RF Signal strength to measure distance

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John Abel
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RF Signal strength to measure distance

Post by John Abel » Wed Aug 31, 2005 1:07 am

While we are on the topic of RF, I thought you might be able to help me with an idea I’ve been brainstorming. That is to use the relative signal strength received of a low power transmitter to determine distance. <p> If a small, low powered transmitter were attached to a moving object, then a stationary receiver would get a varying strength of signal as the transmitter moves towards and away from the receiver. Although this received signal strength probably won’t be very linear, I was hoping some kind of comparator circuit could be used to trigger a buzzer or something once the object/transmitter got to be a predetermined distance/signal strength from the receiver.

I have been experimenting with this idea over a distance of a few feet. Using a very low powered transmitter that operates around 1785Khz and “listening” in on an AM radio, I was hoping the amplitude of the modulation would decrease as the signal weakened. Instead it only seems to be replaced by random static noise with the same amplitude, no matter what stage of the receiver I check. <p> Any feedback and ideas you might have would be great.

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philba
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Re: RF Signal strength to measure distance

Post by philba » Wed Aug 31, 2005 2:41 am

there are a number of reasons why this wont work but foremost is the fact that distance is not the only reason the field strength of the transmitter will vary. For example, if there was a building between the transmitter and receiver, the field strength at the receiver would be much lower than the same distance without the building in the way. <p>The relationship of distance to strength is inverse squared, by the way.

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Clyde Crashkop
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Re: RF Signal strength to measure distance

Post by Clyde Crashkop » Wed Aug 31, 2005 8:47 am

You might look into radio direction finders. Boats used to use them but you have probably seen them used to track animals that have been tagged with a transmitter. They operate on signal strength. You may be able to get a distance and direction out of it.

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jwax
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Re: RF Signal strength to measure distance

Post by jwax » Wed Aug 31, 2005 8:52 am

The amplitude of the modulation bears only a casual relationship to the carrier strength. If you wish to measure signal strength, you have to measure the 1785Khz signal amplitude. That is done in most receivers with AGC, an easy-to-read dc level to indicate signal strength.
Are you reading me 5-9-9?
:D

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Re: RF Signal strength to measure distance

Post by Robert Reed » Wed Aug 31, 2005 9:33 am

RF falls off logarithmically with distance in a clear feild (no other parameters present). I don't think the marine RDF would work well for you here, as they use directional antennas. This would have to be rotated by hand to track the signal, unless it was done electroncally (very expensive). They usually work by nulling out the signal anyhow.Your AM modulation level will never change regardless of distance. The noise you hear isnt a reduction of modulastion, but rather a rduction of signal to the point that ambient noise is stronger than what you are trying to receive.
And this noise will be your main problem when operating in the MF band (0.3 to 3.0 Mhz). Most low end recievers in this range do not even use RF amplifier stages as there is not much to be gained due to the fact that manmade noise is the predominant factor in determing SNR,
Jwax definately has the right approach here( AGC).
The agc is usually developed at the output of the detector stage and then fed back to the receiver front end (which may include IF stages) as a DC voltage. As long as this AGC is operating in its linear range, there will be no difference in audio output (or so it was designed for).By tapping off this AGC voltage, shifting and scaling you might approximate a fairly linear out put with distance. However you neglected to mention the distances involved and relative directions from the transmitting end of your system<p>[ August 31, 2005: Message edited by: ROBERT REED ]</p>

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Re: RF Signal strength to measure distance

Post by Tommy volts » Wed Aug 31, 2005 4:48 pm

John T,<p>That sounds like a fun RF project!<p>Your idea reminded me of another project not related to RF but which involves a distance alarm:<p>In the object, place a GPS unit which transmits the objects coordinates (lat.&long.) to fixed position reciever (these units are cheap and used by equipment/vehicle owners for tracking).<p>Have an MCU read and calculate the differnce in the fixed position coordinates and those of the moving object, then sound a buzzer when the limit distance is exceeded.

John Abel
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Re: RF Signal strength to measure distance

Post by John Abel » Wed Aug 31, 2005 9:08 pm

Sorry for neglecting to tell you the distances I am thinking of are within a range of 1 and 2 uninterrupted yards. Thanks for the advice about the AGC. It sounds like it should be perfect. Do you have any idea where I might find the physical location of this stage on a standard radio? The radio I have taken apart uses an IC for most of the AM stages, but I didn’t seem to find any pin with a varying voltage level like the AGC.

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Chris Smith
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Re: RF Signal strength to measure distance

Post by Chris Smith » Wed Aug 31, 2005 11:04 pm

Even with short distances you have “reflective values” of RF which make RF hard to use as a accurate ranging device. <p>Perhaps TWO RF signals can be triangulated [summed and divided by] and combined at your short distance to triangulate better signal suited for your needs? <p>If not, ultra sonics and light might better suit your needs?

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Re: RF Signal strength to measure distance

Post by John Abel » Thu Sep 01, 2005 12:00 am

Thanks Chris for the warning, but I may have miss worded my last post. I do not intend for this to be an accurate means to measure distance over the range of 1 to 2 meters, but rather to be preset so that the measured field strength at an arbitrary distance (about 1 yard) activates the buzzer or some other circuit. <p> To clarify, I would first set the transmitter and receiver the desired distance apart (in the same system as during the actual operation), and then adjust some kind of comparator circuit to go high at this signal strength. Next, during the actual operation of this setup, the output would remain low as long as the transmitter was farther away than this preset distance. Once the transmitter moves to this preset distance, or closer, the output would go high. A margin of error of about an inch would be acceptable. <p> I am unsure about using ultrasonic or light ranging because the object in motion is a bar with a diameter of less than 2 inches and would move horizontally as much as 6 inches during its motion. <p> I haven’t worked much at all with radio, so if I am misunderstanding the problem let me know.

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jwax
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Re: RF Signal strength to measure distance

Post by jwax » Thu Sep 01, 2005 3:34 am

If you can decifer the part numbers of the chips in the radio, there you go! Google them (or let us know) and you'll find the pinout of the chip.
The AGC pin should be obvious on the pinout.

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Re: RF Signal strength to measure distance

Post by Robert Reed » Thu Sep 01, 2005 8:25 am

Still not sure about required distance.Is this a go-no go type of setup, such asTx (or Rx) moves closer to mating unit until signal triggers a device? Does device have to trigger at a specified distance "X", but not at "X" +2 inches. If so, this would be difficult to discern on AGC voltage. Might be possible with a very restricted antenna system.
I had thought at first post that you wanted to measure distance, but maybe I was wrong as now it sounds like you want to actuate a device at a preset distance.Correct me on this issue.

John Abel
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Re: RF Signal strength to measure distance

Post by John Abel » Thu Sep 01, 2005 10:46 am

Yes, this is a go-no go type of setup as you word it. I was hoping that with a low enough power transmitter there would be enough distance between “x” and “x+2” for a difference to be discerned. <p>The part number on the IC is ANZ024, which I haven’t been able to find a data sheet on. There is more writing that I would have to remove a capacitor to see (the IC is in a tall thin package), but I figured this was just a manufactures code.

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Re: RF Signal strength to measure distance

Post by Bernius1 » Thu Sep 01, 2005 11:59 am

If it's low power, the received signal will be 'heard' gradually appearing from a background of noise. So, if you modulate a 1,000hz tone (arbitrarily) , as loud / soft pulses (tick\tock), then either detecting hysterisis, or using a 555 as a ' missing pulse detector' will give you an output only when the received signal is strong enough to overcome the background noise. (If the HI/LOW pulses are 100 mSec, noise will fill the gaps until the modulated frequency suppresses them ).
What do our compatriots think ? Yeah/Nay?
Can't we end all posts with a comical quip?

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Re: RF Signal strength to measure distance

Post by Robert Reed » Thu Sep 01, 2005 4:36 pm

Nay
Because of the small change in distance involved.No components of the RF signal will show any measurable change. Without change, their is nothing to detect in any case.

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Chris Smith
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Re: RF Signal strength to measure distance

Post by Chris Smith » Thu Sep 01, 2005 4:53 pm

I don’t think you will get that much resolution from a Rf signal. There are ways to focus a A RF beam like a light so its best strength is at a given point, and very wide or low power at other placements, but RF has a tendency to bounce, reflect, and absorb with out notice. <p> It can even get absorbed by humidity which would change your detail/resolution of what your trying to do back and forth with out notice. Near by metal objects are even worse. <p>A light beam can do a better job with one small reflective surface area on the object, and a preset angle, and this will only bounce into the detector when the exact distance/angle is achieved sending the light beam back to a receiver. <p>The simple Trigonometry and angles are set so that a beam goes out at say 45 degrees, and at exactly X distance, it hits the reflective surface and comes back to a sensor exactly based on angles which are preset away from the transmitter, and this angle can only be met at your exact distance, and when it aligns just right, it bounces off the reflective surface and back into the sensor. <p>Further or closer, places the reflective beam to the left or to the right of the sensor. <p>You can even use more than one pick up so that “docking” can be seen on a scale like increasing LEDS. Close, closer, contact.

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