Using a colpitts oscillator to locate a blockage in underground 14 mm plastic ducting.

Thread Starter

Hopeful

Joined Apr 6, 2022
8
Problem :- A 1000 metre length of plastic ducting 14mm o/d and 10mm i/d has been laid around my paddock in a trench 600mm deep and back-filled. This has to run around buildings, trees etc. for the first phase of laying a fibre optic cable. The ducting has to be jointed with compression plastic fittings - 15 in total- and the installers have not been too careful in their work in tightening them, with the result that the i/d has reduced from 10mm to 8mm. Because of the undulating duct lay, a measuring line cannot be used, as when the linear length above ground is measured , the submerged length could be anywhere up to 15 metres longer.
In an earlier posting of Colpitts oscillators and answered by dananak with the ref heading number of 150002/ ( his/her was the second reply ) I shall be using the single transistor CO that he suggested which looked less complicated to build and it all has to fit in a 10 mm i/d pipe !! - complete with batteries ( eight button cells in series).
Questions :-
1.......Will a single transistor oscillator, supply sufficient radiated power to be received on an ordinary VHF 88-108 MHz domestic radio when one is directly above the transmitter, which will be 600 mm below and covered with soil ? - I know I am asking a lot of you all to answer that one, but a vague guess would be most helpful !!.
2........As I require a "blip-tone" every second upon approach to, and the passing past the TX reference, I thought of a aged old friend - the 555 timer - would be splendid to replace the mic. but would the o/p just be AC coupled onto the base of said transistor,or is that too easy ? .
At 79 years old any assistance would be deemed most helpful, as I can still build the odds and ends, but my 27 year old Grandson is getting a trifle peeved at digging the holes !!,
Stay well and safe all of you,
Kind regards,
Hopeful!
 

Alec_t

Joined Sep 17, 2013
12,808
Welcome to AAC!
According to this site, cable/pipe detector frequencies in common use are way below VHF. If you're planning on using VHF you may find it won't propagate well enough through 50cm of soil.
 

Thread Starter

Hopeful

Joined Apr 6, 2022
8
Thank you for taking the time to reply - you have saved me a lot of time- I will now have to use Hall Effect chips to construct a Gradiometer, which I last did in 1978 in Oxford - such is life. I did contact the Inst. of Geo. Sci. but they could not supply the degradation of that frequency through soil/substrate.
Take care, and my thanks once again.
 

Thread Starter

Hopeful

Joined Apr 6, 2022
8
I have just had a thought. In 1976 developed a Pulse induction metal detector, using a square wave pulse of 100Kz with a m/s of 10 :1 - that worked quite well - a 2p piece at 18" as I recall. In your valued opinion would that be possible with a CO - The reason for the question was that my initial thoughts were a cheap possibility as I know very little about a CO'
Kind regards,
 

Alec_t

Joined Sep 17, 2013
12,808
I've no experience with metal detectors but I would expect the frequency used, rather than the type of oscillator, to be the important factor.
 

AlbertHall

Joined Jun 4, 2014
12,023
Many years ago we located shorts in underground cables from the surface by connecting the output of an audio amplifier to the cable and then tracking it with a coil of wire connected to a headphone amplifier. Where the sound stopped was where the short was.
 

Tesla23

Joined May 10, 2009
510
You could look at the techniques that plumbers use to locate problems underground. There is little DIY construction info that I could find, this:
https://www.radiodetection.com/sites/default/files/Theory-Buried-pipe-manual-V10.pdf
gives the basics, but only in outline form (they sell detectors). I think you want to make a sonde, and some way of receiving the signal.

I suspect that you want to keep the frequency low to avoid ground attenuation, but as you are relying on magnetic coupling, the ground attenuation may not be as great as those versed in radio comms may think (as long as you are away from magnetic objects).

I rummaged through what I had on hand and came up with a couple of chokes to run as antennas, https://au.element14.com/ferroperm/1585-500uh-3a-ins/inductor-500uh-3a-axial/dp/1186803. Connecting one to a function generator through 75 ohms and resonating the other with a parallel 33nF polyester cap and connecting to the oscilloscope, at 500mm spacing in air I obtained 1.8mV p-p at the oscilloscope from 14.4V p-p across the transmitting coil at the resonant frequency of 39kHz.

None of this is likely optimal, but it does suggest that it could be the basis for a detector. You would likely send the (significant - in my case 57mA pk) transmitter current into the conduit via the connecting cable.

These coils are about 8mm diameter and 18mm long.

Cranking up the frequency does improve the range in air - omitting the 33n and the system resonates at about 620kHz with the input capacitance of my scope/cable. I measure 40mV p-p on the scope for about 600mm spacing in air. The current in the transmitting coil is about 5mA pk. As the coupling is magnetic, this may pass through the ground OK, in which case you could get away with an AM radio as a detector (with some form of signal strength indicator).
 
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Thread Starter

Hopeful

Joined Apr 6, 2022
8
Many years ago we located shorts in underground cables from the surface by connecting the output of an audio amplifier to the cable and then tracking it with a coil of wire connected to a headphone amplifier. Where the sound stopped was where the short was.
Thank you for your reply, but I may not have made myself clear. The plastic ducting has no fibre-optic cable in it at present, it is the plastic duct that has deformed and is causing a blockage and this is what I require to detect, but as it it 1000 meters long my plan is to fire a self contained sensor up the ducting using 100 psi compressed air, then when that stops due to the blockage a top receiver will be used to locate.
 
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Thread Starter

Hopeful

Joined Apr 6, 2022
8
Thank you for your reply, but I may not have made myself clear. The plastic ducting has no fibre-optic cable in it at present, it is the plastic duct that has deformed and is causing a blockage and this is what I require to detect, but as it it 1000 meters long my plan is to fire a self contained sensor up the ducting using 100 psi compressed air, then when that stops due to the blockage a top receiver will be used to locate.
You could look at the techniques that plumbers use to locate problems underground. There is little DIY construction info that I could find, this:
https://www.radiodetection.com/sites/default/files/Theory-Buried-pipe-manual-V10.pdf
gives the basics, but only in outline form (they sell detectors). I think you want to make a sonde, and some way of receiving the signal.

I suspect that you want to keep the frequency low to avoid ground attenuation, but as you are relying on magnetic coupling, the ground attenuation may not be as great as those versed in radio comms may think (as long as you are away from magnetic objects).

I rummaged through what I had on hand and came up with a couple of chokes to run as antennas, https://au.element14.com/ferroperm/1585-500uh-3a-ins/inductor-500uh-3a-axial/dp/1186803. Connecting one to a function generator through 75 ohms and resonating the other with a parallel 33nF polyester cap and connecting to the oscilloscope, at 500mm spacing in air I obtained 1.8mV p-p at the oscilloscope from 14.4V p-p across the transmitting coil at the resonant frequency of 39kHz.

None of this is likely optimal, but it does suggest that it could be the basis for a detector. You would likely send the (significant - in my case 57mA pk) transmitter current into the conduit via the connecting cable.

These coils are about 8mm diameter and 18mm long.

Cranking up the frequency does improve the range in air - omitting the 33n and the system resonates at about 620kHz with the input capacitance of my scope/cable. I measure 40mV p-p on the scope for about 600mm spacing in air. The current in the transmitting coil is about 5mA pk. As the coupling is magnetic, this may pass through the ground OK, in which case you could get away with an AM radio as a detector (with some form of signal strength indicator).
Thank you for the information - I intend to shoot the self contained sensor ie 3 x 25.4mm x 8mm Neodimium magnets with a 100 psi compressor up the plastic ducting, then walk the track until the magnetic field is detected with my gradiometer - when I have designed it - halve that distance and we should be there, but as always, fingers crossed.
 

AlbertHall

Joined Jun 4, 2014
12,023
Thank you for your reply, but I may not have made myself clear. The plastic ducting has no fibre-optic cable in it at present, it is the plastic duct that has deformed and is causing a blockage and this is what I require to detect, but as it it 1000 meters long my plan is to fire a self contained sensor up the ducting using 100 psi compressed air, then when that stops due to the blockage a top receiver will be used to locate.
If you could fire a loop of cable (twin cable with the underground end shorted) then you could find where the cable stopped using the method I described.
 

sparky 1

Joined Nov 3, 2018
717
In foxhunting a small underground transmitter, the suggestion about mixing audio to a carrier is a good idea.
Another Colpitt oscillator could produce the audio tone. You can experiment finding the audio peak above ground.
The midpoint on an antenna is usually the strongest.
 

panic mode

Joined Oct 10, 2011
2,134
years ago i've built many small oscillators and FM transmitters but never measured how they penetrate media like soil but in short distances (few meters) it will penetrate couple of solid brick walls. the soil will be wet and dampen signal more strongly. i would consider using lower frequency. also one transistor oscillator will produce carrier only and this will be hard to detect. i would definitely modulate it with some audible frequency (1kHz or so). Of course this can be tested before being deployed.
btw 100PSI is quite powerful and may be enough to undo the kink on its own..
 

Thread Starter

Hopeful

Joined Apr 6, 2022
8
If you could fire a loop of cable (twin cable with the underground end shorted) then you could find where the cable stopped using the method I described.
Thank you for your reply. I only have a working dia. of 8mm and a length of 1000 yards, the only way I can send a detector that distance at that diameter is by compressed air, and apart from the sealing problems of 100 psi
You could look at the techniques that plumbers use to locate problems underground. There is little DIY construction info that I could find, this:
https://www.radiodetection.com/sites/default/files/Theory-Buried-pipe-manual-V10.pdf
gives the basics, but only in outline form (they sell detectors). I think you want to make a sonde, and some way of receiving the signal.

I suspect that you want to keep the frequency low to avoid ground attenuation, but as you are relying on magnetic coupling, the ground attenuation may not be as great as those versed in radio comms may think (as long as you are away from magnetic objects).

I rummaged through what I had on hand and came up with a couple of chokes to run as antennas, https://au.element14.com/ferroperm/1585-500uh-3a-ins/inductor-500uh-3a-axial/dp/1186803. Connecting one to a function generator through 75 ohms and resonating the other with a parallel 33nF polyester cap and connecting to the oscilloscope, at 500mm spacing in air I obtained 1.8mV p-p at the oscilloscope from 14.4V p-p across the transmitting coil at the resonant frequency of 39kHz.

None of this is likely optimal, but it does suggest that it could be the basis for a detector. You would likely send the (significant - in my case 57mA pk) transmitter current into the conduit via the connecting cable.

These coils are about 8mm diameter and 18mm long.

Cranking up the frequency does improve the range in air - omitting the 33n and the system resonates at about 620kHz with the input capacitance of my scope/cable. I measure 40mV p-p on the scope for about 600mm spacing in air. The current in the transmitting coil is about 5mA pk. As the coupling is magnetic, this may pass through the ground OK, in which case you could get away with an AM radio as a detector (with some form of signal strength indicator).
Thank you for .carrying out those tests, it does give food for thought, I will do some this end
 

Thread Starter

Hopeful

Joined Apr 6, 2022
8
years ago i've built many small oscillators and FM transmitters but never measured how they penetrate media like soil but in short distances (few meters) it will penetrate couple of solid brick walls. the soil will be wet and dampen signal more strongly. i would consider using lower frequency. also one transistor oscillator will produce carrier only and this will be hard to detect. i would definitely modulate it with some audible frequency (1kHz or so). Of course this can be tested before being deployed.
btw 100PSI is quite powerful and may be enough to undo the kink on its own..
Thank you for that. The 100psi is safe as the leakage passed the sensor will be around 25%. Sadly the wall thickness of the ctrunking is 2mm thick and the plastic is very strong
 

Janis59

Joined Aug 21, 2017
1,383
FM range have harshly bad bunch of habits - weakening, re-radiating, reflecting etc etc. Normally for such, people are using 110 kHz modulated by 1000 Hz. And factory made machinery cost few USD, and certified to be effective until several hundreds of meters. For example https://www.ebay.com/itm/185027183835?hash=item2b147ac4db:g:BHwAAOSw7RBhL0-U or https://www.ebay.com/itm/124995701119?hash=item1d1a53097f:g:okYAAOSwg~dhiJeH or https://www.ebay.com/itm/194921023918?hash=item2d6232c9ae:g:Tq8AAOSwHxpiMsXe
 

Pyrex

Joined Feb 16, 2022
6
It is important to take in mind that duct diameter is limited to 8 mm. It is not easy to make such a small transmitter with sufficient output power.
I guess, it is easier to solve the problem in an another way. Wrapp a coil with diameter of 8 mm, add a capacitor to form a parallel LC tank. LC tank is to be tuned to 50-100 kHz. This LC tank will be used as a transducer and is to be pushed into the duct .
Take a generator , powered from batteries or from the mains- no matter. Set it to the same frequency, as the LC tank was tuned to. Take a coil with diameter of 10-20 cm and connect it to the output of the generator thru resistor ( approx 100 Ohm-1 kOhm) . Use the HF millivoltmeter to control the voltage on the coil.
Walk along the burried duct and continuosly check voltmeter reading. When stand above the transducer, voltmeter readings will fall due to interaction with the LC tank
 

Thread Starter

Hopeful

Joined Apr 6, 2022
8
Thank you all very much for your time and assistance, it is reassuring to know in this day and age that all engineers still help each other. I have considered all of your suggestions but I will go with building a gradeometer as I have at last found a supplier of rare earth magnets and the field will be sufficient 24 inches deep. Thank you all once again and thank "all about circuits" for setting up such an informative sit
 
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