When you first get into radio scanning, one of the first things you tend to look for in a scanner is sensitivity.You want a receiver that can hear weak signals.
That makes sense. After all, what's the point of having a scanner that can't hear the transmissions you're interested in? But there is another specification that is just as important, and often overlooked: Selectivity.
Sensitivity and selectivity are related, but they solve two very different problems. Understanding the difference can make a huge difference to how well your scanner performs, particularly if you live in an area with lots of radio activity.
What is sensitivity?
In simple terms, sensitivity is how well a receiver can hear a weak signal.
A highly sensitive receiver can receive a signal that is very weak at the antenna. Imagine you're standing outside and someone is talking to you from 100 metres away. A sensitive receiver is like having very good hearing. It can still hear that person when they're speaking quietly.
For radio, sensitivity is normally expressed as the minimum signal level required for the receiver to produce a usable signal.
For example, you might see a specification such as: 0.3 µV for 12 dB SINAD
The lower the required signal level, generally speaking, the more sensitive the receiver is.
This is where things get interesting. It is easy to assume:
More sensitivity = better scanner.
Not necessarily. A scanner doesn't operate in isolation. Your antenna is receiving everything around you. If you live somewhere with lots of strong radio transmitters, your receiver may actually have too much to deal with.
This is where selectivity becomes important.
What is selectivity?
Selectivity is the receiver's ability to receive the signal you want while rejecting signals you don't want.
Think of it like trying to listen to one person in a crowded room. Sensitivity is how good your hearing is. Selectivity is your ability to concentrate on one person's voice while ignoring everyone else. A receiver with good selectivity can separate closely spaced signals and reject unwanted signals outside the channel you're listening to.
Imagine you're monitoring a local radio network. The signal you're interested in is relatively weak. Nearby, however, you have a powerful transmitter that is many times stronger.
You need a scanner that can:
- Detect the weak signal.
- Tune accurately to it.
- Filter out signals outside the channel.
- Handle the strong nearby transmitter without becoming overloaded.
- Produce clean, usable audio.
That's much more complicated than simply asking:
"How sensitive is this scanner?"
The best scanning setup isn't necessarily the one that hears the most RF. It's the one that hears the signals you actually want to hear, clearly and reliably.
And once you understand that, you start looking at scanners, antennas and filters a little differently. That's when radio scanning starts becoming less about simply buying equipment and more about understanding how the RF environment around you actually works.
