Leaky Coax Intruder / Perimeter Alert Project

Discussion in 'The Projects Forum' started by kochevnik, Jan 16, 2013.

  1. kochevnik

    Thread Starter New Member

    Jan 7, 2013
    I am a newbie - so please be kind :)

    I am working on a project that uses two parallel 'leaky' coax cable to determine when a human being crosses the wires. This is based on student paper (Simon C Wong) that I found on the internet (see attached pdf).

    In a nutshell, if you run an RF signal thru coax that is designed to propagate signals, then place a second coax wire about a meter away, the signal in the second wire will be altered by the presence of a human - my basic understanding being that the human acts as an antenna. In the paper Wong used a 150 mhz signal (5 watts ?) and then the human body would be act halfwave antenna.

    I'm basically a software guy - I've gotten into digital hardware mcu's and stuff in the last year, this is my first venture into the analog realm. So I bought an old Tek analog scope off ebay, learned how to use it, got an old eico 324 rf signal generator also off ebay and set up two coax wires parallel about 3 feet apart. The first wire (I call the primary) is hooked to the eico red lead and I pumped a bunch of different frequencies down them - tried from 1 to 145 mhz (max on the eico). The secondary coax, acting as the antenna, I hooked to my scope.

    One important thing, since I didnt have any 'leaky' coax, I just stripped two coax wires and wire them up so that the signal is output on the shield of the primary and the oscilloscope probe is hooked to the shield on the secondary. From my reading running a signal down the inner core wire would be useless as coax is designed specifically to shield those signals, by 'flipping' the wiring I assumed that the signal would propagate a lot more and be more easily picked up on the secondary. Oh and the secondary I soldered a resistor from the core wire to the shield. Ground from the scope probe is hooked to the ground on the signal output of the eico.

    So far so good.

    Wired it up, placed the parallel coax on the floor, fired up the eico, sent a bunch of signals thru it from 1 mhz to 145 mhz - and Voila ! when I walk over the secondary cable the signal on the scope literally hops off the screen and then back on. Some freqs seem to work a little better than others, at 36 mhz seems to work best but I think this is because the eico has range knobs and the high range signal is not as strong as the second from the last range.

    So here are my main two questions :

    1) When I say the signal hops off the scope, I mean that from my view of the scope screen (using human eyeballs) the entire waveform maintains its shape, it just appears to jump up off the screen then come back down - but again, to my view, the amplitude does not appear to change. As a complete newbie I don't understand what phenomenon I am seeing - can someone explain please ? And to add I'm not entirely sure what I am seeing is a real signal or not ?

    and 2) Can I use a DAC to 'catch' this signal and process it ? Since from my view the amplitude is not changing, I don't see how the dac would catch this ?

    Appreciate any help / advice anyone can give me.
    Last edited: Jan 16, 2013
  2. Sidra123

    New Member

    Aug 28, 2013

    I am working on same project you have worked on. I am following the same research paper. If you can guide me please on this project, I have just started this project... How much you have done?
  3. kochevnik

    Thread Starter New Member

    Jan 7, 2013
    I did not get very far - I didn't get very far in my tests. I might come back to it in a few months but have other projects I am working on now. The two issues I had were that I couldn't see what kind of effect I should be looking for (spike in frequency or spike in amplitude or even a drop ?). I think the correct cable for teting this is not very cheap either. And if you were looking for a frequency spike then how to see it in a 150 Mhz signal - peak detector is the best idea I came up with.

    There are a few more ideas here : http://www.edaboard.com/thread277390.html
  4. #12


    Nov 30, 2010
    How can Wong present a 28 page paper without a schematic?
    Is there a rectifier anywhere in the detector system?
    Was the oscilloscope set on "AC" coupling so that a sudden DC change would make the trace hop once then drift back to the center of the screen?

  5. kochevnik

    Thread Starter New Member

    Jan 7, 2013
    Human Body Effects Papers
    The body affects the
    characteristic impedance of the transmission lines causing mismatch,
    changes the electrical length, and significantly increases losses, thus
    potentially deteriorating the operation of an RF circuit.
    The close proximity of human body with a high dielectric constant
    and loss are known to have a detrimental effect on antenna input
    impedance and efficiency.
  6. kochevnik

    Thread Starter New Member

    Jan 7, 2013
    If you look at page 11 of Wong's paper, I don't think there WAS any kid of circuit that needed a schematic. He basically hooked up an RF generator to the TX LCX line, then put a bnc connectors on the RX lines and hooked them up to a data acquisition box (National Instruments BNC 2110 12 bit A/D DAQ board), which went to a computer with labview on it. No real schematic needed. Pumped a 5w 150 mhz signal down it (I'm assuing a sine wave) and looked at the signal in labview when he walked across the wires.

    The other thing that I didnt understand is whythe signal amplitude INCREASED when a person walked across the wires. But clearly it did do that in Wong's setup. The human body effects papers above seemed to say the opposite would happen, altho they are for a different setup obviously.

    Attached is the datasheet for the Senstar Omni trax Leaky Coax system - which I am sure cost a LOTof money :) Not a lot of tech infoin there, but some useful info can be gleaned from the specs alone.
    Last edited: Aug 29, 2013