On 25 June 2010 19:57, James Moritz <[email protected]> wrote:

Dear LF Group,

On the subject of VLF signal propagation along buried conductors, and its use by cable locators, I thought it would be interesting to try one out, so I borrowed one (see attached picture)...

The CAT detector part is a passive magnetic field sensor. It has an audible output and operates in 3 modes. "Power" mode detects 50Hz and harmonics from power cables, and also any other conductor that has some 50Hz common-mode current flowing in it, which is most conductors in practice! In "Radio" mode, it is a simple direct conversion VLF receiver with zero beat around 17.8kHz and bandwidth of a few kilohertz. This detects currents induced by signals from VLF utilities - it is quite sensitive, if you turn the gain control up you can hear several MSK signals and the background QRN too. I found the results rather ambiguous, since it is hard to decide if you are picking up a signal from a cable, or just the normal radiated signal.

The "Generator" mode uses the seperate "Genny" unit to inject a signal into the cable you want to trace. It generates an on-off keyed CW signal at 32.768kHz; apparently the output current is 100mA. When the detector is switched to Generator mode, it acts as a 32.768kHz receiver with a bandwidth of a few hundred Hz, so you can hear the "beeps" from the generator. It is also quite sensitive in this mode; the literature says it can detect 5uA signal current 1m away.

As a test, I connected one terminal of the generator to the mains earth in my house, and the the other terminal to some RF ground rods at the end of the garden. This gave a strong signal on the mains cable entering the house, and also apparently the gas main (which despite Mal's proclamations, stubbornly remains of a ferrous nature ;-)). It was possible to trace the various pipes and cables for hundreds of metres - the limiting factors seemed to be that as the pipes/cables branched off to other houses, street lighting, etc, the signal got weaker, presumably as an increasing portion of the signal current was diverted away from the main cable, and also some buzzing QRM appeared, which seemed to be associated with BT telecomms cables.

Apparently, contractors are more or less obliged to have these detectors, so that if they accidentally dig up a pipe or a cable they can say they "took all reasonable precautions" for insurance purposes. But in use, the results certainly need to be interpreted with care - it is often unclear what is or isn't being detected. I suppose if you want to experiment with this kind of VLF transmission before an NoV is available, you could always say it wasn't radio, but a cable tracing experiment ;-)

Cheers, Jim Moritz
73 de M0BMU