Signal theory & frequency basics
The fourth foundations lesson for anyone stepping onto a residential or small-commercial scope after the Foundations 02 surface pre-sweep and Foundations 03 equipment vocabulary are in hand. The lesson covers the electromagnetic signal model a private locator works against on every active-induction locate — the transmitter applies a current, the receiver reads the field gradient the current produces around the conductor at depth — and the discipline of choosing a frequency per conductor that reads what you want and ignores what you do not. The lesson here assumes the demarcation framing from Foundations 01 and the surface vocabulary from Foundations 02, and adds the equipment vocabulary from Foundations 03 already in place, before the advanced track opens up.
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Why this lesson exists
The most common entry-level frequency mistake is to leave the locator on the factory default for every conductor on a mixed-length scope — long mains and short stubs, dense corridors and clean runs. The right frequency depends on the conductor length, the corridor density, and what parallel utilities share the right-of-way, and choosing it per conductor is what separates a clean trace from a combined-field readout the closeout cannot defend. This lesson is the on-site counterpart to the F02 surface pre-sweep: the active-induction frequency pass that sits between the surface reading and the subsurface method the advanced track covers.
- Entry-level field technicians applying active induction for the first time on a residential scope and learning which frequency to pick before any equipment is powered on.
- Customer-side intake staff answering the kick-off call who need a vocabulary for what a frequency pass actually produces on a shared-corridor locate — and why the equipment default is not the answer.
- Project managers and oversight staff who need to read a frequency-decisions log on a closeout report and trust that the depth estimate on the same report was triangulated, not assumed.
By the end of the lesson you should be able to set a per-conductor frequency on a typical residential or small-commercial scope without confusing a high-frequency bleed signature for a clean trace.
- 01Describe the transmitter / receiver field model: the transmitter applies current to a target conductor, that current flows along the conductor and returns via the ground or a second conductor, and the receiver reads the electromagnetic field the current produces around the conductor at depth.
- 02Apply the low-vs-high frequency selection rule of thumb — short stub, high frequency; long main, low frequency — and explain why the rule holds on residential and small-commercial scopes (signal shape, bleed onto parallel utilities, coupling efficiency onto short paths).
- 03Explain how a signal travels along a buried conductor, why the field strength the receiver reads decays with distance from the transmitter, and what a noise-floor drop in the reading at corridor end actually tells you about the conductor (end of run, broken connection, or bleed onto a neighbour).
- 04Recognise signal bleed on shared-corridor runs — the broad / shifting signal shape and the unstable depth readout — and apply the corrective moves (lower transmitter power, switch frequency, re-ground on the opposite side of the corridor).
- 05Estimate depth qualitatively using peak/null triangulation from two passes and the 45° slope method, and spot the six frequency mistakes that show up on early-career scopes — including "use the equipment default" and "stack a high freq on a long main."
The transmitter / receiver field model and the low-vs-high frequency rule
Every active-induction locate works against the same electromagnetic model: a transmitter applies a current to a target conductor, the current flows along the conductor and returns via the ground (or a second conductor), and a receiver picks up the field the current produces around the conductor at depth. The right frequency to apply depends on the conductor length, the corridor density, and the parallel utilities the corridor is likely to share — and the rule of thumb the field holds onto is short stub, high frequency; long main, low frequency.
The signal model is the same on every locate, regardless of the equipment brand or the corridor: apply current, current flows along the conductor, current returns (via ground or via a second conductor), and the receiver reads the electromagnetic field the current produces. The field is strongest near the transmitter because the current is strongest there and decays with distance. The frequency applied to the conductor shapes the field footprint — a low frequency produces a narrower field that bleeds less onto parallel utilities, a high frequency couples more efficiently onto a short return path but bleeds more easily onto neighbours. The decision of which frequency to apply is what the lesson is built around.
The rule of thumb the entry-level pass holds onto is “short stub, high frequency; long main, low frequency.” Long linear mains (a 40 m water-service lateral, a long communications run) read more cleanly at low frequency because the narrow field footprint bleeds less onto the parallel utilities the right-of-way carries with it. Short stubs (a 6 m irrigation lateral, a service branch that ends within ten metres of the transmitter) read more cleanly at higher frequency because the short return path concentrates the current where the receiver can read it. Pairing them wrong is the entry-level frequency mistake the on-site pass exists to prevent. The visual the framework carries is the field footprint over a conductor at two frequencies — the difference between a narrow shape that separates the target from its neighbours, and a broad shape that combines them.
Reading the signal shape on the receive side is what turns the frequency choice from a setting on the equipment into a field decision. A clean peak (sharp, narrow, with a tight null 30 cm on either side) confirms the receiver is reading the target conductor and not a combined field. A broad / shifting shape — peak that moves with each sweep pass, depth that reads 0.6 m at one point and 1.1 m a metre later — is the signature the field reads as bleed, and the corrective move is to drop transmitter power, switch to a frequency the parallel corridor does not share, and re-ground on the opposite side of the right-of-way if the first two moves did not close it out.
From the pre-field frequency hypothesis to the closeout log
The on-site workflow for the frequency pass follows three short phases — pre-field frequency hypothesis, on-site frequency confirmation and depth pass, closeout frequency log — so the customer receives the same frequency decisions the field team wrote down before mobilisation and the depth estimate on the closeout map is the one the two-pass triangulation produced, not the one the single-pass readout suggested.
- Pull the 811 ticket response and the surface pre-sweep from Foundations 02: which utilities responded, which fixtures are visible, which surface indicators flagged a shared corridor or a long lateral.
- For each target conductor, write down the planned frequency before mobilisation: low frequency (8 kHz / 33 kHz family) for long linear mains, higher frequency (65 kHz and up) for short service stubs and irrigation laterals, plus a fallback frequency that the corridor's parallel utilities are unlikely to share.
- Note the parallel utilities the corridor is likely to share (gas along the right-of-way, communications along the same easement, neighbouring water service laterals across the back of the lot) — those are the bleed candidates the on-site pass will check against.
- Set the equipment default to the planned per-conductor frequency; do not leave the locator on its factory default for the on-site pass, because the factory default usually compromises every conductor at once.
- Apply the planned frequency at the lowest transmitter power that still reads the target conductor. The first reading should be a clean peak with a sharp null on either side, not a broad / shifting shape — a broad shape at corridor midpoint is the field signature of bleed.
- If the reading is broad or ambiguous, drop transmitter power to its lowest setting that still reads the target. If the read is still ambiguous, switch to a frequency the parallel corridor does not share (e.g. step from 33 kHz to 8 kHz) and re-pass.
- Estimate depth qualitatively: locate the peak directly over the conductor, step sideways to the null on either side, and triangulate from two passes at a 45° slope to the conductor trace. Two-pass triangulation is what catches the depth-estimation errors the single-pass reading hides.
- Where the signal decays below the noise floor 30+ m from the transmitter, do not read that as "the conductor ends here." Cross-check with a frequency pass from the opposite end of the corridor — if the conductor reads cleanly from the other direction, the original pass read bleed, not the conductor end.
- At closeout, write down the frequency used per conductor and the reading shape (clean peak, broad / shift, decay at corridor end). Without that line, the next contractor cannot tell which frequency the locate was based on.
- Bundle the per-conductor frequency choices with the surface pre-sweep from Foundations 02 so the closeout map reads the same vocabulary the field crew read on-site: what the surface told us, what frequency we chose, what the receiver read at that frequency, and what depth we triangulated.
- If any frequency choice on-site had to depart from the pre-field hypothesis (a parallel utility made 33 kHz too noisy, so 8 kHz was used instead), note the deviation and the reason on the closeout — so the next pre-field hypothesis on this corridor starts from the working frequency, not the default.
- Close the loop with a plain-language summary the customer can read: what frequency we chose per conductor, why that frequency for that run, and what the depth estimate is — so the customer does not have to ask "what does 65 kHz mean in this report."
The single sentence that travels with every residential or small-commercial scope: “We set the frequency per conductor before mobilisation, applied the planned frequency at the lowest power that read the target, and triangulated the depth estimate from two passes — so the trace on the closeout map is the conductor, not the combined field of the corridor.” That sentence, in the kick-off email and on the closeout summary, prevents both the “we used 33 kHz on everything” over-simplification and the “why is the depth different on the same conductor” surprise after mobilisation. It is small, it is plain, and it does most of the frequency-decision work the team needs to do outside of the field report.
Phase 1 — pre-field frequency hypothesis — sets the SIGNAL precondition. Phase 2 — on-site frequency confirmation and depth pass — confirms the frequency against the signal shape on the receiver and triangulates depth. Phase 3 — closeout frequency log — is the verification step on the report the framework promises, so subsequent reads of the closeout can re-run the locate without re-deciding every frequency choice.
Open the engagement frameworkSix frequency mistakes worth flagging
The lesson collects the entry-level frequency calls we see show up on early-career scopes — the kind a written per-conductor frequency choice prevents before mobilisation, and the kind a clean closeout log and a second-frequency cross-check prevents at handover.
A technician runs a 65 kHz signal onto a 40 m linear water-service lateral and gets a clean trace at the start of the run that broadens into a noisy / unstable readout at corridor midpoint — the signature of a high-frequency signal bleeding onto parallel utilities that the lower-frequency pass would have separated.
Drop the long main to a lower frequency (8 kHz / 33 kHz family) where the signal shape is tighter and bleed is reduced. Reserve higher frequency for short stubs where coupling efficiency matters more than separation.
The technician sets a 8 kHz signal on a 6 m irrigation stub and gets a weak / inconsistent readout that does not couple strongly into the short run end — a low frequency on a short path does not concentrate the current where the receiver can read the stub cleanly.
Switch to a higher frequency (65 kHz or higher) on short stubs and laterals. The "short stub, high frequency; long main, low frequency" rule of thumb is the corrective move because the short return path needs the higher frequency to read cleanly.
A locate reports the conductor ends 30 m out from the transmitter because the receiver readout drops below the noise floor — but the conductor still continues underground. The team plans the dig as if the run stops at the drop, missing the rest of the corridor.
Cross-check the apparent end of the conductor with a frequency pass from the opposite end of the corridor. If the conductor reads cleanly from the other direction, the original drop was bleed or connection degradation, not the conductor end.
A team arrives with the locator set to the factory default and applies that frequency to every conductor on a mixed-length campus scope — long mains and short stubs alike. The reads are inconsistent, and the closeout report does not say which frequency was actually used per conductor.
Set the frequency explicitly before each on-site pass and write the choice on the field sketch. The factory default is a compromise that works for no specific run; the on-site pass is the moment to override it.
A team applies a high-frequency signal at full transmitter power onto a conductor that shares the right-of-way with three other utilities. The receiver reads a combined field gradient rather than one conductor, and the closeout map shows three "targets" where one conductor actually is.
Drop transmitter power to its lowest setting that still reads the target. Lower power reduces the field footprint so the receiver reads the target conductor and not the neighbours. If depth separation is still ambiguous, switch frequency as well.
A technician marks the conductor trace from a single 33 kHz pass and reports the locate as complete — but a parallel utility sharing the corridor would also have read at 33 kHz, so the trace is ambiguous and the field sketch does not say which frequency the mark was based on.
Cross-check every locate with a second frequency the parallel corridor does not share. A locate that reads cleanly at both frequencies is much harder to dispute than a single-frequency trace, and the closeout note records which frequencies were used.
Self-check — four questions, reveal as you go
A short authored self-check covering the frequency decisions that show up on the first on-site active-induction pass of any residential or small-commercial scope. Reveal an answer to read the rationale; reset between customers with Hide all.
This is a self-check, not a graded exam. Pick a question, reveal the answer, and use the rationale to decide whether the topic earns another pass before you set the frequency on the next locate.
0 of 4 answers revealed. State is kept on this device only.
- 01
You are on a residential scope with a 40 m water-service lateral feeding a small commercial building, plus a short 6 m irrigation stub branching off the same tap. Which frequency is your default for the lateral and which for the stub, and why?
- 02
The receiver gives a peak reading on the conductor trace at the corridor midpoint, but the signal shape is broader and the depth readout is unstable as you sweep. What is the most likely cause and the first corrective move?
- 03
A signal applied at the meter travels along the conductor but the receiver reads stronger near the transmitter and weaker with distance, eventually falling below the noise floor 30 m out. What does that decay actually tell you about the conductor and the locate?
- 04
A foreman tells a junior technician to "just use the default frequency on the locator" for every conductor on a campus sweep. Why is this a high-risk instruction, and what is the corrective move?
Where to take this lesson
Foundations 04 (track-order) anchors the entry-level frequency vocabulary for the rest of the catalog. The natural next moves are the Foundations 02 return loop (the surface pre-sweep this lesson reads against) and the Foundations 03 return loop (the equipment vocabulary this lesson reads against), or Advanced Module 02 (the subsurface method-fusion that the frequency log and depth triangulation F04 has just produced set up).
Each foundations module ships with its own lesson page, knowledge check, and a stand-alone vocabulary set so the track stays useful even if you arrive mid-sequence.