Theory and Operation of Radio Frequency Locators

The principal of operation of a radio frequency type locator is the same as a radio…where a signal is being transmitted and you pick it up with your radio, which is the receiver.  There are four methods used for utilizing the RF locators.  The conductive, inductive coupler, inductive, and sonde transmitter methods. 

This illustration shows the conductive and inductive coupler method.  The target conductor is directly connected to the transmitter and must have the ability to carry a radio signal, such as a copper wire.  A PVC water line or a non-conductive material would not support this signal.  In our case, a very important feature of completing our circuit is the ground.  Without a good ground, the circuit would not be complete.

Another mode of transmitting an RF signal is the inductive coupler.  It plugs directly into the transmitter and clamps around the target line.  This mode of transmitting an RF signal is better than the straight inductive method, but not as good as the conductive method.  It is used where there is a problem getting a ground, such as in an equipment room.  If there is access to the target it may be the easiest way to locate the line. However if this method does not give satisfactory results, then you must revert to the conductive method.

Often it is necessary to use the inductive method to locate a utility.  In this method, the transmitter is placed on the ground over or near the target, and a radio signal is broadcast into the ground.  Because high frequency locators will induce it’s signal onto any targets in it’s field, we can use this method to induce a signal onto a target where we know one to exist, but do not have access for making a direct connect.  We will discuss the differences in utilizing different frequencies shortly.  Also, this method can be used to locate unknown utilities in an area since if it is broadcast into the ground it will hook onto any utility within its range, which is to about 10-15 feet.  This method can also be used to be sure there are no utilities in the ground, perhaps when a boring needs to take place.  A note of caution here…because of the limitations of this equipment, both in range and type of target it must have, the quality of the clearance is limited.  In most cases it covers the crucial utilities.  The ones not likely to be covered are sewer and drain piping and PVC water lines. 

And how do you know you’re getting good results…by the experience of the operator observing the information provided by their equipment, combined with utilizing all other information available.  If the technician receives an eradicate or inconsistent signal or the signal strength is below that required for a good locate, the technician will question the information. 

I cannot stress enough the importance of the role the technician plays in providing a successful locate.  My opinion is that the quality of the equipment is 25 % and the quality of the technician is 75 % in achieving a successful locate.  While we are on the subject of the technician’s role in a successful locate, I would like to take a minute or two to talk about what the technician should do and know for a successful locate.  The technician must know their equipment…how to use it…how to maintain it…and have a feel for its operation…when its functioning properly and when not.  In addition, the technician should understand systems.  They should be able to read what lines are what coming from an overhead source.  Know how the distribution systems for power, gas, communication, water and other miscellaneous systems are designed.  Not how to design them, but what to expect from a usual distribution system.  The quality of the technician cannot be overemphasized.  We do training continually, that is, every day there are discussions and at least once a week a get together to discuss a unique situation that has arisen, and also to review the standard techniques and the “tricks of the trade”.

Lastly I would like to discuss the sonde.  It is a self-contained battery operated transmitter that is used to locate any type of plastic, vitrified clay, concrete and cast iron soil pipe.  It locates the piping by transmitting a signal that is picked up by a receiver above ground.  Its signal cannot transmit through ductile iron, corrugated metal piping or any piping that would inhibit the signal from transmitting through the walls of the pipe.  Those lines can be located by connecting directly to the pipe.  We use this method most often to locate sewer and drain piping.  Sondes can locate quite deep…we have located lines to fifty feet.