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Modify AM radio receiver to pick up radar-like ping?

Discussion in 'General Electronics Discussion' started by mollwollfumble, Aug 18, 2014.

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  1. mollwollfumble

    mollwollfumble

    4
    0
    Aug 18, 2014
    I have a 173.5 kHz radio animal-tracking transmitter and a new radio receiver. But reception is terrible even at point blank range so I want to modify the receiver by simplifying it.

    The transmitter doesn't output AM but outputs an even simpler signal, a ping, 50 milliseconds of carrier wave. It emits this ping 40 times a minute.

    According to the transmitter manufacturer, the ping ought to be able to be picked up 40 km away but my VHF receiver (Baofeng) doesn't even pick up every ping at point blank range. The signal is totally lost at a mere 50 metres (or less) line-of-sight from the transmitter. I suspect that's because the receiver can't handle such a short pulse because it's optimised for AM voice communications.

    So I want to simplify the receiver by adding and bypassing electronics (such as a big capacitor) to give the loudest ping sound. Help?

    Or failing that build my own ping detector using components (such as aerial, tuning, and speaker) out of the Baofeng receiver. Something like this perhaps? http://www.high-voltage-lab.com/105/economy-radar-detector, but with a proper aerial and tuner?
     
    Last edited: Aug 18, 2014
  2. davenn

    davenn Moderator

    13,617
    1,881
    Sep 5, 2009
    Hi
    welcome to EP :)

    the manufacturer is probably right if the freq is 173.5 kHz
    and if you had a LF receiver instead of a VHF receiver then you could probably receive it

    firstly ... did you err in the frequency you quoted ? is it really 173.5 kHz ?

    Dave
     
  3. JimW

    JimW

    59
    5
    Oct 22, 2010
    I'll go with the assumption that the transmitter and receiver are both compatible. 173.5 MHz, 120.997 MHz, whatever. As long as the receiver is set to the frequency that the transmitter is set to.

    But the problem that I see is that you have a 50 msec burst of detectable energy every 1.5 seconds. So you will need to add external circuitry. A comparator monitoring the output (where ever you can get to on your receiver, maybe even at the speaker). The output of the comparator going into a pulse stretcher. So every blip of detected energy now results into a .5 to 1 second long pulse. The output of the pulse stretcher drives an LED, a sonalert, a toy monkey that claps its hands together, whatever it is that you want to have alert you.

    You will need some test equipment to determine the ambient noise level of your radio receiver, so that you know what to set the comparator detect level at. Or perhaps just make it widely adjustable and then try dialing it in by test and error to see what works best at various ranges.

    JimW
     
  4. davenn

    davenn Moderator

    13,617
    1,881
    Sep 5, 2009
    I considered 173.5 MHz but dismissed it as being in the VHF TV band

    Am not sure how you considered 120.997 MHz ?
    that's right in the middle of the aviation band ... those guys are extremely sensitive (read as ...get upset with) to intruders in that band

    lets wait for the OP to confirm the frequency before proceeding further :)

    Dave
     
  5. mollwollfumble

    mollwollfumble

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    Aug 18, 2014
    > firstly ... did you err in the frequency you quoted ? is it really 173.5 kHz ?

    Oops, sorry. 173.5 MHz.

    > But the problem that I see is that you have a 50 msec burst of detectable energy every 1.5 seconds. So you will need to add external circuitry. A comparator monitoring the output (where ever you can get to on your receiver, maybe even at the speaker). The output of the comparator going into a pulse stretcher. So every blip of detected energy now results into a .5 to 1 second long pulse. The output of the pulse stretcher drives an LED, a sonalert, a toy monkey that claps its hands together, whatever it is that you want to have alert you.

    A "pulse stretcher". Great idea. Ideal output for me would be a signal strength meter, so I can tell how far away I am from the source, but failing that a sonic ping.

    > You will need some test equipment to determine the ambient noise level of your radio receiver, so that you know what to set the comparator detect level at. Or perhaps just make it widely adjustable and then try dialling it in by test and error to see what works best at various ranges.

    Trial and error looks good.
     
  6. mollwollfumble

    mollwollfumble

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    Aug 18, 2014
    So, would the simplest solution be Aerial to Tuner (from Baofeng walkie-talkie) to rectifier-amplifier transistor to capacitor to store up the power from the signal to a buzzer? Then the duration or loudness of the buzzer would give proximity to the source. There are buzzers that run off 3 to 16 Volts, 15 mA max 90 dB. The Baofeng runs off a 7.4 Volt battery.

    I can't see how to do anything else because a 50 ms pulse is too low for the human ear to hear and 173.5 MHz is far too high for the human ear to hear.
     
  7. davenn

    davenn Moderator

    13,617
    1,881
    Sep 5, 2009
    OK for a start ... you are not going to buy a normal receiver that receives 173 MHz

    you would need to look at getting a scanner receiver. They are designed to cover wide frequency ranges
    you just need to look through various models to make sure it covers the 173 MHz region.

    Some scanner receivers include ... Uniden, AOR

    I have an AOR AR8200 .... its an amazing receiver

    Here's a Uniden one on eBay
    http://www.ebay.com.au/itm/Uniden-U...Receiver-/121381158519?_trksid=p2054897.l5660

    you will find other makes and models ... do some searching :)

    one of its bands only just covers your freq of 173.5 MHz
    you will find this common with the cheaper models .... they are restricted in freq range compared to ones like my AR8200

    I cannot vouch for its sensitivity ( the Uniden) at 173.5 MHz when it only goes to 174 MHz ??

    Tho a scanner that covers a wide range, is a bit of an overkill for what you want, your only other choice would be to build a freq specific receiver for 173.5 MHz and unless you have good RF design and construction skills, that would be a very difficult path.
    This makes the scanner option worthwhile ... some one else has done all the hard work !! :)

    cheers
    Dave
     
  8. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    What kind of receiver does the manufacturer recommend? Surely they must say something about the receiver, or possibly even sell one.

    Bob
     
  9. mollwollfumble

    mollwollfumble

    4
    0
    Aug 18, 2014
    They do sell one, but at $800 per receiver it's more than I can afford, and has a three month delivery time. I thought I could get away with the $46 dual band VHF/UHF receiver I bought - but on trying it out I can't, hence the idea of modification.

    Have just bought a buzzer to try out with capacitor.
     
  10. KMoffett

    KMoffett

    719
    73
    Jan 21, 2009
    Again, a 50mS AM burst would have a primary frequency of 20Hz. I think that would likely be outside the audio bandpass for most communications/scanner receivers. And with built-in AGC, might be to fast for that too.

    Ken
     
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