Some scanners will scan channels and scan between the start and stop frequencies (frequency search) with predefined step sizes like 5Khz, 12.5Khz, and 50KHz. The step size is based on the band plan channelization. Frequencies are assigned by FCC or NTIA with defined spacing between assigned frequencies to avoid interference.
A popular older and very capable scanner line the Pro-2006 will scan frequency steps at 26 channels per second. 12.5Khz spacing is normally used in the VHF band. So 80 12.5KHz channels per 1MHz. Times 38MHz is 3040 channels. Divided by 26 channels per second is 117 seconds or about 2 minutes. If you scan 300 steps 12.5Khz spacing per second it would take about 10 seconds.
But that won’t happen. All consumer-grade synthesized receivers have “birdies.” Spurious signals halt the scan. Some receivers like the Pro-2006 will listen for audio for a moment and hearing nothing will continue scanning. Sometimes it hears some noise from the birdie and does not continue. The other problem is saving the active channels. The Pro-2006 has a feature where it stops on an active channel and waits there. The user can press the “monitor” button and save the frequency to a monitor memory then continue the scan. I modified mine to automate the process. Be aware that some inexpensive scanners will miss more transmissions when the scanning speed is high because of the reduced dwell time.
Scanning the entire band is generally unworkable. Picking a band segment between birdies and scanning it for a while, maybe days, then moving to a new segment works. I also use a voice-activated mini-recorder on the audio output to capture what was heard to determine if the frequency is interesting before saving it to a scan channel memory.
Examination of the NTIA red book and FCC frequency allocations narrows the search possibilities so there’s no need to scan the entire band. Unless you’re looking for covert transmitters. Anyone that knows what they’re doing can make searching with a scanner rather ineffective by taking very simple measures, even if they’re not using a subcarrier, burst transmission, spread spectrum, or other means of hiding the transmission. Even baby room monitors have spread-spectrum now and are difficult to detect with a scanner. you might find a spy store-type FM modulated bug with careful scanning of the frequencies they’re known to use, see google for that.
If you’re looking for unused frequency scanning is a bad approach. Some frequencies are dead until there’s a need, a roving agency in the area, or an emergency. Just because the frequency is not in use doesn’t mean no one is monitoring it. There are also data channels, wildlife tracking frequencies, and frequency caches for national emergencies to run afoul with too. The VHF high band often seems mostly empty but it’s not. There are safer places to do… whatever if that’s the plan.
I scan a frequency segment where wildlife tracking transmitters are common. For no reason other than it amuses me. I’ve done this for decades. One day a federal agency popped up in this segment with communications that might have been related to a high-value prisoner transport escort. Never heard them again since that instance.
A wide variety of VHF & UHF scanners can be found on Amazon-