Satellite dish design

[dmascare]

I was wondering if anyone new where I can get information on sizing a satellite dish, satellite dish, satellite dish theory/physics. I have tried google but I havbe not had much luck yet. I basically would like to see if it is feasible to send and recive 125 kHz signals with a satellite dish. I have seen some satellite dishes attached to microphones which claim to make the microphones more directional. I am basically hoping to do something like this but at a slightly higher frequency. <p>Thanks

[Externet]

Nice project !
What transducer you plan to use? From an echosounder ?
A plain audio microphone at the focus may allow to hear a conversation from very, very far... Yes. it becomes unidirectional at a huge gain. So big gain for a 8'Ø dish that your bare ear at the focus will hear a lot.
You find them at backyards behind the spiderwebs in rural areas for next to nothing $.
Edited, added: Avoid the wire mesh ones, get a solid one for your application. The ray bouncing explanation is in trigonometry books.
Miguel<p>[ August 15, 2005: Message edited by: Externet ]</p>

[Chris Smith]

Any standard satellite dish is a lens. To use it for sound, you need one with out mesh. If you use one with mesh, paint it heavily with thick epoxy paint until most of the holes fill up. <p> They all focus to a point of infinity so to speak, [25,000 miles away] but I use a Telescoping slide out of a old children’s telescope to place the microphone so that it become semi focus-able. And it even has a crank knob which is fairly accurate. <p>Use a old Telescope adjuster to mount the mic so that your can move its position plus and minus from the original point where the LNA used to be, and this will allow you to hone in a little better from varying distances. Failing this, use two tubes that fit inside each other, and figure a way to move them accurately and incrementally.<p>The unit I built about 20 years ago had Eleven Crystal mics set into another reverse parabola and set on the adjuster so that I could use it at 100 feet or one mile, but my dish was quite small back then and portable.<p> Regardless, I could hear a whisper at 1000 yards with my FET amp and ear phones. <p>I did run the risk of blowing out my ears as the FET amp was 100 watts per channel,[ no limiter] and one day a child screamed like only a child can do. That hurt considering I was wearing earphones.

[Robert Reed]

Dmascare-
Are you talking sound energy or electromagnetic energy ?
I beleive the dish physics are based on in coming wavelength.
Way too small for 125 Khz RF,but in the ballpark for 125 Khz sonic waves at fractional inch wavelengths.

[dmascare]

THanks for the reply's they were very thought provoking. For this particular application here is my idea. I am talking about electromagnetic waves here. Basically I want to see if it is feasible to use a dish to pump more than the usual amount of energy into a passive RFID tag which operates at 125 kHz. Generally this is done (As I understand it) by inductive coupling with a small loop antenna measuring a couple of centimeters across (about the size of a credit card). I have heard rumors that information could be read from on of these tags from distances on the order of 10's of meters with directional antenna's, but the frequency seemed so low that the Yagi would have to be huge. Basically I am working on building batteryless, wireless sensor networks.

[philba]

I'd be very surprised if you get any gain at that frequency. doesn't the navy use an antenna the size of wisconsin for that wavelength? ok, just kidding but that frequency has a quarter wavelength of just under 2000 feet! I think you'll better off figuring out how to wrap a loop antenna around the target.

[Chris Smith]

The dishes are not wavelength tuned. The feed horn does that. <p>They are suitable for everything from sound, all the way to light, and all RF in between. <p>They merely focus any indiscriminate wave length of energy be it sound, or light. <p>Some dishes are merely better at some frequencies, but all dishes are based on a parabola, one that takes a broad form of energy and focuses it to a feed spot called the feed horn, where the actual frequency is converted into a signal. <p>That feed spot receptor must correspond to the frequency of what you are trying to absorb, be it a LNB of any frequency range, or a sound mic, or even a photo sensor. <p>The bounce frequency of the dish surface area can be altered to suit the energy trying to be received. You can Silver the dish for visible light, close up the mesh for sound, or paint it for other usage, but ALL dishes are merely a lens. <p>The size of the microwave wave length, is larger than the mesh size so that it bounces and doesn’t pass through, but lets the wind pass through and mesh is lighter than a solid, the only real purpose for the mesh. <p>Older dishes were solid, and they also warped or blew over in a Big wind, and snow stuck more often to them. <p>The dish is a parabola, nothing more. <p>I have used them successfully for sound, light, and microwave. <p>If you want a extra hot solar oven, glue small mirror tiles [one inch square] to the dish and watch out. Im sure a ten foot dish could melt lead.

[jwax]

dmascare- Do be careful about "snooping" RFID tags at a distance. There is heightened security about the subject since one can "read" a tags' ID at a distance, and use that info to duplicate the tags' access, for example, at the Mobil gas pump!
Their very limited range was a cheap approach to security, but as technology has improved, snagging another users ID is tantamont to theft.
Keeping your gas tag or work security RFID tag in an aluminum foil wrap wouldn't be a bad idea while you're downing that java at Starbucks!
John

[dmascare]

Thanks for the info everyone that really cleared alot up. I am going to go home and brush up on my ray physics I have some new ideas in my head now. By the way In case any one is wondering, I am currently working on my master's degree in structural engineering. My research is in structural health monitoring (SHM). We are very interesting in finding ways to deliver power to sensors wirelessly. I have heard SHM presentations were people put on the order of 10's of accelerometers in buildings for long term monitoring, and the wireing alone for the accels took up about 70% of the budget. For that reason I am investigatign the use of passive RFID based sensors that do not require battery's or power cables.
My training is in mechanical and structural enginering, so I am trying to see what is out there. Thanks again

[Chris Smith]

Wireless IR is a good way to transmit information. IR bounces the best of all frequencies so If your transmitter is strong enough [IR laser diodes], the wave guide can be a hallway in the building, from one end of a building to the other. <p>A lot of people think a Remote control for the TV has to be pointed at the TV? <p>If the beam is powerful enough, you can bounce it off almost any object in the room, even in the opposite directions to the TV, and it works just fine. With one Remote control that I had a few years back that was very weak, I used the shine off my eye ball to go around corners and change channels and even black objects bounce IR fairly well. <p>A central Receiver on each floor, joined by wire or fiber optics on each of the other floors should minimize the wiring needed. A phone line between floors can gather all the floors information, and relay that to a central point.<p> Use IR lasers diffused to get your power level up and to also be “eye safe” at the same time, and this way the IR will even bounce under a closed door way and still make it quite far. For difficult areas you can add in simple repeaters. <p>Don’t use NEAR IR lasers as these can cause a condition called petite mal siezure in some folks caused from the flashing light, even visible flashing light.

[philba]

feedhorn or dish, neither is anywhere close to even 1/8 wavelength.

[Chris Smith]

Dish Wavelength choices [the ratio of the feed horn to signal frequency] are chosen to set the feed horn length and signal quality, from poor or weak sources. <p> Its called Focal length, and it can be high or low depending on the signal strength at the other end. [satellite or roof top transmitter]<p>
The tighter the parabola, the shorter the feed horn length and the less resolution you have which is why all good dishes have a fairly long feed horn mount. <p>Some are even cat-a-dioptic in that they feed the signal off the dish, then bounce it off the feed horn mount [ a special reflector] and back through a hole in the dish to increase the focal length by folding the signal twice.<p>
But as long as the parabola is curved correctly, and the feed horn is set to its [the dish] ratio, any wavelength of energy will do. <p>The Greater the Focal length, the less sensitive the input signal has to be, but practicality is also another issue in that length is also important to keep the cross section size of the dish and feed horn down. <p>A long feed horn mount, in a high wind, will drift off position and cause a signal loss. <p>All microwave, light, and even sound wave lengths or heights are well within any dish’s handling capabilities, except for mesh types on sound where some sound passes through the mesh. <p>The wave length does not come into play, as long as the feed horn is adjusted for each frequency variable. This can be achieved by a simple movement of the feed horn point in or out a few millimeters. All dishes have adjustment points on the feed horn for fine tuning. <p> Like any Telescope, the gain is a component of one aperture [input/ dish] size over the other [eye piece/ LNB pick up area].<p> The length of the feed horn from the surface area [called the focal length] improves or decreases the quality of the signal in the same way a telescope does with its focal length. <p>A Short focal length means a poorer focus and lower resolution while a longer ones give you a better focus, but, everything has a loss, for a gain. <p>The shorter the focal distance, the less resolution you receive and only energy from a pin point will focus properly onto the feed horn. <p>The longer the distance, the more the signal can be slightly off and still focus most of its energy on the feed horn spot. <p>Any change in frequency, from sound to light for example, slightly deflects differently off the dish in a minuscule way, hence the need for your adjustable feed horn. <p>The amount of difference is in the nano meters which is why the dish works for visable light and sound. <p>Sound probably is slightly more off, but with a high gain ratio it doesn’t affect the overall gain. <p>All parabolas for all antennas and telescopes however are the same, a set equation for the curve and they take energy from a distant spot and bounce it back to another given spot, which is why it works on all frequencies. <p>
Signal Gain comes from the dish size to feed horn ratio.<p>A one inch surface area pick up at the feed horn, and a 15,000 square inch dish surface area gives you a gain factor of 15,000. <p>A smaller dish like the 18 inch Direct TV metal type will give you a gain of approximately 200 to 1.

[philba]

sure that works great for gigahertz frequencies - that dish ain't gonna do squat at 125khz.

[rshayes]

Most efficient antennas (ones that are self resonant) have dimensions on the order of 1/2 wavelength. Adding loading capacitances or inducatnces can reduce the size to the order of 1/10 wavelength with some reduction in efficiency. Wave guide radiators (such as feed horns) have dimensions in the same range (1/2 wavelength or larger).<p>A 1/2 wavelength at 125 KHz is about 3/4 of a mile.<p>A parabolic dish does not radiate energy, but focuses energy that has already been radiated. The radiation can be done with feed horns or other antenna types (dipoles, log periodic antennas, yagis, etc).<p>The dish will only focus the energy that it intercepts. When the radiated energy is spread over nearly 3/4 of a mile, the amount of energy intercepted and focussed by a reasonable sized dish will be too small to be worth while.<p>Energy in the 125 KHz range can be coupled inductively over short distances. The range for inductive coupling depends more on the dimensions of the coupling coils. One application of inductive coupling is in implanted pacemakers, where the coupled power is used both as a data link and as a means of recharging batteries.<p>A loop on the order of a foot square might be adequate and could probably be placed on the outside wall and covered by drywall. Reading data might require bringing another loop within a few feet. This might be workable if readings were only necessary at long intervals. Collecting data for a long period and retrieving it over short intervals might require the use of some type of storage battery. I do not know of any type of battery that could be considered reliable over a several decade time span.<p>If continuous monitoring is needed, it might be better to concentrate on reducing the wiring cost. Doorbell and telephone wiring is cheap to install because the wiring is low voltage and current limited. This type of wiring does not have to be run in conduit or heavily protected, both of which increase the installation cost. A single twisted pair could be used to both power and receive data from multiple measuring instruments.

[jwax]

Thanks for clarifying your application dmascare!
It sounds very beneficial as the countrys infrastructure weakens! Albany, NY just had an interstate highway (I-787) shift on its pier and dropped a few feet!
Are you attempting to transmit power to the tag to energize it to send back information?
Have you considered the technology of X-10 communicators? They use building wiring to switch power on and off in a wired structure. The control signals are in the 100 KHz range. If the structure (A bridge for instance) has no wiring, perhaps use the extensive rebar metal for an antenna?
That gives a large antenna size, albeit difficult to "tune", but will distribute the 125 KHz extensively.
Sidebar- Univ. of NY at Buffalo did research on monitoring DC resistance changes between rebar sections to determine concrete movements! Cool!