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1994-02-28
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CTCSS/PL TONES - by Jason Miles VE3TYG
--------------
Some people regard the CTCSS (Continuous Tone Coded Squelch System)
feature of their radios a real "life saver", and others don't think that it
is a worthwhile feature in the amateur bands.
SO JUST WHAT IS CTCSS, ANYWAYS?
------------------------------
CTCSS (also known as "PL TONES" ) are sub-audible (unable to be
heard by humans) tones that are piggy-backed on top of the TX frequency
provided that the radio is equipped with the ENCODE function. If a
receiver is equipped with the DECODE function, and it was set to receive
the TX'd frequency with the TX'd CTCSS tone, the receiver would "open up"
and let the listener hear the signal. If the TX PL tone was different than
the RX PL tone, the receiver would stay silent.
For Example:
-----------
(1)
Radio A Radio B
TX freq - 146.2800 Mhz TX freq - 146.8800 Mhz
RX freq - 146.8800 Mhz RX freq - 146.2800 Mhz
PL freq - 103.5 Hz <-- Same as 000.1035 Mhz --> PL freq - 103.5 Hz
│ │
┌┴─┐ >>---TX 146.28Mhz---------Rx 146.28Mhz-->> ┌┴─┐
│ │ <<---RX 146.88Mhz---------TX 146.88Mhz--<< │ │
│ │ >>---PL 103.5Hz-----------PL 103.5Hz----<< │ │
└──┘ └──┘
** RADIO A WOULD COMMUNICATE WITH RADIO B **
(2)
Radio A Radio B
TX freq - 146.2800 Mhz TX freq - 146.8800 Mhz
RX freq - 146.8800 Mhz RX freq - 146.2800 Mhz
PL freq - 103.5 Hz PL freq - 127.3 Hz
│ │
┌┴─┐ >>---TX 146.28Mhz---------Rx 146.28Mhz-->> ┌┴─┐
│ │ <<---RX 146.88Mhz---------TX 146.88Mhz--<< │ │
│ │ >>---PL 103.5Hz-----------PL 127.3Hz----<< │ │
└──┘ └──┘
** RADIO A WOULD NOT COMMUNICATE WITH RADIO B **
Why, in example 2, would RADIO A not communicate with RADIO B?
Take a look at the PL frequencies. RADIO A has a PL tone of 103.5Hz and
RADIO B has a PL tone of 127.3Hz. Even though the TX and RX frequencies
match, RADIO B will decode a PL tone of 127.3 where RADIO A encodes
103.5Hz as it's PL. RADIO B stays silent. It is not a good idea to use
the term squelch when referring to PL tones, because when the tones are
encoding/decoding, the squelch circuit is not functional (dead air has no
PL tones!) so even when the squelch is turned right down, the radio is
silent. It is not possible to set a different PL tone for transmit and
receive, the same PL is used for TX/RX.
HOW CAN I USE THIS TO MY ADVANTAGE?
----------------------------------
Have you ever driven through an "intermod alley" and been blasted
out of your car by high pitched tones followed by "John can you please
call the office, John call the office, John call the office please,
John". If you live or drive anywhere near one of these areas, you know
what I mean. If you are listening to VE3PRC, it uses a PL ENCODE frequency
of 103.5Hz. If you set your radio to DECODE 103.5Hz PL tones, your radio
will stay silent until the repeater is transmitting, at which time your
radio will sample the PL frequency, and if it matches, your receiver will
open up and you hear the repeater. When the repeater stops transmitting,
your receiver closes, and you hear nothing (even if your squelch is
turned right down, RIGHT???)
AND NOW THE BAD NEWS!!!
-----------------------
If you are tuned to the repeater, and your PL tone is 103.5Hz
(same as the repeater), and the repeater transmits, you will, of course,
hear the repeater. If, while the repeater is transmitting, a burst of
intermod comes along, you are going to hear the intermod! This is
because your radio is still decoding the 103.5Hz PL tone from the
repeater, and it is keeping your receiver open! Sorry, can't do much
about that except turn off your radio!!!
HOW MANY TONES ARE THERE, AND WHAT ARE THEY??
---------------------------------------------
There are 33 tones available, listed below.
All values are Hertz (Hz)
67.0 146.2
71.9 151.4
77.0 157.7
82.5 162.2
88.5 167.9 Although these frequencies
94.8 173.8 were obtained from a reliable
100.0 179.9 source, the accuracy of them
103.5 186.2 can not be guaranteed. If
107.2 192.8 you get an error while trying
110.9 203.5 to program these tones into
114.8 210.7 your radio, it is not a valid
118.8 218.1 tone.
123.0 225.7
127.3 233.6
131.8 241.8
136.5 250.3
141.3
Part of the fun of amateur radio is being able to experiment. Have
fun with trying the examples that I have shown, and be creative and come up
with some examples of your own, and show me in future editions of the
HOD. The purpose of this article is not to be technical, but to give
an understanding of the principles behind Continuous Tone Coded Squelch
System.