What is a third-order intermodulation product?

It is a spurious signal created when two or three transmitter signals mix in a non-linear stage, at frequencies of the form 2A − B or A + B − C. Third-order products matter more than any other order because they land very close to the original carriers, which puts them inside the same block and inside your receivers' passbands.

Why can't I just space my frequencies evenly?

Because evenly spaced carriers generate intermodulation products that land on the same regular grid, and therefore on top of the carriers themselves. Three transmitters at 500.000, 500.400 and 500.800 MHz produce third-order products at exactly 500.000, 500.400 and 500.800 MHz. Coordinated frequency sets look irregular for this reason.

Where does intermodulation actually happen?

In any stage that is not perfectly linear: transmitter output amplifiers when two transmitters are physically close, receiver front ends when strong signals arrive together, antenna amplifiers and distribution amps, and occasionally corroded metal junctions. On a location sound bag, transmitters sitting against each other are by far the most common source.

Does intermodulation get worse with more channels?

Yes, and far faster than linearly. The number of third-order products grows roughly with the cube of the channel count, so four transmitters produce around two dozen, eight produce a couple of hundred, and sixteen produce nearly two thousand. This is why hand coordination stops being viable somewhere around four to six channels.

Planning a shoot

Intermodulation, explained without the math

RF ScoutUpdated July 26, 20265 min read

Intermodulation happens when two or more radio signals mix inside a non-linear stage — a transmitter output, a receiver front end, an antenna amplifier — and produce new signals at frequencies that nobody transmitted. The ones that matter in practice are third-order products of the form `2A − B` and `A + B − C`, because they land close to the original carriers and therefore inside the block you are working in. The count grows roughly with the cube of the channel count: four transmitters produce about two dozen third-order products, eight produce over two hundred, sixteen produce nearly two thousand. That growth, not the arithmetic itself, is why coordination software exists.

Intermodulation is the reason a frequency plan can be perfectly clean on the scan, perfectly legal, perfectly spaced, and still fall apart the moment all the transmitters are switched on at once. The interference is not out there waiting for you. Your own gear makes it.

Where the new signals come from

Every amplifier is linear up to a point and then is not. Feed two strong signals into a stage operating anywhere near that limit and it does not simply add them together — it multiplies a little as well, and multiplication in the time domain produces sums and differences in the frequency domain. Those sums and differences are intermodulation products, usually shortened to IM or intermod. They are real signals at real frequencies, radiating out of real antennas, and a receiver tuned to one of them cannot tell it apart from an honest transmitter.

The mixing happens in more places than people expect:

Transmitter output stages. Two bodypacks close together — in a bag, on a belt, taped to the same actor — each push signal into the other's final amplifier. This is the dominant source on a location sound bag and the reason physical separation matters.
Receiver front ends. Strong signals arriving together at an input stage generate products inside the receiver, which then hears them perfectly. See why wireless drops out near a TV tower.
Antenna amplifiers and distros. Anything with gain and a finite linear range qualifies. Running an amp harder than it wants to be run turns it into an intermod generator.
Corroded connectors and loose metalwork. The classic rusty-bolt effect. Rare, maddening, and worth remembering when nothing else explains the symptom.

Orders, and why third order is the villain

Products are classified by order, which is just the sum of the coefficients in the formula that produces them. A + B is second order. 2A − B is third order. 3A − 2B is fifth order. Two things vary with order: how strong the product is, and where it lands.

Second order — A + B, A − B: Strong, but they land nowhere near the original carriers. Two mics around 500 MHz produce second-order products near 1 GHz and near DC. Ordinary receiver filtering deals with them and nobody loses sleep.
Third order — 2A − B, 2B − A, A + B − C: Weaker than second order, but they land right beside the carriers that made them — inside the same block, inside the receiver's passband, inside the spectrum you just paid to coordinate. This is the order that ruins days.
Fifth order — 3A − 2B and friends: Weaker again and further out, but still in band. Coordination software usually offers to include them; on dense multi-channel days, let it.

A worked example with three carriers

Take the tidiest frequency plan imaginable: A = 500.000, B = 500.400, C = 500.800 MHz. Evenly spaced, easy to remember, easy to write on tape. Now run the third-order forms.

Third-order products of an evenly spaced three-channel set (MHz)
Form	Arithmetic	Lands at	Consequence
2A − B	1000.000 − 500.400	499.600	Just below the set — usually harmless, but it is now occupied
2B − A	1000.800 − 500.000	500.800	Directly on carrier C
2B − C	1000.800 − 500.800	500.000	Directly on carrier A
A + B − C	1000.400 − 500.800	499.600	Piles onto the first product
A + C − B	1000.800 − 500.400	500.400	Directly on carrier B
B + C − A	1001.200 − 500.000	501.200	Just above the set

Three carriers, three of them hit. Every channel in the plan is sitting on top of a product generated by the other two. Nothing was illegal, nothing was badly spaced by any intuitive standard, and the set is unusable the moment all three transmitters are live in the same bag.

Why it gets bad so fast

Adding a transmitter does not add a fixed number of problems. 2A − B involves every ordered pair of carriers; A + B − C involves every choice of one carrier plus a pair from the rest. Both counts grow far faster than the channel count does.

Third-order product count by channel count (before removing duplicates and out-of-band results)
Carriers	2A − B products	A + B − C products	Total
4	12	12	24
6	30	60	90
8	56	168	224
12	132	660	792
16	240	1,680	1,920

Many of those products fall outside the block, several coincide with each other, and plenty are far too weak to matter. The shape of the curve is what counts: doubling the channel count multiplies the products by roughly eight. Going from eight channels to sixteen does not double the coordination problem, it octuples it, which is exactly why the sixteen-channel day feels categorically harder than the eight-channel day rather than twice as hard.

So use the software

This is a search problem with a large answer space and a simple test, which is precisely what computers are for and precisely what human intuition is bad at. Every major wireless manufacturer publishes a free coordination tool, and third-party packages handle mixed fleets and shared venues. Give it the block, the occupied TV channels from your scan, the number of channels and the system type, and it returns a set that is clean to third order and usually fifth as well.

Two things the software cannot do for you. It cannot know what is on the air at your location — that comes from a scan, and from checking the broadcast picture before you travel. And it cannot fix intermod generated by two transmitters taped together on the same actor, because that is a physics problem, not a frequency problem. RF Scout handles the first of those; it does not yet compute intermod-free sets, which stays with your manufacturer's tool for now.

Coordination works best inside a block that is already clear. Score your location, then let the software place the carriers.

Find clean spectrum first

Common questions

What is a third-order intermodulation product?: It is a spurious signal created when two or three transmitter signals mix in a non-linear stage, at frequencies of the form 2A − B or A + B − C. Third-order products matter more than any other order because they land very close to the original carriers, which puts them inside the same block and inside your receivers' passbands.
Why can't I just space my frequencies evenly?: Because evenly spaced carriers generate intermodulation products that land on the same regular grid, and therefore on top of the carriers themselves. Three transmitters at 500.000, 500.400 and 500.800 MHz produce third-order products at exactly 500.000, 500.400 and 500.800 MHz. Coordinated frequency sets look irregular for this reason.
Where does intermodulation actually happen?: In any stage that is not perfectly linear: transmitter output amplifiers when two transmitters are physically close, receiver front ends when strong signals arrive together, antenna amplifiers and distribution amps, and occasionally corroded metal junctions. On a location sound bag, transmitters sitting against each other are by far the most common source.
Does intermodulation get worse with more channels?: Yes, and far faster than linearly. The number of third-order products grows roughly with the cube of the channel count, so four transmitters produce around two dozen, eight produce a couple of hundred, and sixteen produce nearly two thousand. This is why hand coordination stops being viable somewhere around four to six channels.

Sources

RF Scout — methodology, limitations and roadmap

Related notes

Field Notes are general reference, not legal advice or a substitute for the rule text. Spectrum rules change; verify anything you are relying on against the current source before you transmit. See the methodology and disclaimer.