An Electronic Gate-Keeper; Limiting Access; Frequency Sharing - Baofeng UV-82 User Manual

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An Electronic Gate-Keeper

A CTCSS decoder allows you to choose which signals are heard in your transceiver's speaker. When you activate the decoder,
your radio will fall silent. Only the signals that carry the CTCSS tone you've selected are passed through to your receiver's audio
amplifiers and, ultimately, to the speaker. All other signals are ignored. They're still there, but you won't hear them.
By activating the CTCSS decoder you're making your transceiver behave like a tone-protected repeater. But why would anyone
want to limit what they hear? There are three good reasons:

Limiting Access

In the beginning, amateur repeaters used CTCSS to restrict access to certain groups or individuals. (Only those who knew the
correct CTCSS tone frequency could use the repeater.) Although some repeaters still use CTCSS in this fashion, they are the
exception. After all, it's relatively easy these days to determine which CTCSS frequency is in use on a particular repeater. Some
modern radios will even display the frequencies of received CTCSS tones. All you have to do is monitor the repeater input frequency
and, when a user is within range so you can copy him direct, decode the tone from his transmission.
However, CTCSS is still a good way to limit access to other devices such as simplex autopatches, remote bases and so on. If
you are going to use your mobile rig as a temporary cross-band repeater (another feature found in many radios today), it's a good
idea to use a CTCSS decoder on the input. This will limit access to only you and those you've chosen to operate the system.
When used in conjunction with DTMF (touchtone) tones, CTCSS can be a more effective tool to limit access. In fact, many
"closed" repeaters require users to send specific DTMF tones, in addition to a constant CTCSS tone.

Frequency Sharing

To some extent, CTCSS can make it easier for different groups to use the same frequencies without bothering each other. This
application is seen most often in repeater networks.
In heavily populated areas it is not uncommon to find repeaters sharing the same frequency pairs. Coordination groups try to
arrange it so that these systems are separated by a considerable distance, but coverage areas often overlap. This means that some
stations are able to access two or more repeaters at the same time (see Figure 1). By installing CTCSS on both repeaters, stations
are limited to accessing only one repeater at a time. They must send the correct subaudible tone to use the repeater they desire.
(Modern FM transceivers make this easy by allowing you to specify particular CTCSS tones when you program repeater frequencies
into memory.)
CTCSS can also be a big help on simplex. For example, one of the FM simplex nets that I frequent here in southern California
shares the frequency with another group about 75 miles away. Many of us can hear them, and this can be more than a little
annoying!
The solution? Everyone on our net uses CTCSS decoders set to 100 Hz and everyone sends 100-Hz tones whenever they
transmit. As a result, we hear only each other! The only drawback is that it's difficult for newcomers to join the net unless they know
our system. If they attempt to break in without sending a 100-Hz tone, we probably won't hear them.