Transmitter for line/pipe location sensing

Thread Starter

keithwins

Joined Jan 28, 2014
8
So: I am sort of a newbie, just enough knowledge to ask the wrong questions. That said, I'm 60 yo with a Physics degree, so... I'm looking for help thinking about this. I'm not very familiar with some of this science (i.e. radio transmissions & antennas, I guess). Trying to get smarter here.
I'm about to buy an underground line locator: a Ridgid SR-20. It's a fairly expensive ($1500 used) unit with antennas and circuits to try to tell you the depth and location of underground power and water lines. The water lines have to be metal, and are energized with an associated Transmitter. A number of transmitters are available, in the Ridgid line that would include the ST-305 and the ST-510, battery-powered units rated at 5 & 10 watts each.
Each of those transmit by attaching to the metal line, and to a nearby ground, and transmit several different frequencies: generally, lower frequencies work better apparently, by remaining more local to the line/less dispersion (might be improper use of a technical term).
Anyway: I'm a cheapskate, especially after having dumped $1500 into the SR-20, so I want to fake the transmitter (the ST-305/510 retail for $800/$2000 or something). But I can't quite figure out what I'm looking for: is this a sine-wave signal generator? Some kind of power-limited blah blah... uh, I'm in unfamiliar territory. I can tell that I'm trying to get the pipeline to function as an antenna, unless I'm even wrong on that. Any help thinking about this will be appreciated.
Thanks!

Oh, I meant to mention: these units transmit at specific frequencies, with 512 Hz often identified as ideal, up into the few thousand kHz.
 

MisterBill2

Joined Jan 23, 2018
6,998
A good "cheating trick" would be to get a manual, or even a service manual, for one of those Ridgid brand transmitters. The service manual should contain a circuit diagram and operating conditions and from that you can produce a similar device.
Otherwise, possibly an audio amplifier, since 512 Hz is in the audio range. You would need to add an impedance matching transformer.
 

strantor

Joined Oct 3, 2010
5,329
I can't quite figure out what I'm looking for: is this a sine-wave signal generator?
[...]
Oh, I meant to mention: these units transmit at specific frequencies, with 512 Hz often identified as ideal, up into the few thousand kHz.
It could be a sine wave but I doubt it. Sine waves are not the easiest waves to produce with battery powered devices. My money is on a square wave. That may or may not matter. The receiver may only be looking for a frequency, not a specific waveform. Or, to reduce false positives, may be looking for a specific waveform. My money is on not.

In this situation I think what I would do, is get a function generator, or an arduino, or a 555 circuit, whatever is the simplest means at my disposal to get a 512hz square wave, attach it to a dummy pipe, and see if the receiver will register it. You'll probably need some sort of amplifier circuit though.

I second the suggestion to scour the internet for a service manual. That will probably tell you what kind of wave you're looking to replicate.
 

MisterBill2

Joined Jan 23, 2018
6,998
Now I wonder if the frequency is 512 Kilohertz? And the other thing is that to transmit through the ground takes a lot more power. But those prices for the equipment seem very high, almost in the military equipment price range. So I am wondering if the TS would share the intended application, since there may be alternatives that are not generally known to everybody.
 

ronsimpson

Joined Oct 7, 2019
910
I built a line/pipe location sensing circuit. The receiver was a battery transistor radio. (don't remember if it was AM or FM) For the transmitter I used simple wireless microphone transmitter. Must be AM because the radio has a ferrite stick antenna. If the radio was held pointing in the direction the wire/pipe goes the radio picked up well, but if the radio is at a right angle then no signal.
 

MisterBill2

Joined Jan 23, 2018
6,998
I built a line/pipe location sensing circuit. The receiver was a battery transistor radio. (don't remember if it was AM or FM) For the transmitter I used simple wireless microphone transmitter. Must be AM because the radio has a ferrite stick antenna. If the radio was held pointing in the direction the wire/pipe goes the radio picked up well, but if the radio is at a right angle then no signal.
How deep was the pipe? That matters quite a bit.
 

MisterBill2

Joined Jan 23, 2018
6,998
There are a nomber of ways to track a pipe underground, depending how deep it is buried, how big it is, if it is ferrous, other metal, or non-metallic. Now that the TS has the receiver it is sort of important to know what the transmitter needs to be sending.
 

MisterBill2

Joined Jan 23, 2018
6,998
Theory: https://www.radiodetection.com/sites/default/files/Theory-Buried-pipe-manual-V10.pdf

512 Hz is a frequency

Buying the xmitter and improving the receiver migh have been the better way. Audio amplifiers might be in the right frequency range, but don't generally employ VI or current limiting.

A $20.00 scope, a cheap AM radio for detectors.
Many transformerless audio amplifiers can deliver full power up to 33 KHz and so they could drive a pipe in the ground very well if adequate impedance matching is provided. And a receiver tuned to a prequency can be very sensitive if it can avoid broadband noise. So the plan would be to know what frequency the receiver can tune to.
 

MisterBill2

Joined Jan 23, 2018
6,998
The manual in the link certainly provides a detailed description of what the transmitter does, while the demand that servicing be done by only a company service location indicates that internal details are not available. But knowing the anticipated load impedance and the max output voltage and the power levels it is possible to know in fairly close detail what the circuit must provide. All of those extra convenience features and functions are not really needed to do tracing, and they are inside the microprocessor program in the device, as well as covered by a patent or several.
But duplicating the outputs in an independently designed system avoids any patent issues, and so the creation of an alternative transmitter can proceed.
 

MisterBill2

Joined Jan 23, 2018
6,998
The file I posted in post 8 had the typical frequencies used and why.
OK, so now we know just about everything needed to be able to specify and then design a useful transmitter to go with that receiver. An oscillator that can be set for the specific frequencies, an amplifier with at least 10 watts output capability, and a matching arrangement to couple the signal to the pipe to be traced. The hard part would be putting it in an easy to use reliable package.
 

Thread Starter

keithwins

Joined Jan 28, 2014
8
Hey, thanks everyone! I got busy with other things for a few days.
It seemed to me that the receiver was the complicated piece of equipment, and hence I bought it. If you look at videos of how the SR-20 works, perhaps you'll understand what I mean. It's doing some fancy math in there, I think.
However, I may have underestimated what the transmitters are doing, since they are suspiciously pricey also (as someone pointed out). Now, they should be built to handle fairly challenging (dirty, wet, messy) working conditions, so that will explain a little of it... but they might be doing all kinds of impedance matching etc.
As I reflected on it further, I realized some of the challenge of the situation: in principle, I can use this system to trace my water pipe (in fact, that's just what I'd hoped to do). However, water pipes are routinely used as one of the two required grounding systems for the electrical system of your house, here in the US. That means they are counting on a good electrical connection between the pipe and the earth. I think that makes the operation of said pipe as an antenna that much harder... So I don't know if these transmitters are also doing some crazy math hinging on the impedence of the boundary layer between the pipe and earth, or some such craziness. It would help explain the pricing, though not really: the 5W Ridgid unit (ST305) is $700 right now on Amazon, and the 10W unit is $1700. They both have to be doing the same math, so that has to be a "what the market will bear" calculation, I think. Amplifiers don't cost $200/W.
Well, like I said, I was looking for help thinking about it, and you got the gears spinning a bit. Not sure quite where I'll take it. I might see an oscilloscope in my future though.
 

Thread Starter

keithwins

Joined Jan 28, 2014
8
Oh, I forgot to mention: my receiver just arrived today.
It's a Ridgid SR-20: here's the frequency breakdown, from the manual:
Active Line Trace Frequencies *128 Hz, 1 kHz, 8 kHz and 33 kHz.
Passive Power Trace50 Hz, 60 Hz, <4 kHz Broadband.
Passive Radio Trace4 kHz-15 kHz, 15 kHz-36 kHz.
Sonde Frequencies *16 Hz, 512 Hz, 640 Hz, 850 Hz, 8 kHz, 16 kHz, 33 kHz.
 

Thread Starter

keithwins

Joined Jan 28, 2014
8
Oh, I forgot to mention: my receiver just arrived today.
It's a Ridgid SR-20: here's the frequency breakdown, from the manual:
Active Line Trace Frequencies *128 Hz, 1 kHz, 8 kHz and 33 kHz.
Passive Power Trace50 Hz, 60 Hz, <4 kHz Broadband.
Passive Radio Trace4 kHz-15 kHz, 15 kHz-36 kHz.
Sonde Frequencies *16 Hz, 512 Hz, 640 Hz, 850 Hz, 8 kHz, 16 kHz, 33 kHz.

So the 512 only really applies to Sonde frequencies. That would be a radio beacon you insert in a sewer line or something: they sell 3 for $140 or something, flushable little pods. I guess they will hopefully let you find an underground clog.
I think I was really thinking of the Active Line Trace part, which makes sense now I guess. Interestingly enough, the receiver sort of goes wonky at 512, so I don't know what that's about... it just sort of buzzes hums and cackles in all directions.
I
 

MisterBill2

Joined Jan 23, 2018
6,998
While the transmitter does have some sort of complex functioning, it is iindeed the receiver that would be more effort to duplicate. A basic transmitter seems to me is simpler just because amplifiers and impedance matching are common to many things and so much more information should be available about generalized cases of those functions. Certainly rugged abuse resistant packaging takes some effort, no question there. But the transmitter gets connected to a pipe and stays in one place while the receiver goes off tracing that pipe all over.

AND, while a water pipe is connected as a ground for the mains power system it really is a secondary connection, the power can work fairly well if that connection is lost. At least under normal conditions.
 
BTW, the DSO150 can sometimes be had assembled for less than $30.00. it's single channel, 200 kHz bandwidth. Mine came with a probe, but no power supply. 9 V. There's no battery compartment either. Mine was assembled with a x1 x10 probe. Do not exceed 9V.

I strapped a 9V protected battery pack on the back with a rubber band and used a commercial connector.

You can't go wrong. it's a lot more than a toy. https://jyetech.com/dso-150-shell-oscilloscope/47

Remember, assembled for <$30.00 USD, with a probe and no power supply.
 
https://cdn2.ridgid.com/resources/media?key=d803f3e3-275a-4dff-af0b-241257049a10&languageCode=en&countryCode=US&type=document#page=5.

So, it suggests 320 ohm load nominal, 10W, 25 mA and 63 VRMS Two frequencies at the same time.

Unfortunately, take (63)*(63)/8 and you get 500W or so.

A telephone coupling transformer 600:600 ohm might not be a bad place to start. https://cdn2.ridgid.com/resources/media?key=d803f3e3-275a-4dff-af0b-241257049a10&languageCode=en&countryCode=US&type=document

But frequency response is very limited.

High voltage OP-amps are expensive, but you don;t need too much current.

Tube audio transformers might be the way to go. https://www.tubesandmore.com/search/node/transformer?page=1

https://www.apexanalog.com/products/linear_selector.html makes high voltage, high power OP-amps and they are not cheap.

You also may need to mix two frequecies together.

A lot of the audio transformers probably average around 20 mA max current.

Just throwing ideas out with very little merit.

but yea, a 500 W into 8 ohm amplifier has the voltage even though it's putting out little power.

So, two oscillators, a mixer and a 500W solid state amp with say a metal oxide series resistor is probably cookbook overkill.

A valve amp is probably better suited.

Transformers obey the VpIp=VsIs relationship. Primary power = secondary power.
And you need up to 63 Vrms into 320 ohms.

EDIT: maybe not. I built the Leach Amp. http://leachlegacy.ece.gatech.edu/lowtim/ It has +-50V rails. it could probably reach 63/2*1.414 or +-42V or so without drawing much current. Voltage gain would have to be higher.

it's not an amp to sneeze at. Definately has the bandwidth and then some.
 
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