Satellite dish design

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rshayes
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Re: Satellite dish design

Post by rshayes » Tue Aug 16, 2005 10:51 pm

At 125 KHz, 1 wavelength is 7,857 feet. At this frequqncy, a 10 foot dish is flat to about 1/1000 wavelength. The Arecibo dish would be flat to somewhere around 1/10 wavelength. These are fairly good as optical flats go. Optical flats do not focus energy. They are basically a plane reflector.<p>A waveguide for 125 KHz would be on the order of 4000 feet wide by 2000 feet high, for a total area of 8 million square feet. The area of a 10 foot dish is about 79 square feet, for an efficiency of .0000098, or .00098 percent. The other 99.99902 percent of the energy is not focussed.<p>If the waveguide is made significantly smaller, it becomes a waveguide below cutoff, and the attenuation in the waveguide becomes quite high.<p>A 1/2 wave dipole would be even worse.<p>The recommendation in the U3280 data sheet is for two resonant coils that are inductively coupled. In effect, these are loop antennas, and this is consistant with LF antenna practice.<p>The "Antenna Engineering Handbook", published by McGraw-Hill has gone through several editions. The original editor was Jasik, but subsequent editions had other editors. Most of the various editions should have chapters on loop antennas. Since you are at a college, you should have access to an electrical engineering library, which should have several books on antenna design that will cover loop antennas.

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Tue Aug 16, 2005 11:36 pm

If these were the only facts relating to antennas, the receiver end of any standard RFID antenna would also be as big as Arecibo. <p>Funny how its not? And it even works!<p>Just place this tiny guy at the feed horn position of a dish for a gain and presto, it works even better. <p> But since a dish is not an antenna, it will reflect any and all frequencies to focus towards the actual antenna at the feed horn position. <p>And like all antenna, the actual length rarely parallels the actual frequency length for good reason. <p>But then a dish or reflector cares little about this fact, because its not an antenna.

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jwax
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Re: Satellite dish design

Post by jwax » Wed Aug 17, 2005 3:46 am

"Jwax, You seem to have never experimented in this field?
Your comment on 125 kHz is with out qualification.
This very experiment has already been done, and the hackers are quite happy with the results."<p>Finally you got a fact correct Chris! I have NOT experimented in this field. I experiment with doables dealing with facts like resonance with my antennas.
Speaking of qualification, kindly provide reference to "the hackers who are quite happy with the results" of their 125 KHz dish experiments.
I think we agree that the dish is only a part of a type of antenna system (the lens), it also needs a feed device, but does an antenna system not require resonance? What part of a 125 KHz antenna dish system resonates?

Ron H
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Re: Satellite dish design

Post by Ron H » Wed Aug 17, 2005 8:43 am

<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>
First, let me make it clear that I am not an antenna expert. I just got curious after following this thread and decided to do a little research.
Chris, I don't know where you get your definition of "antenna". Most definitions of "satellite dish" that I could find refer to it as an antenna. It has many of the characteristics of other antennae - gain, aperture, and efficiency.
Regarding gain vs frequency: <p>This paper and this one have the gain equation for a parabolic dish. When I went through the math, I concluded that gain drops 20dB for each decade of frequency drop, e.g., if a dish has X dB of gain at 1.25GHz, it will have (X-80) dB of gain at 125kHz. I don't dispute that it will still be better than a little whip antenna. I just think that this fact (unless I'm wrong, in which case it isn't a fact) has been sorta glossed over here.<p>[ August 17, 2005: Message edited by: RonH ]</p>

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Wed Aug 17, 2005 9:39 am

Unfortunately a dish is still NOT an antenna. If you know anything about a dish, you know where the antenna is. Its called the feed horn section, and it’s a LNA or LNB within the feed horn that does the antenna function. <p>[radiation and absorption, the definition of any antenna]<p> The dish merely assists this function and gain through the usage of Focus. <p>Yes, you can refer to the whole package as a antenna, but it is a passive part of the whole, it has no power attachments and no emissions are received, absorbed, or radiated from the dish it self. <p>It merely reflects the signal much like any mirror lens. It is a component of the whole antenna assembly, IE., the reflector part of the antenna. <p> Gain is a component of efficiency from the focus that the dish can allow, [or disallow] if the feed is properly in focus. <p>All of the rules of a optical reflector apply to a dish, the rules that apply to a antenna only overlap in some corners of this scenario. <p>For example, a FLAT was mentioned earlier [optical], but a flat plane can not gather or focus or amplify a signal. <p>At best it can reflect a single of incidence with out any gain because only one line of sight can be bounced to another single point of absorption. <p>A dish can focus many points to a single point for gain. It can also restrict wasted energy for a short period from a collimating effect, [and reducing the emission field in half] especially over short distances of transmission. <p> The “dish” operating in the MW arena is a whole, and the antenna and the feed horn couldn’t operate with out it. However, when used as a simple reflector, a simple gain is had. <p>Were not talking about making the impossible happen to a 125khz signal, were simply referring to a control over the situation to get a small boost and all from focus. <p>Were not talking about not tunning up the rig to work in perfect harmony and gain at 125khz?<p> The same way a coffee can on a CB radio transmission or reception makes the signal directional and more useful, so can a dish on a RFID tag signal by simply removing 50% of the equation and focusing energy in one direction. <p>Over the short distances involved, a slight gain will be had not because of tune or perfect harmony but because of RAW radiated energy and focus. <p>ARE these tuned components, NO, they are a tool to improve a weak signal. <p>Don’t confuse MW and a tunned Dish assembly with a “lens” that aids a weak signal. <p> Improper usage of Electronics is used every day if a plus can be had. A 125khz signal and a dish will never make it into any major article involving electronics, unless you happen to be a hacker, and you happen to show every one else that what they were taught doesn’t apply. <p>Colorful usage of equipment has led to many discoveries, in fact all of electronics came from a colorful use of the mind. Rules are fine, if your trying to copy some one else.

rshayes
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Re: Satellite dish design

Post by rshayes » Wed Aug 17, 2005 9:50 am

There are nonresonant forms of antenna (beverage, terminated rhombic, and log-periodic to name a few) but in general they are larger than resonant antennas and have low gain for their size. The beverage and rhombic are usually on the order of tens of wavelengths long. The log-periodic is about 1/2 wavelength at it lowest frequency limit.<p>Resonant antennas are usually about 1/2 wavelength in some dimension unless lumped elements are used to resonate shorter elements. The effiency of the smaller antennas is usually lower.<p>Loop antennas can be made with dimensions considerably shorter than a wavelength, but their efficiency is usually very poor unless care is taken to minimize ohmic losses in both the antenna and the resonating and matching networks.<p>The data sheet for the RFID part suggests the use of loop antennas. The range is so short that there is a question as to whether these are actually antennas or are coupled inductors forming a loosely coupled transformer.<p>As a communication link, the RFID tags are incredably poor. You can get better range and bandwidth with two tin cans and a piece of string.<p>[ August 22, 2005: Message edited by: stephen ]</p>

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Re: Satellite dish design

Post by Robert Reed » Wed Aug 17, 2005 5:32 pm

Stephan
As to your post regarding the dish as being "flat" at 125 Khz and therfore making it a mere reflector. This brings to mind that Gain antennas that employ reflectors Are usually spaced 1/2 wavelenth apart for proper phasing to the "true "element in these arrays. This would mean that the dish in this case is just a small hunk of "tin" inadvertantly bouncing RF (125Khz) back to a almost meaningless feedhorn as far as pickup goes.
This feature makes the 125Khz/dish system having even less credability.
Chris
You keep insisting that the dish is not an antenna. Most of us are quite aware of this fact, as well as velocity factors (wire/ resonance length), so that doesn't need repeating.
As to antenna efficiencies, Ive seen everthing from coat hangars to scraps of romex wire and even human bodies used for this pupose. And they all work, but they do not work at all welll. Why would we go to the expense of using a dish antenna at these low frequencies when we could use simpler,cheaper devices to achieve the same, albeit inefficient, purpose?. But now you have spiked my curiosity on this subject. Have you seen any such antenna sytstem (dish) ever used at this or any other VLF frequencies? If not ,what is the lowest Known frequency for dish operation that you have heard of?

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Wed Aug 17, 2005 6:45 pm

The answer to this is simple. Direction, focus, and gain.<p>A dish like a metal wall, can stop radiation from going in the wrong direction. <p>This is a 50 % gain, in theory.<p>A dish can take a antenna and focus its radiant energy forward towards another receiving antenna. <p>This is a gain. <p>Dishes aren’t expensive, I have a junk pile of them out side, even 10 foot dishes for free, so cost is irrelevant to experiments such as the original poster requested. <p>Loss of signal?
This can be off set by any gain in due to focus. <p>When ever I do optical experiments and the signal is weak, I use a lens, which is the same as any dish. <p>For me to say “oh, its too expensive or it wont work” would put my experiments at ZERO for success. <p>If I worked off “formulae only”, and the lens I used was incorrect, [but it works] should I stop there? <p>“No imagination” will get you exactly that far in science, “no where”.<p>[ August 17, 2005: Message edited by: Chris Smith ]</p>

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philba
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Re: Satellite dish design

Post by philba » Wed Aug 17, 2005 8:33 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by stephen:
...
The data sheet for the RFID part suggests the use of loop antennas. The range is so short that there is a question as to whether these are actually antennas or are coupled inductors forming a loosely coupled transformer.<p>As a communication link, the RFID tags are incredably poor. You can get better range and bandwidth with two tin cans and a piece of strins.
<hr></blockquote><p>My understanding of RFID is that the current crop of 125Khz cards and readers are inductively coupled. Look at the specs of the readers, I think the best HID one (proproxII, iirc) works not farther than 3 inches from the reader. Inside the card are about 30 loops of wire 2.5"x1.5" That says induction to me.<p>Phil

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Wed Aug 17, 2005 8:40 pm

RFID with low noise amps, a dish, and a few other tricks can be read up to 20 yards.<p>Normally they claim to be readable only up to 20 feet absolute max. <p>But hackers know better because hackers break all the rules by burning the books that say” it cant be done”. [See the latest Hackers convention] <p>Inductive and even magnetic transmissions can also be focused with a dish especially for short distances.

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jwax
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Re: Satellite dish design

Post by jwax » Thu Aug 18, 2005 3:52 am

dmascare, instead of experimenting with new, unrealistic concepts like a transmitting/receiving 125 KHz dish, you'd be ahead by considering alternative paths to your goal. Have you considered lithium battery powered, burst microtransmitters? It would click on every minute, or hour, or whenever you decide to send data to a logging receiver. Battery life would be determined by space available, and yield perhaps years of operation. Doable now.
John

rshayes
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Re: Satellite dish design

Post by rshayes » Sun Aug 21, 2005 9:29 am

A system for monitoring structual health in buildings will need to have a life time of several decades to possibly a century. The main application for such a system would be for industrial buildings, and they tend to be used for a long time.<p>A fair number of buildings around here were built for wartime production during World War II. They are around 65 years old now, and will probably be uased for another couple of decades. Other buildings were built in the 1920s and a few go back to the late 1800s.<p>The real need for a monitoring system in a building will probably be 50 years from now, when the building is starting to age.<p>I don't know of any battery systems that I would consider reliable on this time scale. Primary batteries, such as lithium, might last for a decade or so if the current drain was low enough. Self discharge might limit life to less than that. The batteries are too new to know for certain.<p>Rechargable lithium batteries might be a possibility, but are too new to really tell.<p>A passive device with power coupled in inductively might work, but will probably require a coupling coil to be brought into close proximity (a few feet) to couple sufficient power. Coupling at any frequency may be inefficient if the structure contains much metal. The frequencies allowed for RFID tags may change.<p>This is why I previously suggested that it might be more fruitful to investigate minimizing the installation cost of a wired system.

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Re: Satellite dish design

Post by thesprocket » Sun Aug 21, 2005 4:35 pm

DMASCARE:<p>Lets take a tangential turn here and look at the project again:<p>Tens of accelerometers, this is per building?<p>Structured cabling is taking 70% of the budget?<p>You plan on eliminating both cables and batteries by powering your sensor, micro controller, and RFID transceiver via the RFID pulse. <p>I think you may need to re-look at the construction costs:<p>You can get an 8 conductor, 4 pair, UTP cable (cat3 for cheapness) installed indoors at about $125/run just about anywhere in the US if you put a little effort into shopping around.<p>If a cost like this reflects 70% of the budget, that tells me that you have only $178 per sensor install. With about $50 for each sensor and a few dollars of each 178 dedicated to a centralized collecting/communicating device.<p>Now what you are trying to do for the same 178 is emplace a sensor, datalogger, and RFID device, AND a LARGE DISH PER SENSOR clustered at the receiving end possible tens of meters away. As each sensor/RFID remote device will need its own dish precisely aimed at it. <p>A do-it-yourself mount for a 7.5 foot dish; a hole in the dirt, a 10foot stick of 4” IMC conduit, and 10 70lb sacks of concrete will cost you about $60 dollars and a days hard work. Not to mention, that the building that you are trying to monitor will now have an array of tens of large parabolic dishes parked all of twenty meters away for everyone’s viewing pleasure. <p>I have to touch on this as well: what stable reference voltage do you plan on providing to the accelerometers, now that we have eliminated batteries and cables? Not the shared output pulse of the RFID tag. Plus, I have worked with accelerometers recently. The multi axis ones sensitive to detect building shift over time cost more that the derived $178 per.<p>Wow, you got me monologing!<p>My real point is that I think you have more budget than you claim, and cabling cannot be such a high percent of budget, or that someone not familiar with structured cabling was suckered on that first project. <p>Finally, a structure of the size needing tens of sensors represents a 7+ digit investment by someone, I can’t imaging the cost of ten or twenty cable runs amortized over the 75 to 100 year life of the structure to be the sort of deal breaker implied.

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Chris Smith
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Re: Satellite dish design

Post by Chris Smith » Sun Aug 21, 2005 7:27 pm

Actually, fiber optics replace all of what he proposes.<p> Most bridges and just about everything today incorporates fiber optics into the concrete structure, for this very purpose. <p>I threw away all my “Transportation Tech Briefs” on the subject otherwise I would have given him the alternate and even the names and companies that specialize in this very thing. <p>But even NASA tech briefs occasionally has articles on accelerometers and embedded fiber optics, and its free to any one who asks for that free subscription.<p>Personally I believe he is on the wrong track, but I wont discourage it because many great things have come from this type of thinking.

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Re: Satellite dish design

Post by dmascare » Mon Aug 22, 2005 1:19 pm

Thanks for the input<p>But, please remeber this is a research project. I am also very aware of the fiber optic sensing. My advisor is an expert in fiber optic strain gages. We actually recently built some testing frames for building fiber optic arrays that would use strain measurements to work out the shape of a cable. <p>For me personally still being in school this is mostly a project where I would like to see what is out there. If you can't make mistakes in school where can you make them?
No one I work with really has much of an electronics or RF background, so I am trying to look at different directions we can go. My boss suggested the RFID. There are things about the RFID I question everyday, but as I develop the rest of the project, it should be relatively easy to use other methods to transmit the data. I have found a number of interesting things out in the course of this project, and even if they don't work out, I feel my time has been well spent.

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