For this type of monitoring, one of the most useful features available on a modern scanner is CloseCall.
CloseCall changes the way I approach monitoring. Instead of starting with a large database of known frequencies and waiting for activity, I can take the scanner into an area and see what signals are actually being transmitted nearby.
My approach is simple: Get up close. Exclude the known. Focus on the unknown. Use my eyes and ears.
When I am doing CloseCall monitoring, I particularly like concentrating on the UHF bands. UHF is a great area for local signal hunting because there can be a huge variety of users packed into a relatively small frequency range. Depending on where you are, this can include UHF CB, commercial two-way radio systems, industrial communications, data transmissions, telemetry, security systems and other short-range services.
Rather than trying to monitor everything at once, narrowing the search to a particular section of UHF makes it much easier to understand what is happening around you.
Use CloseCall as a Discovery Tool
I don't treat CloseCall as a replacement for a frequency database, Instead, I use it as a discovery tool.
If I already know that a particular frequency is regularly active in an area, there is little value in recording it again every time I visit. The interesting signals are the ones that I don't already know about.
This is where the idea of excluding the known becomes important. If I arrive somewhere with a list of known frequencies, I can quickly identify the signals I already recognise. Once those are accounted for, I can concentrate on anything new that appears.
A CloseCall hit is only the beginning. The real challenge is working out what the signal actually is.
Get Up Close
CloseCall works particularly well when you get physically closer to the source of a signal. If I am walking around an industrial area, shopping centre, transport facility, car park or other busy location, I am constantly looking for clues about what might be generating the signals I am hearing.
A strong CloseCall hit can provide an approximate indication that a transmitter is nearby, but the scanner alone doesn't tell the whole story.
Moving around can help. If the signal becomes stronger in one direction, I can start working out where the transmitter might be located. Sometimes simply walking or driving a few hundred metres can make a significant difference. This turns radio monitoring into a bit of a signal hunt.
Use Your Eyes
One of the biggest mistakes a scanner enthusiast can make is concentrating entirely on the scanner display.
If I hear a strong UHF signal, I start looking for things that might explain it.
The physical environment can provide important clues about what I am hearing. Sometimes I will see something first and then hear the associated radio traffic. Other times, I will hear a signal and then start looking around for the source.
The two approaches work together.
Use Your Ears
Listening is just as important as watching. Not every CloseCall hit will produce an obvious voice transmission. Some signals may be short bursts of digital data, telemetry or other machine-generated traffic.
The sound itself can still provide useful information.
Is it an analogue voice transmission?
A digital signal?
A very short burst?
A repeating transmission?
Something that occurs every few seconds?
Something that appears only when a particular vehicle or person is nearby?
These patterns can help determine what you are actually looking at. I also listen for repeated callsigns, channel references, locations, vehicle descriptions and other information that can help identify the user of a frequency.
Don't Immediately Log Everything
One of the advantages of CloseCall is that it can produce a lot of hits. The disadvantage is that it can produce a lot of hits.
Not every hit deserves to go straight into the database. A frequency might be a brief transmission from a passing vehicle. It might be a harmonic, an incidental signal, a device transmitting for only a few milliseconds, or something that is already known.
I prefer to investigate a hit before adding it to my permanent database.
I'll look for:
- Frequency
- Signal strength
- Location
- Time
- Type of transmission
- Whether it repeats
- Whether it can be heard again
- Possible user or organisation
- Any visual clues
- Whether the frequency is already known
The more observations I can make, the more useful the eventual database entry becomes.
Drive, Walk and Stop
CloseCall monitoring doesn't have to mean sitting in one location. In fact, moving around can be one of the most useful techniques.
A scanner in CloseCall mode can be running while driving, although of course the driver should not be interacting with the scanner while the vehicle is moving. I can monitor from a safe position and investigate interesting signals after stopping.
Walking is even better. When I am on foot, I can move towards or away from a signal and watch how the signal strength changes. A strong signal that suddenly appears when I walk past a particular building is far more interesting than a random hit that I cannot reproduce.
Build the Picture
The ultimate goal isn't simply to collect frequencies. It is to build a picture of the RF environment around you.
A single frequency tells you very little. A frequency that repeatedly appears at the same location, at similar times, with the same type of traffic and associated with a particular activity is much more valuable.
Over time, these observations can reveal patterns. You may discover that a particular business uses a frequency you had never previously identified. You may find a local data transmission that appears at regular intervals. You may discover previously unknown UHF activity simply because you happened to be standing in the right place at the right time.
That is what makes CloseCall monitoring interesting.
My Basic CloseCall Approach
When I am doing a dedicated CloseCall session, my approach is generally:
1. Choose an area: Pick somewhere with a good chance of local RF activity.
2. Concentrate on UHF: Narrowing the search makes it easier to identify and understand signals.
3. Run CloseCall: Let the scanner identify strong nearby transmissions.
4. Identify the known: Compare hits against my existing frequency information.
5. Exclude the known: Don't waste time repeatedly investigating signals I already understand.
6. Investigate the unknown: Pay particular attention to frequencies that are new or unusual.
7. Move around: Change position and see how the signal behaves.
8. Use your eyes: Look for vehicles, buildings, antennas, businesses and other potential sources.
9. Use your ears: Listen to the actual transmission and look for patterns.
10. Verify: Try to hear the frequency again before treating it as a confirmed discovery.
11. Log it: Record the useful information while it is still fresh.
12. Add it to the database: Once I have enough information to make the entry worthwhile.
The Fun Is in the Hunt
For me, CloseCall monitoring is less about simply sitting down and scanning a database and more about going out and hunting for RF.
You never quite know what you are going to find. A short drive, a walk through an unfamiliar area or even stopping somewhere for a coffee can result in a completely new discovery.
The scanner gives you the starting point, but it is your observations that turn a frequency hit into useful information.
So when I am out monitoring, I try to remember four simple things:
Use CloseCall.
Focus on UHF.
Get up close and exclude the known to focus on the unknown.
Use your eyes and your ears.
That combination turns an ordinary scanner outing into an RF signal hunt, and that is where I find a lot of the fun in radio monitoring.
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