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T o
ne access is the best way to protect
repealer inpu ts when two or more
repeat ers share a common in pu t frequency.
In ma ny cases the coverage of the repeaters
overlap each o the r and a mobile trying to
access o ne may bring up several others to o.
This situatio n is highly undesirable as well as
annoying to the monitors o f the other
repeaters, particularly if they have auto ma tic
loggin g. Tone access can he effective in
eliminating these unwanted key ups and is
used in three different modes: Co ntinuous
Tone Coded Squelch (CTCS), Tone Burst ,
and the " Whistle-Up."
Of all the tone access systems CTCS is the
most positive as it wil l even work when the
input signa l to the receiver is quite noisy . In
a CTCS system a small encoder is mounted
in the mobile of base transmitter and su pplies
a subuudible tone in the range of 80 to
180 Hz to the audio input o f the transmitter.
Eac h time the c res equipped are
placed o n the air the c rcs tone is continuously
transmitted at a level that is about
'20% of the maximum speec h deviation. In a
typical narro wband system whe re the ma ximum
speech deviation is +5 k Hz the CTCS
deviation level is adjusted be t ween +750 Hz
to I kHz. T he re peater input receiver in tu m
is fitted wit h a CTCS decoder and this is
used to control the repeater transmitter in
place of th e usual Carrier O perated Relay
A PRI L 1972
,
Ed Webb W4FQMIl
Technical Editor
73 Magazine
(CO R), Thus the repeater ca n only be
brought up by an input signal having the
proper c rcs tone frequency. and when this
signal goes off the air the repeater will drop
out eve n tho ugh there is another signal o n its
inp ut freq uency . It is the CTCS tone that
not o nly brings the repeater up but also
holds it up and when there is no CTCS tone
the decoder cuts the keying voltage o ff to
the transmitter. (Of course there is no rmally
a few seconds d rop o u t time but the repeater
does drop as this type of control is very
positive. A CTCS system is fairly secu re as
the d eco der will o nly respond to LICS
signals that are with in + I liz o f t he LIes
de coder freq uency.} CTCS is widely used in
Moto rola calls their syste m " Private Line"
o r P. L., and Gene ral Electric calls theirs
" Chan netGuard " or C.G. For those amateurs
having commercial rigs the installation of a
crcs encoder is o nly a matter of wiring up
the manufact urer's encoder in t he unit. But
for the ham who has an imported unit o r a
piece o f older comme rcial gear the installation
of a e TCS encoder has po sed so mewhat
of a problem. But recen tly some small CTCS
encoders have become available in t he price
ra nge of $ 15-$25. T hese types of encoders
use reso na nt reeds as the frequency determining
device and you ca n figure o n spending
an additio na l $18.50 for the req uired
reed .
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