Should all transmitters be off when scanning for frequencies?

Yes — every transmitter under your control, including spares, IFB, comms and anything left powered in a case. A scan is meant to show what other people are putting on the air; your own carriers and the intermodulation products they create will mark clean spectrum as occupied and push you into worse frequencies.

Why is a scan at the truck unreliable?

Because it measures the RF environment at the truck rather than at the set — different position, different height, different obstructions, and usually a metal box full of your own electronics. Scan with the receive antennas already rigged where they will work from, and at roughly the time of day you will be shooting.

How do you walk-test wireless microphones?

Put the transmitter on a body the way it will actually be worn, then walk every position the scene touches — marks, doorways, stairs, vehicles, and the spots the director has not thought of yet — while watching RF level and diversity activity on the receivers, not just listening. Turn around at each position too, because body absorption at UHF can cost you significant signal when talent faces away from the antennas.

How often should you re-scan on a shooting day?

At minimum after the other departments have finished rigging, at every company move, and any time a previously stable channel starts taking hits. LED fixtures, video links, wireless lens control and comms all arrive after the morning scan and can raise the noise floor enough to shorten range noticeably.

Does a frequency scan replace checking for local TV stations?

No, they answer different questions. A scan shows what is radiating at that antenna in that moment, while a licence database lookup shows what is authorised to transmit near the location, at what power and from which direction, regardless of whether it happened to be on during your sweep. Professional practice uses both.

Planning a shoot

Scanning and walk-testing on location

RF ScoutUpdated July 26, 20265 min read

A useful scan has three conditions: every transmitter you own is switched off, the receive antennas are already in the position and at the height they will work from, and the scan happens at the location and roughly the time you will actually be shooting. A scan taken at the truck, at the cart, or with your own gear radiating describes a different radio environment than the one your talent will walk through. After coordinating, walk-test every position in the scene with the transmitter worn the way it will be worn, then re-scan once grip, video and comms have arrived, because they change the picture substantially.

A scan is a measurement, and like every measurement it is only as good as the conditions it was taken under. Most bad scans are not wrong about the spectrum. They are right about somewhere else.

The three conditions for a scan that means something

Everything of yours is off: Every transmitter, not most of them. Bag packs, plant mics, spares in the case, IFB, comms, the transmitter still live in yesterday's bag on the truck. A single forgotten unit blacklists a perfectly good frequency, and worse, its intermod products blacklist several more.
The antennas are where they will live: Same position, same height, same cable, same amp settings, same orientation. Antennas half a metre lower or on the other side of a wall can produce a materially different picture, because that is exactly what antennas are for.
It is the right place at the right time: Scan at the location, not at base camp, and ideally near the hour you will be working. Broadcast is constant; almost nothing else is. The venue's own wireless, another crew across the lot, a stadium event three miles away — all of that has a schedule.

Why the scan at the truck lies to you

The scan at base camp is comfortable, warm and useless. The truck is usually parked somewhere convenient rather than somewhere representative, often behind a building or below a ridge that shadows exactly the transmitter you needed to know about. It is a metal box with its own generator, its own inverter noise and its own wireless. It is a hundred metres from the set and possibly on the other side of it. And it is quiet, because your gear has not been distributed yet — which means the scan flatters you.

What a scan can and cannot tell you
A scan sees	A scan misses
Signals present at that antenna, at that moment	Anything intermittent that happened to be off
Energy strong enough to register above the noise floor	A weak interferer that still beats your talent at 60 feet
The environment at the antenna position	The environment at every position talent will occupy
What is on the air now	What is licensed to be on the air later — that comes from a database lookup
Occupied channels	Which of your own frequencies will collide with each other once powered up

Running the scan

Rig the antennas properly first. Height, sightlines, spacing, clear of metal and bodies. The scan measures whatever you have built, so build it before you measure.
Kill every transmitter on the show. Say it out loud to the department. Check the case.
Sweep the primary block, then the fallback block. Both are worth having on record before anyone is dressed.
Let it run more than once. One sweep catches a moment; several sweeps, or a scan left running while you rig something else, catches the walkie that keys up every ninety seconds.
Save or photograph the scan. When something goes wrong at hour ten, the morning's scan is the only evidence of what changed.
Coordinate from the scan, using your manufacturer's software rather than by eye. See coordinating frequencies for a multi-channel bag.

Walk-testing: the part people skip

Coordination gets you frequencies that do not fight each other. Walk-testing tells you whether those frequencies survive contact with the location. They are different questions, and the second one is answered with your feet.

Wear the transmitter the way it will be worn. On a body, under wardrobe, with the antenna running the direction it will actually run. A pack held in the hand at chest height is a test of nothing.
Walk every mark in the scene, then walk the ones nobody has mentioned yet. Doorways, stairwells, the hallway they will inevitably add, behind the refrigerator, inside the car, out to the balcony.
Turn around. Body absorption at UHF is real and one-sided. A position that is solid facing the antennas can drop out when talent turns their back on them.
Watch the receivers, do not just listen. RF level meters and diversity indicators show you the margin you have left. A position that sounds fine on a single antenna with no headroom is a position that will fail on take four.
Note where it gets thin. If a mark is marginal, say so before the shot exists rather than after. A plant mic, a second antenna position or a slightly moved antenna is cheap at rigging time and expensive at 4 p.m.

Re-scan after the crew arrives

Your clean 7 a.m. scan describes a location that no longer exists by the time the first shot is up. Between those two moments the set acquires LED fixtures and their drivers, dimmer packs, a generator, video village, wireless follow-focus and lens control, camera hops, a walkie repeater, wireless DMX, laptops and hotspots, and whatever the venue's own systems do when the building wakes up. Some of that is broadband noise rather than a discrete carrier, which is why it shows up as a raised noise floor and shortened range rather than as an obvious spike.

Re-scan after the rest of the departments have rigged, before the first rehearsal if you can get the sixty seconds.
Re-scan at every company move, however short. A block away can be a different broadcast picture, and it is certainly a different building.
Re-scan when a channel that was solid starts taking hits — that is new information about the environment, not bad luck.
Keep the fallback block coordinated and ready. Being able to move the whole bag in two minutes is worth more than being right in the morning.

The one thing you can settle before any of this is which part of the spectrum to bring in the first place. Broadcast does not change between your scan and your wrap, and it is a matter of public record.

See which TV channels are occupied near the address, so your scan starts in the right neighbourhood.

Scout the location before you pack

Common questions

Should all transmitters be off when scanning for frequencies?: Yes — every transmitter under your control, including spares, IFB, comms and anything left powered in a case. A scan is meant to show what other people are putting on the air; your own carriers and the intermodulation products they create will mark clean spectrum as occupied and push you into worse frequencies.
Why is a scan at the truck unreliable?: Because it measures the RF environment at the truck rather than at the set — different position, different height, different obstructions, and usually a metal box full of your own electronics. Scan with the receive antennas already rigged where they will work from, and at roughly the time of day you will be shooting.
How do you walk-test wireless microphones?: Put the transmitter on a body the way it will actually be worn, then walk every position the scene touches — marks, doorways, stairs, vehicles, and the spots the director has not thought of yet — while watching RF level and diversity activity on the receivers, not just listening. Turn around at each position too, because body absorption at UHF can cost you significant signal when talent faces away from the antennas.
How often should you re-scan on a shooting day?: At minimum after the other departments have finished rigging, at every company move, and any time a previously stable channel starts taking hits. LED fixtures, video links, wireless lens control and comms all arrive after the morning scan and can raise the noise floor enough to shorten range noticeably.
Does a frequency scan replace checking for local TV stations?: No, they answer different questions. A scan shows what is radiating at that antenna in that moment, while a licence database lookup shows what is authorised to transmit near the location, at what power and from which direction, regardless of whether it happened to be on during your sweep. Professional practice uses both.

Sources

RF Scout — methodology and limitations

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.