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Varying the distance radio wave travels (Urgent)

  1. Feb 2, 2012 #1

    So i am planning to make this project in which i want to use two transmitters and two receivers.

    I want to make one of them to transmit waves that go only for say 3+ feet and another than can only transmit around 1 feet. 4

    Is it possible?
    I know frequency plays an role and so does the power, but how should i exactly get the distances my wave can travel?
    Is there a precise way? If not how i should work it out?

    if i get the frequency i can work on the component values of my wave generator.

    Please need help fast.

    Is there a simple way in which i can know whether the wave i transmitted is being recieved.. i want to know it on the transmitter side.. (maybe getting it back from reciever).. But exactly how? Is there any other method?
  2. jcsd
  3. Feb 2, 2012 #2


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    In free space radio waves go on forever but they just get weaker as they spread out.

    So, once you transmit them, there is really no way to stop them unless you put some sort of barrier in the way.
    This would normally be a large sheet of metal. Metals that are good conductors make better shields.

    Another approach is to make the signal so weak that anyone further away than a few meters will not be able to hear it. This is the sort of approach Bluetooth uses.

    Is there a simple way in which i can know whether the wave i transmitted is being recieved.. i want to know it on the transmitter side.. (maybe getting it back from reciever).. But exactly how? Is there any other method?

    If the receiver was very close, like in the same room, you would be able to hear it.
    Otherwise you could send a signal back using wires or another radio transmitter.
  4. Feb 2, 2012 #3
    I plan to reduce the power the waves. And the distance i am talking about is like one transmitter for around 3 feet and another for around 1 feet. Is it feasible?
    And the right power will be known using hit and trial method?

    I want one LED to glow when the receiver receives the sent wave. How is it possible? What is the easiest way to do so?

    Sorry for really basic questions, Thanks in advance :)
  5. Feb 2, 2012 #4
    I doubted you can make the distance that exact. I think you better off design using a more directional antenna so the ones not in the direction receive much weaker signal. But you are opening up a different can of worm.

    You did not explain what you trying to accomplish. If you just want the two don't interfere each other, there are much easier ways. They have communication protocol that encode the data so one will not interfere with the other. It is like Blue tooth and 802.11x are all run at 2.4GHz and you can have multiple devices running in the same room and they are not interfering each other. The separation is inside the data stream. You need to look into communication theory and find out how. I don't know enough in this, but it works as I am on the Wi-Fi and I use my wireless mouse only inches apart from each other. In fact, I have two WiFi laptops that connect to the same station, I have had both laptops sitting next to each other both have wireless mouse working next to each other, both on the internet and work perfect. You better off looking in this direction. Go blue tooth so the signal don't travel far.
    Last edited: Feb 2, 2012
  6. Feb 2, 2012 #5
    I think its my fault i am not properly giving the information.
    So here it is,
    i want two transmitters floating on water. Both if different waves which travel different distances. Now, there are two receivers on the same level at some distance above the water level.

    I am trying to make a simple water level estimator.
    If the receiver receives the stronger wave, it means the water level is low..and if both the receivers receive the signals implies that the water level is high.

    Can you now help? Thanks in advance.
  7. Feb 2, 2012 #6
    Perhaps you don't need two transmitters and two receivers. Perhaps you only need one transmitter and one receiver and compare the signal strength to various thresholds.

    How much accuracy do you need?
  8. Feb 2, 2012 #7

    That could be done, but what is the crudest way to do it?

    Well i need to make a small prototype. So..
    I will have two stages.
    1. When the tank is almost empty, that is the distance between the rec and trans would be around 2-3 feet.
    2. When the tank is almost full, that is the distance is less than a foot.

    Does that solve the query?
  9. Feb 2, 2012 #8
    What is your tank made of, metal?

    What is your liquid? Is it conductive or non-conductive?

    How can you assure your transmitter will stay directly beneath the receiver and not float to the other side of the tank?

    How will this be better than just having a float on a tether that activates one switch when the level is high and a different switch when the level is low?
  10. Feb 2, 2012 #9
    Liquid is water.
    The tank will be made of plastic.

    Its a water tank level checker.

    In the small prototype that i am making, the tank(bucket) will not have a very big diameter.
    So the transmitter signal has to reach the receiver.

    How will this be better than just having a float on a tether that activates one switch when the level is high and a different switch when the level is low?

    Sorry but can you elaborate it?

    Thanks for being patient. :)
  11. Feb 2, 2012 #10
    See Attachment.

    Attached Files:

  12. Feb 2, 2012 #11
    OK, I am just talking out loud:

    There got to be a way to transmit and start a counter and wait for the reflection from the surface. That's how Radar, sonar and ultra sound scanner stuff work. You don't need to have transmitter and reciever. Air and water is very different in density and definitely can get reflection.

    For cry out loud, flash an LED and start counting and when you detect the reflection of light from the reflection and trigger to stop the counter, you can get a number of counts that reflect the distance. Look into lazer distance measurement, this is nothing new.
  13. Feb 4, 2012 #12
    Electromagnetic fields continue like matter-- they keep going untill they're attenuated or reflected. consider a light bulb-- it travels forever like stars do. If you ONLY want an emission to go a certain distance, you simply use an absorber or reflector. What you use is determined by the frequency, a 1/2" thick piece of 12" steel will stop almost any.

    Keep thinking of light-- You want a receiver, use a device that responds to whatever you're emitting the distance is only determined by what your measuring device can detect. Again think of stars zillions of light years away---and increasing, the light emission is just another portion of the electromagnetic spectrum as is infra red and sound
    Michael Rogers

    Last edited: Feb 4, 2012
  14. Feb 4, 2012 #13


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    We could take advantage of the plastic tank. Plastic allows radio signals to pass.

    Suppose you had a small transmitter on a float near one side of the tank and a receiver almost above it, but on the outside of the tank.

    The closer the transmitter got to the receiver, then the stronger the signal would get.

    There would be no need to have the transmitter running all the time so pulses sent every 10 minutes or so would be very economical on the battery of the transmitter. A small cheap micro could generate the pulses and another, in the receiver, could store the magnitude of the received pulses and present them on a display. .

    The transmitter and a small antenna could be built into plastic plumbing fittings that were large enough to become a float. Waterproofing would be helped by the use of plastic plumbing fittings.
    It could be tethered at each end by large rings around vertical ropes.

    An alternative would be to have the receiver on a small trolley running up and down the outside of the tank until it found the position of strongest signal.
    Intriguing, but maybe an overkill.
  15. Feb 14, 2012 #14
    whats wrong with simply measuring the mass of the water? could you bounce a sound wave off the surface and measure the time delay? (by using a cross correlation function or whatever)

    you could tap on the bottom of the bucket and measure the vibration with a sensor near the bottom and near the top to detect the presence of the water since the vibration will travel much more effectively through the water. Actually a simple buzzer at the bottom and a microphone at the top attached to a power meter could detect the water level.

    put a little float with an LED and a photodiode and measure the intensity of the light reflected off the bottom?
  16. Feb 14, 2012 #15


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    Somewhere I saw a list of about 20 ways of measuring the depth of water in a tank.

    They even included resonating the air column above the water with audio signals.

    So there are lots of ways of doing it and many of these have been discussed here before. This is a fairly common question.

    In this case the question was whether you could use radio signals to do it. And it is probably possible to do this.
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