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Finding broken wires underground

by Q_Goest
Tags: invisible fence wire
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Q_Goest
#19
Apr1-14, 09:46 AM
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Quote Quote by sophiecentaur View Post
This is PF, remember.
Yes, I have my cat hearding license from the ASPCA. They teach you that constant course corrections are needed to keep any thread on track!

Perhaps if I shoot out observational blurbs, something will ring a bell, so here goes..

Ok, I don't know what attenuated, folded dipole, pulse repetition blah bla bla means. I'll have to look those up... But here's another observation...

I know if I don't have the radio tuned to the right frequency, around 600 khz, it won't work. Just a tiny bit above or below that and the radio doesn't pick up a signal. Why?

It must have something to do with the RF choke and I suspect something other than this particular RF choke won't work. Radio Shack has lots of different RF chokes, and I haven't tried any others, but I suspect there's a reason the instructions say to use this particular one and it has something to do with the frequency the wire will transmit at. In fact, this particular RF choke and this particular AM frequency is intended for use on ANY underground wire, so I don't think claims about soil capacitence or wire length have anything to do with how this works. Further, I suspect this particular RF choke has something to do with the ability to detect the wire near the break. Perhaps this particular choke produces a 10' gap while other chokes produce a larger gap.
AlephZero
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Apr1-14, 10:22 AM
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Quote Quote by Q_Goest View Post
I know if I don't have the radio tuned to the right frequency, around 600 khz, it won't work. Just a tiny bit above or below that and the radio doesn't pick up a signal. Why?
From your first OP, the system is effectively a radio transmitter with the wire loop as the aerial, and a receiver in the dog collar. The collar is presumably activated by the strength of the signal received, i.e the distance from the wire.

So you have to tune the radio to the transmitter frequency to pick up the signal. If the sound is just "static", it is probably transmitting an unmodulated signal (constant amplitude and frequency). Radios are usually designed to mute the audio output when there is no signal received. When the radio is tuned to the signal, that noise suppression is switched off. The static noise you hear is the random noise generated by the radio's circuits, unrelated to what is being transmitted.

I don't expect the exact type of RF choke is critical, but giving a Radio Shack part number is convenient for people with no background knowledge. Anything with an inductance the same order of magnitude as the unbroken wire loop would probably work fine.
Q_Goest
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Apr1-14, 11:28 AM
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Quote Quote by AlephZero View Post
From your first OP, the system is effectively a radio transmitter with the wire loop as the aerial, and a receiver in the dog collar. The collar is presumably activated by the strength of the signal received, i.e the distance from the wire.

So you have to tune the radio to the transmitter frequency to pick up the signal.
Are you saying you believe the 600 khz AM signal is what the system normally puts out and is picked up by the dog collars? If that's true, then I should be able to pick it up on my AM radio now that it's repaired, right?
sophiecentaur
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Apr1-14, 12:04 PM
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1. What do you actually hear on your mf receiver? I imagine the box produces pulses (bursts) of 600kHz rF which activate pooch's pain collar (did you ever see Twin Peaks?) and which may come through as clicks (or something) on the speaker when the receiver is tuned to the right frequency (as with any received RF signal). This collar needs to be a bit selective in what signals will activate it or a local radio broadcast could send pooch wild!
2. The choke need not be very accurately specified and I guess it is largely there to fool the box into believing there's a complete conductor. Your distance limit near the end may depend on actual routing / lengths / depth buried etc. etc. as well as the choke value. Two wires, fed at the centre, behave like a dipole and they will also operate if you bend them round into a circle - you just get a different impedance. If the added choke acts as a bit of improvement in the matching the broken system then it cannot be highly critical as the break could be anywhere and the routing is unknown.

Also, your loop / wire system is not an 'aerial' as such because it is not required to 'radiate' its power (better not to, in fact). It just couples with a nearby receiver and its near field operation is what counts.
Q_Goest
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Apr1-14, 12:28 PM
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What's an MF receiver? I had an AM radio tuned to 600 khz. I haven't tested it yet with the wire intact. I'll check that tonight and report back.
sophiecentaur
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Apr1-14, 01:07 PM
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Quote Quote by Q_Goest View Post
What's an MF receiver? I had an AM radio tuned to 600 khz. I haven't tested it yet with the wire intact. I'll check that tonight and report back.
It's what you have but I am using a better defined term for it. AM refers to the (Amplitude) Modulation system. MF refers to the frequency band, which is more relevant here. AM radios cover a vast range of frequencies - from 60kHz to many GHz.
Your "wireless set" should detect the signal - possibly better, when the unit is working properly. And so should your dog's collar - no more nights out on the town with the local ladies.
meBigGuy
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Apr1-14, 03:01 PM
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WOw, how many times do I have to explain the choke. The choke is simple. All it is is a loop simulator. It lets the unit think it is driving a loop so you can track the signals down the broken wire.

Yes, you will receive 600KHz (or a close frequency) when the loop is repaired. That is how the system works.

As for driving the loop at resonance, I have built loop antenna systems that automatically resonate the variable inductance loop. I'm not saying this unit does that, just that it isn't all that hard. Having a decent match is probably not that critical in this application. I would expect a fixed tuning capacitance in the unit such that a typical loop would be near resonance.
Q_Goest
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Apr1-14, 07:39 PM
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Quote Quote by sophiecentaur View Post
1. What do you actually hear on your mf receiver? I imagine the box produces pulses (bursts) of 600kHz rF which activate pooch's pain collar (did you ever see Twin Peaks?) and which may come through as clicks (or something) on the speaker when the receiver is tuned to the right frequency (as with any received RF signal).
I haven't seen Twin Peaks, no. I must be missing something again <sigh>.

Anyway, when tuned to 600 kHz, there is a strong, static noise with a bit of a high frequency popping to it.

I tried searching for the wire tonight with the radio and found the same noise. In fact, it seems every radio station up and down the AM dial comes in significantly stronger when the radio is near the wire, but I guess that has to do with the wire acting as a receiving antenna somehow as well. My house has aluminum siding and similarly, signals come in stronger when the radio is held near it as well so that's no oddity.

As a side note, I also tried searching for a signal on FM but didn't come up with anything.

So yea, I guess the choke simply 'fakes out the circuit' into transmitting as it normally would. The fact it's transmitting on an AM wavelength is a bit strange though, local radio stations and all as you say...

Thanks for everyone's inputs. I realize I'm not the easiest student but I'm still learning!
meBigGuy
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Apr1-14, 11:28 PM
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I'm still learning too. Learned how to use a choke to fake out a loop transmitter.

FM radio is in the ~100Mhz band.

I suppose either the signal from the loop can overpower or be detected through the local stations if one happens to fall on the same frequency, or the signal is broadband or sophisticated enough to not be affected by an AM station.

I sure would like to know how the radio behaves as you approach the break with one lead disconnected (That last 10ft drop-off). (I guess I'm asking you to undo your fix, though).
AlephZero
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Apr2-14, 10:13 AM
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FM radio frequencies would not be a good idea here. The wavelength is only a few meters, so a "random" shape of the loop could lead to problems with the signal strength example if the loop has sharp 90 degree bends at the corners, or doubles back on itself.

On the other hand at 600 kHz the wavelength is about 500 meters, so any reasonable sized loop will act as an untuned wire.

The specified RF choke (100 uH) has an impedance of about 380 ohms at 600 kHz. Presumably that is low enough for the transmitter to think the circuit is unbroken, but high enough so the broken loop in parallel with it still gets most of the power from the transmitter.

As somebody said earlier, this system works using the "near field" radiation from the wire. That is high close to the wire but typically falls off as ##1/d^3## compared with ##1/d^2## for the far field or ##1/d## for a directional antenna. The far field signal will be too low to interfere with a broadcast AM radio on the same frequency.
sophiecentaur
#29
Apr2-14, 10:29 AM
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Perhaps it's because I was in Broadcast Engineering for a long time but I find it strange that many people refer to RF frequencies in terms of the Modulation Systems that are often used at those frequencies. In the 'VHF' bands - which extend from 30HMz to 300MHz (ITU designation) various forms of modulation are used and that use is subject to change. Also, AM is used in some VHF channels. I think it would be a good idea on PF, at least, if we used the more precise terms, in the interest, as usual, of avoiding confusion.

I realise that what's written on peoples' radio receivers may be not what I am recommending but the same goes for a lot of what we find in 'popular' sources of knowledge.

@AlephZero
I am not sure what you are referring to when you talk of a simple inverse law for a "directional antenna". Afaik, there is nothing to interfere with the Inverse Square Law for any EM source at great distance. Could you explain the circumstances for 1/d drop off, please?


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