Why are parabolic wifi antennas not solid

[russell2002]

Hi,

Parabolic wifi antennas normally are made up of a grid with spaces between rather than a solid dish.

Why is this. And why are satalite dishes not the same.

Thanks.
Russell.

 

[Born2bwire]

Originally posted by: russell2002
Hi,

Parabolic wifi antennas normally are made up of a grid with spaces between rather than a solid dish.

Why is this. And why are satalite dishes not the same.

Thanks.
Russell.

As long as the gaps appear electrically small at the signal's frequency, they are for all intents and purposes, solid. Satellite dishes, at least the small Dish Network styles (the older larger style dishes were not always solid), could be solid to ensure the best reception since the transmitter has to penetrate the atmosphere and be powered locally on the satellite. Still, the dishes may not be solid at all since they have an exterior housing I have never seen what the inside looks like.

 

[russell2002]

Thanks.

Just seen that sky (uk) mini dishes have hundreds of small holes !

 

[stevf]

because they do not need to be. It has to do with the size of the hole vs the wavelength the antenna is designed for. The opening is so small in comparison to the wavelength that it doesnt significantly affect the gain of the antenna.

As to why satellite dishes arent(assuming you are talking about the small tv ones). Dont know exactly why they are designed that way. it is probably more of a design choice of cost/sturdiness vs gain. Perhaps the dish is small so it needs all the surface it can get or perhaps it is the material it is made of, or the designers just like solid design. Dont know what freq those operate at, but if it is high freq(small wavelength) as indicated by the antenna size then perhaps they cant make a suitable mesh with small enough holes not to affect the gain significantly

Steve

 

[highwire]

Originally posted by: stevf
because they do not need to be. It has to do with the size of the hole vs the wavelength the antenna is designed for. The opening is so small in comparison to the wavelength that it doesnt significantly affect the gain of the antenna.

As to why satellite dishes arent(assuming you are talking about the small tv ones). Dont know exactly why they are designed that way. it is probably more of a design choice of cost/sturdiness vs gain. Perhaps the dish is small so it needs all the surface it can get or perhaps it is the material it is made of, or the designers just like solid design. Dont know what freq those operate at, but if it is high freq(small wavelength) as indicated by the antenna size then perhaps they cant make a suitable mesh with small enough holes not to affect the gain significantly

Steve

A big reason for NOT having solid antennas is wind load. Aesthetics may also play a part for large antennas. Below a certain hole size at a given frequency, a perforated antenna is electrically solid anyway, so there is little downside. Additionally, if a single polarization is used, the reflector can simply be be closely spaced strips and this ,too, will act as a solid surface.

The comparisons of the antennas mentioned are wifi at 2.4 ghz and sat antennas at ~12ghz. With dinky sat dish antennas, there is little wind load, so solid is good.

 

[Modelworks]

The same reason satellite dishes started out solid then became wire mesh.
The signal can't pass through the wire mesh holes, so you can build the dish lighter and cheaper by using wire mesh.

Its also the same reason you can look through the door of your microwave and watch the food cooking without getting cooked yourself.

Microwaves are small in length, but big on radius.

 

[Born2bwire]

Originally posted by: Modelworks
Microwaves are small in length, but big on radius.

..........

The holes in the microwave oven doors are waveguides that allow ready transmission of visible light frequencies but only allow highly evanescent modes in the microwave region.

 

[Biftheunderstudy]

I don't think waveguide is the right term here. The microwave oven door is a Faraday cage, ie a conductor which prevents electric fields from propagating through it. The only waveguides in a microwave are the rectangular TEM ones which attach to the magnetron(I really like that word). Since microwaves have large wavelengths compared to the holes in the door the mesh appears to be a solid surface to the microwaves. A similar technique is used in satellite dishes where the mesh reflects the larger wavelength radiation.

 

[highwire]

What he said.
An interesting addition. A quality metal-to-metal contact around the microwave door to complete the cage is not practical. To prevent harmful levels of leakage at the door, a "choke joint" ( no, not bad weed :roll: ) is used. It is a clever feature anyone who has worked with waveguide plumbing is familiar with.
http://www.tpub.com/content/ne...14183/css/14183_51.htm

 

[jjzelinski]

Yeah waveguide would be a bad term to use for a microwave screen door. Waveguide is a RF conductor thats makes use of the "skin-effect" S band and above induce on conductors. The higher the frequency the more the RF will travel through the shield of a cable rather than the center conductor. If you're working with S band RF or above why not just get rid of the center conductor. Hence, waveguide

As for perforated antennas, as long as the radius of the hole doesn't exceed 1/4 the wavelength of the signal you're trying to contain (or reflect) it is electrically solid (as was mentioned earlier.) As for why antennas (or reflectors in many cases) are perforated there are several good reasons like: weight, surface area (wind), aesthetics (kind of like a microwave screen door,) etc. Next time you go to an airport, take a moment to check out its ASR (airport surveillance radar, big guy spinning around atop an 80ft tower) and how you can see through it's reflector. The frequency of that radar is between 2.7 to 2.9 Ghz so if you want to take moment you can Google how to determine the maximum radius of it's perforations

 

[Born2bwire]

Originally posted by: Biftheunderstudy
I don't think waveguide is the right term here. The microwave oven door is a Faraday cage, ie a conductor which prevents electric fields from propagating through it. The only waveguides in a microwave are the rectangular TEM ones which attach to the magnetron(I really like that word). Since microwaves have large wavelengths compared to the holes in the door the mesh appears to be a solid surface to the microwaves. A similar technique is used in satellite dishes where the mesh reflects the larger wavelength radiation.

The holes in the door's screen are cylindrical waveguides. You can sit down and solve for the wave solutions of the waveguide and notice that only evanescent solutions will propagate and thus have an estimation of the amount of power that will leak through. Waveguide is a rather general term, it's a structure the confines and guides the propagation of electromagnetic waves or sound waves. It does not need to be the classical rectangular or cylindrical PEC waveguide as coaxial cables are waveguides, traces on a PCB are waveguides, etc.

 

[HVAC]

Modern satellite dishes may very well be mesh encapsulated in plastic or fiberglass. Don't know...haven't cut one up or studied them.
But I do know that it is difficult to print and read a marketing logo on a mesh. Much better for Dish and DirecTV to paint dishes with solid surfaces.

 

[jjzelinski]

Originally posted by: Born2bwire
<div class="FTQUOTE"><begin quote>Originally posted by: Biftheunderstudy
I don't think waveguide is the right term here. The microwave oven door is a Faraday cage, ie a conductor which prevents electric fields from propagating through it. The only waveguides in a microwave are the rectangular TEM ones which attach to the magnetron(I really like that word). Since microwaves have large wavelengths compared to the holes in the door the mesh appears to be a solid surface to the microwaves. A similar technique is used in satellite dishes where the mesh reflects the larger wavelength radiation.</end quote></div>

The holes in the door's screen are cylindrical waveguides. You can sit down and solve for the wave solutions of the waveguide and notice that only evanescent solutions will propagate and thus have an estimation of the amount of power that will leak through. Waveguide is a rather general term, it's a structure the confines and guides the propagation of electromagnetic waves or sound waves. It does not need to be the classical rectangular or cylindrical PEC waveguide as coaxial cables are waveguides, traces on a PCB are waveguides, etc.

You sound like you're a more informed about the subject than I am but I still question why you would choose the term "waveguide." The holes appear designed to block the RF the magnetron in the microwave is emitting while allowing light to pass through unblocked (shorter wavelength of course.) Doesn't seem much more complicated than that.

 

[Biftheunderstudy]

Ultimately, both waveguide and faraday cage are just terminology. In this case both words can be used and be technically correct. The reason for this is that both phenomenon are easily described using Maxwell's equations--this is really what should be used to describe whats happening, not trying to label or categorize microwave doors or satellite dishes.

 

[Born2bwire]

Originally posted by: jjzelinski
<div class="FTQUOTE"><begin quote>Originally posted by: Born2bwire
<div class="FTQUOTE"><begin quote>Originally posted by: Biftheunderstudy
I don't think waveguide is the right term here. The microwave oven door is a Faraday cage, ie a conductor which prevents electric fields from propagating through it. The only waveguides in a microwave are the rectangular TEM ones which attach to the magnetron(I really like that word). Since microwaves have large wavelengths compared to the holes in the door the mesh appears to be a solid surface to the microwaves. A similar technique is used in satellite dishes where the mesh reflects the larger wavelength radiation.</end quote></div>

The holes in the door's screen are cylindrical waveguides. You can sit down and solve for the wave solutions of the waveguide and notice that only evanescent solutions will propagate and thus have an estimation of the amount of power that will leak through. Waveguide is a rather general term, it's a structure the confines and guides the propagation of electromagnetic waves or sound waves. It does not need to be the classical rectangular or cylindrical PEC waveguide as coaxial cables are waveguides, traces on a PCB are waveguides, etc.</end quote></div>

You sound like you're a more informed about the subject than I am but I still question why you would choose the term "waveguide." The holes appear designed to block the RF the magnetron in the microwave is emitting while allowing light to pass through unblocked (shorter wavelength of course.) Doesn't seem much more complicated than that.

Exactly, and in that sense the holes are waveguides. The higher frequency visible light propagate through the cylindrical holes without any power loss because they are of a wavelength small enough to have numerous modes of propagation. But the physical dimensions of the holes are such that the lower frequency microwaves cannot pass through the holes without significant loss in power. There are two ways at looking at it, the first being that the holes are small enough to assume that the screen forms an electrically solid PEC. But if you were to actually design a microwave, you would have to treat the holes as waveguides to calculate the actual amount of microwave radiation that will leak through (in addition to the leakage from the seams of the door and choke joints). There are guidelines to the amount of radiation that can be emitted by microwaves and one would need to design their product ahead of time with an estimation of the amount of leakage.

 

[f95toli]

Also, in more "critical" applications where EMI suppression is REALLY important (e.g. shielded rooms) the fact that the holes are waveguides with a specficic cut-off frequency and frequency dependent attenuation becomes very important.
E.g. the fact that the SHAPE (and not only the size) of the holes is important, only becomes evident when the full waveguide solutions are used.
In most shielded rooms honeycomb patterns are used instead of round holes in vents etc.

 

[highwire]

Originally posted by: Born2bwire
The holes in the door's screen are cylindrical waveguides. You can sit down and solve for the wave solutions of the waveguide and notice that only evanescent solutions will propagate and thus have an estimation of the amount of power that will leak through. Waveguide is a rather general term, it's a structure the confines and guides the propagation of electromagnetic waves or sound waves. It does not need to be the classical rectangular or cylindrical PEC waveguide as coaxial cables are waveguides, traces on a PCB are waveguides, etc.

The fact that evanescent energy is present on the outer surface of a perforated EM shield is not a sign of "leakage". It should be thought of as being bound to the surface or the wave within. The very rapid decrease in intensity of this energy at even very small distances is a clue that energy lost to radiation not a necessary condition.

While, as was mentioned, everything in this realm can be reduced to Maxwell's Equations, this does not mean that everything can be called anything. I think it is a good thing to respect technology and keep in mind functional descriptions of components. Thus, the box in which I heat my coffee with radio frequency energy in the morning is not a waveguide. It is just a cavity with a lot of standing waves, and hopefully not very leaky. On the other hand, since the functional attribute of "waveguide" generally means conveyance of energy, I very much endorse that in circuits where EM fields are important, that twisted pair, coax cable, circuit traces, really really are waveguides as well.

 

[Born2bwire]

Originally posted by: highwire
<div class="FTQUOTE"><begin quote>Originally posted by: Born2bwire
The holes in the door's screen are cylindrical waveguides. You can sit down and solve for the wave solutions of the waveguide and notice that only evanescent solutions will propagate and thus have an estimation of the amount of power that will leak through. Waveguide is a rather general term, it's a structure the confines and guides the propagation of electromagnetic waves or sound waves. It does not need to be the classical rectangular or cylindrical PEC waveguide as coaxial cables are waveguides, traces on a PCB are waveguides, etc.</end quote></div>
The fact that evanescent energy is present on the outer surface of a perforated EM shield is not a sign of "leakage". It should be thought of as being bound to the surface or the wave within. The very rapid decrease in intensity of this energy at even very small distances is a clue that energy lost to radiation not a necessary condition.

While, as was mentioned, everything in this realm can be reduced to Maxwell's Equations, this does not mean that everything can be called anything. I think it is a good thing to respect technology and keep in mind functional descriptions of components. Thus, the box in which I heat my coffee with radio frequency energy in the morning is not a waveguide. It is just a cavity with a lot of standing waves, and hopefully not very leaky. On the other hand, since the functional attribute of "waveguide" generally means conveyance of energy, I very much endorse that in circuits where EM fields are important, that twisted pair, coax cable, circuit traces, really really are waveguides as well.

Evanescent waves are not confined to surface propagation. They can certainly propagate off of a surface or through a waveguide. One classic example of them propagating away from the surface is the evanescent surface waves produce by dipole antennas on top of a conductive medium (one mode of this propagation is called the Zenneck Surface Wave). Long range radio wave transmission is accomplished by reflections off of the ionosphere but in closer proximity to the antenna the propagation mode is the evanescent wave coming off of the surface of the Earth. The receiving antenna does not need to be on or in contact with the Earth's surface as the Zenneck mode does propagate up off the surface (but again it does so evanescently). There's a good paper by Collin in one of the IEEE journals that dealt with his rederivation of the Zenneck mode.

I am not saying the microwave is a waveguide, I would consider that to be a resonant cavity. The holes cut in the screen of the door are cylindrical waveguides (they have a finite thickness) and as I have stated before, are of such dimensions to support lossless (or low loss, depending on how you model the conductivity and dielectric of the window) modes for the higher frequencies like visible light. The microwave frequencies excited by the microwave's source are below the cutoff frequency of the holes in the screen and thus propagate through the holes in evanescent modes. Once through the holes, the microwaves will propagate off into the room. These waves will contribute to the measurable electromagnetic radiation outside of a microwave and is subject to certain government regulations. In the same vein, computer cases are required to provide a certain degree of shielding. The grids that cover the air vents are designed to allow the readily flow of air but at the same time prevent the microwave radiation from the computer from propagating out of the case. Intel has anechoic chambers on site to perform these measurements of the radiation from computers to ensure that they satisfy regulations.

 

[asearchforreason]

This is really quite clear. The holes in a microwave screen are certainly waveguides, just below cutoff as born2bwire has explained several times. Assuming you are an engineer for a microwave manufacturer, how else do you analyze the leakage from the screen without considering it as an array of very short waveguides? I guarantee you they do this analysis.

There seems to be some confusion though between a surface wave and an evanescent wave. A surface wave may be evanescent in the non-propagating direction (perpendicular to the surface), but for a surface wave to propagate it must have a real k vector along the surface and therefore must not be evanescent. On the other hand, an evanescent mode like that found in a waveguide below cutoff has a complex propagation vector so the wave is quickly attenuated in the direction it propagates. I'm not familiar with the Zenneck wave in particular but if I understand you correctly, the wave is propagating along the air/ground surface and the antenna is excited by the evanescent tail above the surface. In my opinion, it is a misnomer to say that the evanescent tail is propagating up off the surface. It exists off of the surface, but it is not propagating in that direction.

 

[highwire]

Originally posted by: asearchforreason
This is really quite clear. The holes in a microwave screen are certainly waveguides, just below cutoff as born2bwire has explained several times. ...

No, not at all "just below cutoff", more like lambda/100. Thus, an essentially non-radiating point with extremely weak coupling to produce an EM wave. (OK, it won't be zero)

BTW, since the teeny hole "waveguide" has aprox zero length, the strength of the evanescent field at the hole should be the same as the internal field, and the ~1/1000000 reduction in far field will be due almost completely to the near non-existent couplings of the evanescent fields.

There seems to be some confusion though between a surface wave and an evanescent wave. A surface wave may be evanescent in the non-propagating direction (perpendicular to the surface), but for a surface wave to propagate it must have a real k vector along the surface and therefore must not be evanescent. On the other hand, an evanescent mode like that found in a waveguide below cutoff has a complex propagation vector so the wave is quickly attenuated in the direction it propagates.

Agree in toto. The evanescent energy in a cutoff waveguide is being attenuating exponentially, e.g. 2db/mm. (useful for sig generator attenuators) . Since an evanescent field decreases in this fashion, it is impossible for it to propagate more than a few wavelengths away from the surface. It is always associated with a surface and cannot be "out there" in the middle of the room if it originates from the holes in the door of my 12.5cm microwave except as an EM wave through coupling.

I'm not familiar with the Zenneck wave in particular but if I understand you correctly, the wave is propagating along the air/ground surface and the antenna is excited by the evanescent tail above the surface. In my opinion, it is a misnomer to say that the evanescent tail is propagating up off the surface. It exists off of the surface, but it is not propagating in that direction.

I saw a ref to Zenneck and several other names lumped together as the same "surface waves". Dunno. I need to review some of this.
To satisfy my own curiosity about the shape of a loran C wave front, I spent a little time comparing GPS and loran at various altitudes from about ground level to 12k. It must have been inconclusive, because I don't remember the results now. Loran C is 100khz and strictly ground wave. I do remember my theory that since various surfaces retard the wave greater or lesser amounts, the wave would arch ahead at altitude, but that might be wrong. Maybe the wave and ground currents interact to keep the wave vertical.

 

[Blefuscu]

Originally posted by: asearchforreason
This is really quite clear. The holes in a microwave screen are certainly waveguides, just below cutoff as born2bwire has explained several times. Assuming you are an engineer for a microwave manufacturer, how else do you analyze the leakage from the screen without considering it as an array of very short waveguides? I guarantee you they do this analysis.

No. There is no need to calculate it that way. For any reasonable conductor much thinner than the wavelength of radiation, it is just simple Fraunhofer diffraction. There is no mode cutoff. All of the exact modes for a cage with holes in it have field amplitude outside the cage. Cages with slightly smaller holes have slightly less field out there. This is not an evanescent field problem, by the way. Power leaks out and radiates to infinity. Modelling the loss as the transmitted radiation diffracting through a thin screen gives the losses due to the holes. The exact transmission coefficient T = (power emanating from hole/power incident on hole) depends on the shape of the hole but for a round hole much smaller than one wavelength it's of order T=1/3*(k*a)^2*cos(alpha) where k is the incident wavevector for a hole-less cavity, a is the hole radius, and alpha is the angle between the wavevector and the surface normal. Since the holes on my microwave have a=0.05cm and the wavelength is 12.24cm, microwaves at normal incidence will transmit at about T=2*10^-4. For wavelengths much smaller than a, the diffraction analysis gives T = 1, as expected.

Including the thickness of the cage (I assume this is what the other guy meant by a "waveguide") makes the calculation much harder and gives you about the same answer.

 

[antillean]

Originally posted by: Born2bwire
<blockquote>quote:
Originally posted by: russell2002
Hi,

Parabolic wifi antennas normally are made up of a grid with spaces between rather than a solid dish.

Why is this. And why are satalite dishes not the same.

Thanks.
Russell.</blockquote>

As long as the gaps appear electrically small at the signal's frequency, they are for all intents and purposes, solid. Satellite dishes, at least the small Dish Network styles (the older larger style dishes were not always solid), could be solid to ensure the best reception since the transmitter has to penetrate the atmosphere and be powered locally on the satellite. Still, the dishes may not be solid at all since they have an exterior housing I have never seen what the inside looks like.

I think those satellite dishes are solid because it's cheaper. It's not a housing-- the front surface you see is painted metal I think. They just fold back the sides so it looks thick. Which do you think is easier to make, a sturdy mesh on a stiff structure, or a piece of formed sheetmetal?

 

[Born2bwire]

Originally posted by: antillean

Originally posted by: Born2bwire
<blockquote>quote:
Originally posted by: russell2002
Hi,

Parabolic wifi antennas normally are made up of a grid with spaces between rather than a solid dish.

Why is this. And why are satalite dishes not the same.

Thanks.
Russell.</blockquote>

As long as the gaps appear electrically small at the signal's frequency, they are for all intents and purposes, solid. Satellite dishes, at least the small Dish Network styles (the older larger style dishes were not always solid), could be solid to ensure the best reception since the transmitter has to penetrate the atmosphere and be powered locally on the satellite. Still, the dishes may not be solid at all since they have an exterior housing I have never seen what the inside looks like.

I think those satellite dishes are solid because it's cheaper. It's not a housing-- the front surface you see is painted metal I think. They just fold back the sides so it looks thick. Which do you think is easier to make, a sturdy mesh on a stiff structure, or a piece of formed sheetmetal?

They could just stamp it out from a sheet of aluminum or other light metal and then sandwich it between a plastic housing to provide structural support. I could imagine it being done in two pressings, one to cut out excess metal, the second to form the metal around a die for the parabolic shape. The dish part is purely a reflector, all it needs to be is a sheet of metal. The plus side with using a housing is that it would allow you to decrease the weight of the unit to allow for securer mountings. It's been a while since I handled a dish but it was my recollection that it was plastic or some other material when my folks had theirs installed.