Leaky Coax Perimeter Project

kochevnik · Jan 17, 2013

I am a newbie to electronics and since this is an rf system albeit a strange one I thought you guys might have some answers for a project I am working on.

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

**broken link removed**

Full project pdf at the bottom of that page.

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.

jiripolivka · Jan 17, 2013

kochevnik said:
I am a newbie to electronics and since this is an rf system albeit a strange one I thought you guys might have some answers for a project I am working on.

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

**broken link removed**

Full project pdf at the bottom of that page.

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.

From your description it looks like you directly feed the VHF signal to the scope.

I would prefer to use a RF detector before the scope. The "leaky cable" structure as you described passes the RF signal from one line to another with some propagation loss.
Human body changes the propagation loss; it may decrease or increase, the change indicates body presence between the lines. YOu may even observe the body if it appears on the other side of the "receiving" line if it is close to it (or the transmit line, too).

By using a RF detector, you obtain a voltage which will vary due to body presence. Then you can use an ADC to process it like the scope shows. Also, with the detector you can better use the scope sensitivity (and ADC.too) as for low-frequency detector output the load can be 1 MOhm, not 50 Ohms without a detector.

kochevnik · Jan 17, 2013

Thanks for the help !

Actually what I was doing is using the scope probe to check the returned rf signal - so the probe on x10 that gives me 1Mohm yes ?

But I will definitely need an RF detector for the final system - thanks for the head's up I didnt know these existed.

From the googling I did it seems that what I want is an rms rf detector ?

Do you have any idea what process is in play when human body affects the Rx signal ? I was told the body acts as a lossy delectric and this affects the capacitance in the Rx wire / antenna and that this in turn would change the amplitude of the signal that is being received. Does this make sense ?

Thanks again, appreciate the help.

jiripolivka · Jan 17, 2013

kochevnik said:
Thanks for the help !

Actually what I was doing is using the scope probe to check the returned rf signal - so the probe on x10 that gives me 1Mohm yes ?

But I will definitely need an RF detector for the final system - thanks for the head's up I didnt know these existed.

From the googling I did it seems that what I want is an rms rf detector ?

Do you have any idea what process is in play when human body affects the Rx signal ? I was told the body acts as a lossy delectric and this affects the capacitance in the Rx wire / antenna and that this in turn would change the amplitude of the signal that is being received. Does this make sense ?

Thanks again, appreciate the help.

You are right that human body is a block of lossy dielectric that can attenuate, reflect or guide the VHF waves. The effect may be calculable but is quite complex over frequency.
The "Perimeter fences" were conceived long ago, later the "lossy coax" was added.
In fact, if you send pulsed RF signals along the line, by checking the delay one can estimate the position of the human body from the transmitter. Again, as the lossy lines are laid on or under the ground, the position can only be estimated.

It may be possible to directly digitize the RF signal as you did with the scope but I do not see any profit of such complexity. A simple RF detector and comparator or ADC should do well.

- - - Updated - - -

For the RF detector in this application I would not buy any costly one. You can make one from an old (or new) Germanium diode like 1N34 or 1N60 which are good to >200 MHz. At video output use a small, ~100-300 pF capacitor to ground to "catch" fast-moving bodies (a running guy or a pet). Try a simple comparator afterwards like LM311 with adjustable threshold, and have fun e.g. with a blinking LED.

kochevnik · Jan 17, 2013

OK that makes sense. I had gone to all the 'leaky coax' perimeter detection companies and looked at their websites - I didnt bother to ask price LOL as I assumed it would be many tens of thousands of dollars. I was aware from their sales literature that they could pinpoint to within a meter or so where a person crossed their lines, for me I'm not looking to do anything even remotely like that, all I want to know is if a human (or I suppose a large animal) crosses the perimeter wire anywhere along it's length. Altho, the idea of sending pulsed signals kind of intrigues me - but for now what I am trying to figure out is how best to set up the very simplest Tx + Rx coax wire system that I could hook to my scope to see a human body effect on the Rx wire.

On another forum a poster said that the effect was based on the change in capacitance between the Rx and Tx wires - does that makes sense ? In the Wong paper I cite above it seemed like they were not really sure what caused the human body effect and came up with a couple of diff explanations - at the same time they noted a 1.7 meter tall human might act as a half wave antenna for a 150 mhz signal.

Since I'm going for simplicity here, and as you stated the body can reflect attenuate or guide the VHF waves, I'm thinking of just picking a couple of fixed frequencies 70 mhz 100 mhz 120 mhz etc. My main goal is just to demonstrate that some kind of human body effect can be picked up on a scope.

So based on all that, if there was an effect, would it appear on my scope as a change in the peaks of my waveform - i.e. the peak to peak voltage should either increase or decrease and it should do it for a fairly long period of time (1/2 second +) as the speed of human movement is fairly slow. Does that make sense ?

The diode and capacitor setup is a good idea - I was going to ask you if that was how the rf detectors worked.

Thanks again for you help.

- k

jiripolivka · Jan 19, 2013

kochevnik said:
OK that makes sense. I had gone to all the 'leaky coax' perimeter detection companies and looked at their websites - I didnt bother to ask price LOL as I assumed it would be many tens of thousands of dollars. I was aware from their sales literature that they could pinpoint to within a meter or so where a person crossed their lines, for me I'm not looking to do anything even remotely like that, all I want to know is if a human (or I suppose a large animal) crosses the perimeter wire anywhere along it's length. Altho, the idea of sending pulsed signals kind of intrigues me - but for now what I am trying to figure out is how best to set up the very simplest Tx + Rx coax wire system that I could hook to my scope to see a human body effect on the Rx wire.

On another forum a poster said that the effect was based on the change in capacitance between the Rx and Tx wires - does that makes sense ? In the Wong paper I cite above it seemed like they were not really sure what caused the human body effect and came up with a couple of diff explanations - at the same time they noted a 1.7 meter tall human might act as a half wave antenna for a 150 mhz signal.

Since I'm going for simplicity here, and as you stated the body can reflect attenuate or guide the VHF waves, I'm thinking of just picking a couple of fixed frequencies 70 mhz 100 mhz 120 mhz etc. My main goal is just to demonstrate that some kind of human body effect can be picked up on a scope.

So based on all that, if there was an effect, would it appear on my scope as a change in the peaks of my waveform - i.e. the peak to peak voltage should either increase or decrease and it should do it for a fairly long period of time (1/2 second +) as the speed of human movement is fairly slow. Does that make sense ?

The diode and capacitor setup is a good idea - I was going to ask you if that was how the rf detectors worked.

Thanks again for you help.

- k

I think you are right and understand the system operation. A human body is a nice chunk of lossy impedance, and if located around antennas and lossy cables, it does affect the wave propagation. In the early radio times, radio operators were skilled in tuning their antennas for the best reception, often by hands or bodies moved around the tuned circuits.
Now we observe similar effects with mobile phones as their tiny antennas are close to the hands, heads and bodies of the users.
As you have started experimenting, you will see what works best. When choosing the frequency, be sure you use "unlicensed" ones. If you cause an interference to other licensed frequency users, you could be prosecuted by law. Use also the lowest output needed; it makes no sense to radiate watts when a milliwatt can do the job.
Most good detectors are composed of one diode and the output capacitor. RF input is mostly a 50-Ohm resistor to make the device wideband, and the video output is assumed to be high impedance, like your scope input, 1 MOhm/ 30 pF.

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Leaky Coax Perimeter Project

kochevnik

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jiripolivka

Advanced Member level 5

kochevnik

kochevnik

Newbie level 4

jiripolivka

Advanced Member level 5

kochevnik

kochevnik

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jiripolivka

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