EMRFD Message Archive 7463

Message Date From Subject
7463 2012-04-30 08:04:18 dnorbury Underground wire detector
I think I have a cut wire feeding some lights in my front yard. Since the lawn is looking really good for the first time in years I'm not too excited about randomly digging it up to find the broken wire. I think this is a great opportunity to build a homebrew version of the transmitter/receiver equipment that the utility services use to find underground pipes and wires. I've seen some on the web that operate at low power (10 mW or so) at 447 kHz. Others, designed for deeper applications I think, use frequencies around 33 kHz (power unknown). My application should be less demanding than those that the commercial equipment are designed for. The wire I'm looking for is only buried a few inches and is less than 100 feet long. It carries 12 VAC from a transformer at the house. I would connect a low power transmitter to the transformer output and walk around with a narrow band detector. My question - has anyone tried to do this or seen articles discussing it? I can't justify buying a $1000+ unit for the little use it would see and building it would be a whole lot more fun.

Thanks for any suggestions,

73, Dave, KD6A
7465 2012-04-30 09:45:59 AD7ZU Re: Underground wire detector
years ago I built several hetrodyne metal detectors.
After much experimentation I found the best results to be around 400Khz for the search loop frequency.
to locate a wire break you could couple a low power signal into the wire and with a portable receiver you should be able to trace the wire along the ground.  A portable AM radio would work  if you AM modulate the trace signal and tune the generator to a quiet spot in the AM band (550 - 1600khz).  try to find the quiet spot at the low end.

7466 2012-04-30 10:14:26 Andy Re: Underground wire detector
I suppose the effectiveness of doing this might depend on the type of
fault. If only one wire is broken, will the signal continue along the
other wire?

If that happens, I suppose a homebrew TDR of some sort might be a
viable alternative. Launch a pulse into the wire pair, and measure
the time for the reflection to come back. Of course you'd need to
figure out the effective dielectric constant (for the propagation
velocity) of your buried wires.

7473 2012-04-30 21:34:29 Marc Franco Re: Underground wire detector

It is very easy to do. We recently got a dog and we had the invisible fence already buried but broken in several places.

I simply connected the output of an IFR500 sig gen at 500 KHz amplitude modulated with a 400 Hz tone to the buried wire. I then followed the signal with a portable AM radio. The sig gen level was around -30 dBm for good reception, although the wire loop covers 2.5 acres.

73, Marc N2UO

7474 2012-05-01 04:44:19 Dave Norbury Re: Underground wire detector
Thanks for the inputs! Randy/Marc - the idea of a signal generator and AM
radio makes perfect sense. I don't get to build anything but it is probably
the quickest and easiest way to go. Andy, the idea of a TDR is really
intriguing. But assuming I could accurately guess the effective propagation
constant I could determine the distance to the break but still wouldn't know
where it was since I don't know the exact wire route.

Thanks again,


[Non-text portions of this message have been removed]
7476 2012-05-01 07:41:02 AD7ZU Re: Underground wire detector
You could build a low frequency AM modulated source .. there may be some design ideas from the folks that are building AM transmitters for 80 or 160.
Then of  you could build a simple direct conversion or maybe one of the simple superhet receiver designs ..there are quite a number of them floating around.
Then with the new low frequency band 479khz?  if I remember from the last issue of QST coming soon all those wire finding projects could then be put on the air vs put in the ground ;)
--just a thought

7477 2012-05-01 08:15:16 Brooke Clarke Re: Underground wire detector
Hi Dave:

The telephone company has been doing this for years using DC. For example the ZM-4 bridge:
But I expect you could do the same thing with a modern DMM.

See: http://en.wikipedia.org/wiki/Murray_loop_bridge
The ratio of Rx and Ry will tell you where the problem is.

Have Fun,

Brooke Clarke, N6GCE

emrfd@yahoogroups.com wrote:
> Underground wire detector
7480 2012-05-04 17:11:25 Eldon Brown Re: Underground wire detector

I once did some underground fault locating with a portable lamp with
Dimmer, and an AM radio with loop stick antenna.

Plug the lamp in near the transformer, set the lamp to medium Brightness,
set the AM radio to an unused frequency on the dial. The static from the
Lamp Dimmer will null with the orientation of the loop with respect to a
near wire (even those underground). A noticeable drop in Dimmer static will
be heard after passing the underground fault.

This worked very successfully with a wire which was about 2 feet deep.

Eldon Brown

72 - Eldon - WA0UWH - CN88xc - http://WA0UWH.blogspot.com/