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Thread: Building a small radar

  1. #1 Building a small radar 
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    Hi,

    I am interested in building a small radar capable of picking up signals within 200 meters - 1km. It should be within the 2 - 5.2 ghz spectrum and can be either Bi-directional (Scattered) or Directional (Point). I know an entire setup of radar can be quite expensive with the right equipment but was wondering if there was an alternative such as using transmitter + receiver + Antenna + Display screen setup.

    The idea i have = The transmitter itself will be the pulse generator whilst the receiver captures the rebound sound and displaying it on screen.

    Any suggestions are welcome. Hoping the entire project will only last 1 or 2 months in total.


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  3. #2  
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    Hi,
    An option could be to build a passive radar : you just use existing public TV or FM broadcast as transmitter. You will need two receivers and two antennas : one receiving the direct signal and another one receiving the signal reflected by the target. Computing the difference of phase between the direct and reflected signal will give you the distance of the target.

    Patrick.


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  4. #3  
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    Quote Originally Posted by caKus View Post
    Hi,
    An option could be to build a passive radar : you just use existing public TV or FM broadcast as transmitter. You will need two receivers and two antennas : one receiving the direct signal and another one receiving the signal reflected by the target. Computing the difference of phase between the direct and reflected signal will give you the distance of the target.

    Patrick.

    Ok, so Receiver A receives the direct signal (which i understand). And receiver B receives the signal reflected by the target (Which i don't understand because receiver's are suppose to receive, aren't they? Unless you're saying that the receiver emits a pulse which reflects off the target and is then received by the same receiver emitting the pulse, but what is the point of having 2 receivers?)
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  5. #4  
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    The idea was to separate the direct (strong) signal from the reflected (weak) signal. So, I was thinking to use separate directive antennas (yagi with many elements for instance) raccorded to separate receivers. Of course, main transmitter and target must be in different directions, seen from the your point of reception;
    If you use only one omnidirectionnal antenna and one receiver, I fear that the reflected signal will be to weak to be detectable and will be covered by the direct signal.

    Eventually, you may use only one highly directive antenna positionned in a direction such as the direct signal is totally eliminated. If a target is present in the field of your antenna, a reflected signal will be detected. But you won't be able to compute the distance to the target with only the reflected signal.

    Here is a simple experiment that I did a few years ago with only one omnidirectional antenna and one receiver : in my country, we have a powerfull military radar enlighting verticaly the sky. The purpose of this radar is the detection of foreign military satellites. The transmitter is in the VHF range. If you listen this frequency with a SSB receiver, and if a meteorit is moving over at hight speed, you can hear a signal from your loudspeaker, a sort of tweet sound due to the doppler effect. If you connect your receiver to a computer with a spectrograph software (waterfall display), you can see a track on your screen and detect weak signals. My receiver was located at 200 or 300 kilometers from the transmiter, out of the range of VHF direct propagation, so only reflected signals where detected.
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  6. #5  
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    Thanks caKus for talking with me on this.

    Could i use this to listen to the sky at night? What if i use a 22 cm Antenna which is pointed directly at the centre of a parabolic bowl (such as a 50 cm diameter metal salad bowl), connect that to an amplifier and then feed the received signal out to a computer with a spectrograph software, would it make a difference in terms of making a weak signal strong? And does the receiver come before or after the amplifier and the metal antenna salad bowl?
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  7. #6  
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    Quote Originally Posted by Sci_Research View Post
    What if i use a 22 cm Antenna which is pointed directly at the centre of a parabolic bowl (such as a 50 cm diameter metal salad bowl), connect that to an amplifier and then feed the received signal out to a computer with a spectrograph software, would it make a difference in terms of making a weak signal strong? And does the receiver come before or after the amplifier and the metal antenna salad bowl?
    Using a directive antenna (parabol or Yagi) as two advantages : 1) it usualy gives a gain of amplitude of the signal. 2) It reduces the noise (atmospheric, industrial, other transmissions). The global result is a better signal / noise ratio, so a better ability for the receiver to discrimine the signal from noise.

    A 22 cm antenna will give you an approximative frequency in the 1100 / 1200 Mhz range (Just a guess, I didn't do the calculation). If you want to create a passive radar, you will need an existing source in that range.

    As a first experiment, I would suggest to use a UHF TV antenna (Yagi like). They are cheap, have a gain better than 10dB and you have plenty of sources around you : the TV stations.
    You may have a little trouble connecting the TV antenna with an impedance of 75 Ohm to a radio receiver (generaly 50 Ohm). But if the cable is short, you won't have too much loss. Or you can build a simple impedance transformer.

    The use of a preamp between the antenna and the receiver is an option. It may improve significantly the result, but not always : some (wide band) preamp ampifies the noise with the signal so the result is stronger but not better.

    As a source of inspiration here is a very simple and interresting experimentation (I didn't do it myself but it was reported by sites on the web) :
    It uses a standard FM radio receiver connected to a good external antenna. The receiver is tuned on the frequency of a broadcast transmitter that is out of range (over the horizon). Normally, the receiver don't get any signal. When a meteorit cross the sky, it may reflect a part of the signal and during a brief moment (1 second or so), you can hear the radio station.
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    nice. I think picking up tv stations as a first experiment would be good for me. I had a kit which allowed me to tune into/take over a radio station channel and say whatever i liked within the lower bands (88 - 91 Mhz) but only within 1 km. So i think picking TV stations by adjusting trimcaps in that range would suit a small antenna. Not sure though, what do you think?
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  9. #8  
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    I am not sure to understand what your "kit" is. It seems to be a transmitter in the FM range. If it is the case, I fear that these sort of devices are illegal in most countries.
    Anyway, you don't need it : the interest of passive radars is to use wave sources provided by other people. So, they are undetectable and ... less expensive.
    I think that the main diffculty is to eliminate totaly the direct signal issued by the source. A Yagi antenna is directive but its diagram shows that it has some sensibilily from both sides and from the rear. See thisFichier:MMANA-Yagi-10m-Diagram.JPG - Wikipédia
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  10. #9  
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    Ah, yes you are correct. The kit is a Fm transmitter and is from a store called Jaycar: here is the link Three Stage FM Transmitter - Jaycar Electronics. It is legal here in Australia but it still has its restrictions.

    I was wondering if it could be possible that the Yagi antenna can reduce its sensibility but rotating 360 degrees (powered by a small motor running off a rechargeable Li-ion battery of course)?
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    Actually, with a Yagi, you will have the greatest attenuation of the direct signal when the axis of the antenna is doing an angle of 90° with the direction of the wave source. This page about UHF Yagi contains a few sensibility diagrams examples that illustrate this.
    Antennas - UHF Yagi Antenna (406-512 MHz) | Yagi Antennas | COMPROD Communication


    Another trail that you might explore (if your goal is to identify and locate aircrafts) is the reception of signals emitted by the planes. Most large aircrafts embarks an anti-collision system that reports periodically the position and some other data. See ADS-B system on the wiki. You can receive these signals at a long distance (up to 200 Nm) and decode them with a rather simple equipment (dedicated radio and small antenna, a computer with a decoding software). The display of results is done on a computer screen and looks like the radar screen of an air traffic controller.
    This is not a radar, from a technical point of view, but it gives impressive results. I can tell you more on this if you are interested. Just let me know.
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  12. #11  
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    Thanks caKus, i might get the info about the aircaraft emitting signals later on from you after the TV project.

    I might try both and see which sounds best (between parabolic and Yagi). Are Indoor Yagi Antenna's better then Outdoor Yagi antennas? Because i thought they both worked the same.... Also do you think some program script writing is needed to make the dot appear on screen within a range of 0 - 5 km?
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  13. #12  
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    Hey have a look at this and tell me what you think:

    http://radiojove.gsfc.nasa.gov/teles...cvr_manual.pdf

    I am interested in building it as well.
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  14. #13  
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    Quote Originally Posted by Sci_Research View Post
    Are Indoor Yagi Antenna's better then Outdoor Yagi antennas
    Forget indoor antennas they wont be sensible enough.

    The RadioJove project is interresting because you can build everything yourself and it is not very difficult to build electronic equipement in the 20Mhz band. It would be much more difficult to work in the gigaHz frequencies.
    You must be concious that the result depends on the quality on the antenna. The RadioJove site recommands an array of dipole antennas. So
    1) you must live in an area "clean" of any electro-magnetic perturbations. If you are in the "outback" is't pretty good. If you are in the center of Sidney ... don't know what you will get.
    2) an array of dipoles in the 20 Mhz is an impressive installation (site the picture on RadioJove site). It may not be to the taste of your wife / girl friend / parents / neighbours / dogs ...

    I could find similar projects using UHF frequencies with TV antennas :
    280 MHz Radio Interferometer
    LA RADIOASTRONOMIE
    Sorry, the last one is in french, but you can have a look at the pics and schemas if you don't speak french.

    I remember I also saw a system using a simple satellite parabolic antenna connected to a small satellite finder. The antenna was directed to the sun. The satellite finder was modified in order to "hack" an exit voltage that was send to a computer. And you get a "radio telescope" able to measure the activity of the sun !

    Good luck.
    Patrick.
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  15. #14  
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    Quote Originally Posted by caKus View Post
    Forget indoor antennas they wont be sensible enough.
    Hi CaKus, are you French? Just thought I would point out that "sensible" is a faux amis; the correct English word would be "sensitive". (This catches out my Italian friends as well.)
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  16. #15  
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    Strange, you guessed it ! I had a doubt when writting it but was too lazy to check.
    Thanks.
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  17. #16  
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    Hi caKus,

    i have this site found this site for the receiver kit:

    Receiver Kits

    They say model 1056 is only $45. So thinking that's a good start. Coupled with the antenna and some components changed on the kit, it should work.
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  18. #17  
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    Hi,
    Beware of the bands of frequency available with this receiver : the max band is 10meter, that is 28Mhz. Practically, it is difficult to buid a highly directive antenna in these frequencies (a very cumbersome installation : think of a yagi antenna with a width of 5 meters and a lenght of 20 or 50 meters depending on the number of elements ). It is the reason why I recommend UHF.

    If budget is an issue (it is in most great scientific project), you might consider to buy a used UHF scanner. You will probably be able to find a good one for less than 100 USD. Check with ebay.
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  19. #18  
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    Ok still looking around for parts, but will consider what you have said. Thanks.
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  20. #19  
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    You must have a look at SDR (software defined radio). They allow you to change your computer into a radio receiver.
    Google SDR or have a look here
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