Satellite dish design

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dmascare
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Re: Satellite dish design

Post by dmascare » Tue Aug 16, 2005 7:53 am

Bascially I am going to have a sensor node that consists of an impedance measurement chip, an atmel microprocessor, and an atmel U3280 RFID transponder. Ideally we would like to be able to deliver enough power to the sensor node so the microprocessor can localy analyse the data and then send a signal back to the reader to indicate what amount of damage exists. I don't have any number yet on how much power this will require. I am still in the process of learning C to programs the microprocessor.
Anyways, this discussion brings up another question that has kind of been bugging me. What is the reason that they even made RFID tags operate at 125 KHZ in the first place? I know some active tags operate at 13.56 MHz, 860-930 Mhz, and some at 2.4 Ghz. I am starting to think that the 125 Khz transponder may be a lost cause. I think I am going to see if I can find some transponder that will operate at the higher frequencies. That seems to be a better way to go.

dmascare
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Re: Satellite dish design

Post by dmascare » Tue Aug 16, 2005 7:56 am

Bascially I am going to have a sensor node that consists of an impedance measurement chip, an atmel microprocessor, and an atmel U3280 RFID transponder. Ideally we would like to be able to deliver enough power to the sensor node so the microprocessor can localy analyse the data and then send a signal back to the reader to indicate what amount of damage exists. I don't have any number yet on how much power this will require. I am still in the process of learning C to programs the microprocessor.
Anyways, this discussion brings up another question that has kind of been bugging me. What is the reason that they even made RFID tags operate at 125 KHZ in the first place? I know some active tags operate at 13.56 MHz, 860-930 Mhz, and some at 2.4 Ghz. I am starting to think that the 125 Khz transponder may be a lost cause. I think I am going to see if I can find some transponder that will operate at the higher frequencies. That seems to be a better way to go.

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Externet
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Re: Satellite dish design

Post by Externet » Tue Aug 16, 2005 8:52 am

Hi.
Guessing, that frequency was/is the used at the exit of stores to detect shoplifters, where you see the short handrail looking loops near the door.
Miguel
- Abolish the deciBel ! -

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jwax
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Re: Satellite dish design

Post by jwax » Tue Aug 16, 2005 11:35 am

RFIDJournal describes frequencies used-
http://www.rfidjournal.com/article/arti ... What-28258
125 KHz because it's cheap.

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Tue Aug 16, 2005 12:12 pm

All parabolas are reflectors, even at 125 kHz. <p>The part that emits the radio frequency is not the antennae dish, it’s the element. [feed horn]<p>And once the RF has left the proper element, any dish will reflect the signal and concentrate it more than free air. Same goes for recieving unless the dish is too tiny to be effective at gathering.

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jwax
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Re: Satellite dish design

Post by jwax » Tue Aug 16, 2005 4:10 pm

Unless you have access to Arecibo, no dish of a practical size is going to give any substantial gain (directivity) at 125 KHz.

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Tue Aug 16, 2005 4:23 pm

Wrong, the antennae for RFID is not Arecibo in size and it works as small as it is, and it can be placed in front of any parabola for a focused gain. The parabola or dish is not the antenna, it is a lens.

dmascare
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Re: Satellite dish design

Post by dmascare » Tue Aug 16, 2005 5:16 pm

Hello Mr. Smith,<p>Let me make sure I understand what you are trying to say. Basically you are saying that as long as my RFID loop antenna is at the focus of the parabolic antenna, all of the electromagnetic waves from the loop antenna that manage to hit the satellite will basically be focused out to infinity. <p>Now from my antenna handbook it says that a small loop antenna will have a dircetivity pattern in the shape of a figure 8. So I figure it would be best to orient my loop at the parabolic focal point, so one lobe of the figure 8 is pointing toward the dish. As I understand it, by doing this, what ever portion of that lobe of the figure 8 that intersects the dish, will be reflected out to infinity. Is this correct by your estimation.<p>Also, last night I got ahold of my roomate's optics book, and it discussed the spherical, parabolic, and eliptical dish's. The eliptical dish caught my eye becuase it took the reflections from one focus and then concentrated them again at another focus. This seems even better than the parabola, because I am interested in concentrating the energy at a point a finite distance (tens of meters) away. I did not see any equations in his book describing where the focal points were at, but I was wondering are there any commercially available elliptical dishes with a focal point tens of meters away? I notice that there appear to be eliptical Direct TV dishes out there, but I am assuming their focal point is out in space somewhere. I am guessing maybe some kinds of flashlights might have something like this in them. Does anyone have any other ideas of possible canidates? Most of my search results get drowed out in Direct TV hits so it has been a little difficult to find other possible alternatives.

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jwax
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Re: Satellite dish design

Post by jwax » Tue Aug 16, 2005 5:36 pm

Big coils provide the coupling to 125 KHz tags, at close range. If you want gain at 125 KHz, you need big dish. Sure, some "directivity" will be observed with a tiny dish, mostly due to the shielding from the backside of it.
When was the last or first time anybody ever saw a 125 KHz dish for sale? Please.

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Tue Aug 16, 2005 7:14 pm

Yes, it will improve the signal focus if done right.

The antenna dish is not the transmission point, it is a lens for the RF only, and it can focus any RF [or sound/light] energy if done properly. <p>Many systems incorporate many different means to keep the beam tight, the dish is but one way.<p> For your application specific, Which I assume is not that far in distance, I would experiment with a small dish and see if you get any gain. Over your standard coils in open air, you will see a gain but for how far a distance between Receiver and Transmitter, only the experiment will prove out.<p> Dishes are not limited to MW-RF.<p>A Parabola takes in [or sends out] all places on the dish surface, and sends them to a point on the feed horn. This also works in reverse, out to a single finite point. <p>If your pickup coil was extensive is size, perhaps one of the other types of dishes might have more gain?
But if your feed antenna coils or lop is only couple inches across, id stick with the parabolas.

To best understand a dish function, read up on Schmidt Cassgrain telescopes, both the dioptic and cat-a-dioptic. Remember Light is RF too.<p>Jwax, You seem to have never experimented in this field? <p>Your comment on 125 kHz is with out qualification. <p>This very experiment has already been done, and the hackers are quite happy with the results.<p>LNAs were used in the reception with a dish.<p> Open your mind, not everything you learn comes from a book or from wearing blinders. Any time some one said to me It cant be done, I went out and did it.<p>There is no such thing as a 125khz dish, because the dish is not radio frequency chosen. A small telescope looks into the sky exactly the same manner as the Space telescope in orbit. It amplifies exactly the same as the small one only better. The same goes for any dish. It’s a lens, and amplifier and concentrator of RF, its not the antenna, and its not the transmitter, and it has one purpose, to focus. Confusing the dish with any radio frequency is like confusing the dish with a transmitter. Two different subjects.<p>[ August 16, 2005: Message edited by: Chris Smith ]</p>

Robert Reed
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Re: Satellite dish design

Post by Robert Reed » Tue Aug 16, 2005 9:31 pm

Hi chris
I woked in 6 Ghhz microwave systems for several years.
To my knowledge "focal point only referred to relaton of feedhorn to parabola that it was mounted on. I have never heard of "focal point in relation to transmission path distance. Our links ran 2 miles to 20 miles with the same 6 ft. dish at all locations. The only thing different dishes did was change directivity and gain. The standard dish we used had a whopping gain of 60DB and a beam width of 3 degrees making alignment very critical for system operation. All of the Rf enegy emitted should go out in a direct line and not be focucused to any point according to the parabola physics. The same for reciving energy in a direct line. As RF energy hits the parabola , it is then focused or directed to the feedhorn which is then concentrated at one point and this accounts for the hi antenna gain. All the sytems I woked on had transmit powers of one to four watts which was more than enough with these types of antennas. Beam width was not a problem as this was strictly point to point communications
I am not sure if I missed something here as this sounds very strange to me.
Also I have a hard time visualizing small dishes bing useful at these low frequencys, because I was always under the impression that the dish diameter should always be greater than the wavelength its intended to receive. Wouldn't a small dish intercept such a small portion of a 125 Khz wave front as to make it terribly inefficient?

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Re: Satellite dish design

Post by Ron H » Tue Aug 16, 2005 9:42 pm

I had posted a couple of references to some equations here, but I think my brain was fogged by the lateness of the hour. I'll rethink it in the morning.
[ August 16, 2005: Message edited by: RonH ]<p>[ August 16, 2005: Message edited by: RonH ]</p>

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Tue Aug 16, 2005 9:57 pm

Robert
Any parabola has a small amount of adjustable focal point. <p>Because almost all Dishes are made for infinity and for MW RF, they have a small amount of movement for the feed horn, and most of that is for assembly the first time you set up the dish.<p> However, If you understand the principal of any parabola, or even telescope lens, you know that by moving your focal point on the dish this will broaden or reduce the focal point off in the distance. [ Beam width] <p>For example if you broaden your beam to the size of your receiving dish, all of the dish surface area at the receiving end will gather the signal equally and better than if you focus the beam tight and dead center, mainly because it will bounce off the out side of the recieving feed horn and weaken your signal. <p>If you pin point a laser 100 feet away, hoping to illuminate a whole subject in your field, good luck. <p>However if like a mag light, you spread out the beam, you illuminate the subject more efficiently especially back into a dish.<p>
Dishes normally aren’t sold or designed for “dmascares” purpose but this doesn’t mean that he cant utilize a dish as a reflector and lens for his speciality purpose. <p>Any signal concentration and gathering at the other end will benefit his purpose considering the fact that RFID is normally made for less than 20 feet.<p>High gain receivers [LNA] and dishes have been able to pick up RFID signals at over 20 yards so far so, so this in it self is a huge gain. <p>Now if a dish can improve the incoming signal, a dish on the sending end will focus the Rf a little more and should improve the link some what even further.

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Tue Aug 16, 2005 10:03 pm

Ron, A dish is not a antenna. <p>It’s a reflector, a lens, a way to concentrate the energy after it leaves the actual antenna. <p>In a dish system the feed horn contains the antenna usually called a LNA or LNB for receiver dishes. <p>A transmitter antenna for MW has a microwave diode or other means to transmit the RF, and this too is focused onto the dish, then relayed out into the airway.

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Re: Satellite dish design

Post by Ron H » Tue Aug 16, 2005 10:34 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Chris Smith:
Ron, A dish is not a antenna. <p>It’s a reflector, a lens, a way to concentrate the energy after it leaves the actual antenna. <p>In a dish system the feed horn contains the antenna usually called a LNA or LNB for receiver dishes. <p>A transmitter antenna for MW has a microwave diode or other means to transmit the RF, and this too is focused onto the dish, then relayed out into the airway.<hr></blockquote><p>Chris, I edited my post, for the reasons stated above. As I said, I'll rethink it in the morning.

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