CTCSS on the Repeaters.

 

CTCSS- Continuous Tone Controlled Squelch System- is now used on most UK repeaters.

The system is used in addition to the conventional 1750Hz toneburst access system, not as a substitute, its use is at the discretion of each Repeater Group, except on 6 metres, where it is compulsory.

The official RMG specification for CTCSS is:-

There is no need for any groups to fit CTCSS to their repeaters, but if they wish to do so they must adhere to the RMC plan which is based on geographic areas (see below)
CTCSS when available is in addition to the standard 1750Hz toneburst access NOT a substitute.
The current standard remains unchanged. It expects a normal receiver sensitivity of 0.3µV
In practice, this means that a signal of about 0.6µV is required to open the squelch which usually has a 6dB hysteresis. However when the repeater's receiver detects its own CTCSS sub-tone, the squelch can open at a lower level, providing a better service for those equipped to take advantage of it, while not affecting the service to others.
The repeater has to radiate the same sub-tone but only when it is in talkthrough. Thus stations which monitor the repeater with receivers using CTCSS need not hear the regular morse identification.
Where a repeater is fitted with CTCSS it must identify the sub-tone used by appending a word gap (7 dot spaces) and then the sub-tone identification letter to its morse identification so that all users are made aware of the sub-tone required.
The recommended deviation for the CTCSS tone where used shall be 500Hz ± 200Hz

 

Nine different sub-tone frequencies are allocated for UK repeater use:-

 

Tone A = 67.0Hz

Tone B = 71.9Hz Tone C = 77.0Hz
Tone D = 82.5Hz Tone E = 88.5Hz Tone F = 94.8Hz
Tone G = 103.5Hz Tone H = 110.9Hz Tone J = 118.8Hz

 

The subtone frequencies are allocated on a nationwide basis in such a way that no two adjacent  repeaters on the same frequency share the same sub-tone. 
Generally, this means that all the repeaters in a local area will have the same sub-tone, as they will not be on the same frequency.

All the Repeaters in Lincolnshire are allocated to Tone B; 71.9Hz.

Advantages of CTCSS

On GB3FR and GB3SO, the receiver squelch is overridden on incoming signals which have the correct CTCSS tone, this means that dropout is less likely to occur on weak or fluttery signals.  Also, there is no need to use a 1750Hz toneburst, so no need to press an extra button to access the repeater, and no annoying bleeps at the beginning of each over, as there used to be in the days of auto tonebursts.

Implementing CTCSS

Almost all modern commercial rigs have CTCSS fitted, and it is very easy to add to older rigs, or ex-pmr equipment.

The simple Encode circuit shown below, designed by Adrian Knott, G6KSN has been found to work well, it can be built using the PCB layout shown, or built up on a piece of Veroboard. Unfortunately the pcb is not available commercially.

Component List

Resistors Capacitors
R1         1M
R2          5k6
R3          270k
R4          18k
R5          39k
R6          6M8
R7          18k
R8           47k
P1           5k
P2           10k

 

 C1        100nF polyester
C2        100nF polyester
C3         47pF Ceramic
C4         10µF 10V Electrolytic
C5         10µF 10V Electrolytic
C6         10µF 10V Electrolytic

IC           TL074

TL074 pinout

A printable version of this circuit (zip file) may be downloaded by clicking here.