the current in the wires. So I don't know how that relates to what you say. The composite antenna would have a higher radiation resistance (one way of looking at it). More power would be radiated. Is this another of those "what is really happening?" questions? Is it only the current in a dipole that is the radiating agent?

"I have been an RF experimenter for 60 years incidentally" doesn't surprise me at all. It shows in many of your comments.

 

[davenn]

I think the rods and cones of the eye are dielectric antennas. The incoming radiation causes electrons in them to create a chemical reaction directly, without the need for any metal.

no, there is still a metal antenna element behind them see drawings

the dielectric sections are purely for beam-forming ... producing the desired radiation pattern

essentially, they are acting as waveguides

 

[davenn]

In your model, what is the difference between the feed of a dielectric antenna and the feed of an antenna with conventional metal parasitics? Plus, are you sure that the dielectric structure (particularly the parts right next to the drive) must not be considered to be part of the 'radiator'?

well for a start a dielectric DOESNT conduct current (RF or otherwise) You cannot put the 2 conductors of the coax cable onto one end of the dielectric rod and hope that RF is generated ... it won't be. And that is what makes the difference between the radiator and the beam forming section
The metallic radiator launches an EM wave into the dielectric which then shapes that EM wave into the desired pattern

In particular, in the case of a resonator antenna, does your model still hold.
I really shouldn't have got so involved in this because Does it really matter what we call the radiator? Any hard rule is bound not to fit all cases and it can only confuse the innocent.

of course it does ... repeat my above commentI'm not sure why several of you guys are finding this difficult

 

[Tom.G]

Rule 1) The Boss is always right.
Rule 2) If the Boss is wrong, see Rule 1.

Looks like too many "Bosses" here.

(Of course there is a much more crude way of expressing the above, but it involves description of a bodily function.)

Anyhow, it's entertaining!

 

[sophiecentaur]

You cannot put the 2 conductors of the coax cable onto one end of the dielectric rod and hope that RF is generated

Yes, of course that's right. The waves have to be matched into the dielectric just as with any antenna. If it's a dielectric resonator antenna, the impedance to match to could be very relevant.
Edit: PS just connecting a coax to any old part of any antenna doesn't guarantee it will radiate significant power, whether it's made of metal or a dielectric. It has to be done 'appropriately'. Which part of a regular antenna do you call the 'radiator' and which part do you call the beam-former?

 

[Baluncore]

Which part of a regular antenna do you call the 'radiator' and which part do you call the beam-former?

It might pay to consider if there is a transducer, something that transforms one form of energy into another form. A transmit antenna is a transducer that adapts one end of a non-radiating feedline into an EM wavefront. A receive antenna is a transducer that performs the reciprocal conversion.

So where is the transducer in an antenna? Where does energy change form?
A transmission line is not a transducer.
A waveguide is not a transducer, it is a hollow transmission line.
A dielectric lens is not a transducer.
A horn on a waveguide is not a transducer.

A parabolic reflector is not a transducer, even though it changes the hand. Or is it? The propeller on an aircraft or boat is often considered to be a disc transducer that converts torque into thrust. How much change of energy form is required? Is the transducer the coupling loop into a waveguide, or is it the horn at the radiating end of the waveguide? It is all EM waves, even in a twisted pair. Where do we draw the line?

 

[sophiecentaur]

A receive antenna is a transducer that performs the reciprocal conversion.

That's one way of looking at it. To my mind, the only transducers involved are the transmitting output device and the receiver front end. They actually do change the 'form' of energy. Wires, wave-guides, antennae are only constraining and directing EM Power that's already been generated elsewhere.
But there's no end to this subject because we all have our own model in our heads.
Why can't everyone see things my way?

 

[tech99]

I would like to buy an argument. Consider a resonant half-wave dipole in free space. Model it as an inductive wire in the middle with capacitance between the ends. The current at the centre will be a maximum as the voltage between the ends passes zero. V and I are therefore in quadrature. But since the intrinsic impedance of free space is real, with a value of about 377 ohms, an EM wave in space has the E and H components in phase.

What gives?

The current in the element seems to generate the magnetic component. That H then creates the perpendicular E electric field which permits the wave to proceed through space. It seems the voltage on the dipole is irrelevant.

My take on this is based on experiments which I undertook as an amateur scientist. There are some imperfections still in understanding. The inductance and capacitance are not part of the radiation mechanism. They store reactive energy and create induction fields which are in quadrature. At switch-on, these fields charge up over a few cycles and at switch-off they similarly decay. The L-C resonance makes it easy to drive a current into the antenna, only the Radiation Resistance being involved if losses are small.
The radiation is caused when the voltage at the feedpoint accelerates electrons in the wire. This causes the static fields of the electrons to be tilted, creating a transverse radiated E-field. When this electric wave passes a magnetic sensor, as a consequence of Relativity we see a magnetic field.
The acceleration also causes the electrons to have a velocity, and this equates to a current. The magnetic field of this current is initially indistinguishable from the B radiation, as it is in-phase and falls off with 1/r. However, at a distance of lambda/2 pi, it is found that this field begins to decay more rapidly, whereas the radiated B field continues outwards as radiation, falling with 1/r for ever.
The radiated E-field remains constant out to lambda/2pi, but retards in phase. It then falls with 1/r for ever. The induction E-field does not retard in phase with distance - I believe Hertz noticed this.
Very close to the feedpoint we see a local E-field caused by the feeder voltage. This voltage is equal to the volt drop in the Radiation Resistance, and may be equated to the transverse E-field generated by the accelerating electrons. The induction E-field from the dipole ends is strong near the ends but small near the equatorial plane of the antenna.
In a general case, if we bring probes near a radiating dipole, we see that the E-field initially increases at 6dB per octave of distance as we approach, but remains about constant from lambda/2 pi until very close to the antenna. Very close we see a 6dB increase due to the feeder voltage.
With a magnetic probe the field increases 6dB per octave all the way until we touch the centre of the antenna, so may be typically 20dB or so greater than the E-field when touching the conductor.
If we use a terminated dipole as a probe, the received power remains constant from lambda/2 pi until we physicaly touch the antenna. When we touch the antenna there is no jump in received power. Closer than lambda/2 pi the received power is always half the transmit power, so the path loss between two dipoles never falls below 3dB. This is because when they touch, they form one antenna and half the power is radiated and the other half delivered to the receiver.
.

 

[Baluncore]

The inductance and capacitance are not part of the radiation mechanism. They store reactive energy and create induction fields which are in quadrature.

What is all this sinusoidal stuff with λ and 2π ? Ever since Sir Oliver Lodge, back in 1897, wandering the gas-lit streets of London, to invent frequency tuning, we have been fighting an uphill battle to spread the spectrum. Only the designers of cheap λ/2 dipole antennas need sinusoidal waves. The few of us who are over tuned circuits, are content to get it right by launching the radiant energy at the first attempt. We do not have to tune everything to resonance, then hope that maybe a little bit more will leak out next time, with the inevitable progressive loss of signal coherence. Who in their right mind would stick with inefficient radiators, sinewaves and limited bandwidth.

All an antenna needs to do is to match the impedance of the feedline to that of free space. There is no requirement that the signal be sinusoidal, nor that the antenna be a tuned quarter wave transformer.

OK, I did say I wanted to buy an argument. I am a discerning buyer and believe the one I have here may be of a premium quality, at lower cost. It is as easy to understand as is falling off a log.

When you nudge an electron one way, on the surface near the midpoint of a conductive dipole, there are two equal but opposite electromagnetic disturbances that travel away towards their respective ends. Those disturbances are EM waves guided by their own Narcissian reflection, seen in electrons on the surface of the conductor. The E and M fields of each disturbance are in phase since they are traveling in opposite directions along a non-reactive line. At some point the disturbance must reach a point where the impedance on the dipole surface passes the intrinsic impedance of free space, Zo = Uo * c ≡ 4π * 29.97924580 ≈ 376.73 ohms. At that point the EM wave will be unable to differentiate between the dipole surface and free space. It is not surprising that, once it encounters that perfect match, the wave falls away from the dipole, to radiate outwards into space. The two disturbances traveling away from each other, sum in phase broadside to the dipole, to create the typical dipole radiation pattern. There will be some radiation from all parts of the dipole, but it will be limited by the impedance mismatch between the conductive line and free space.