20151221 - Why 123.0 Hz CTCSS

123 Mother Caught a Flea

One, two, three,

Me mother caught a flea,

She put it in the teapot

And made a cup of tea.

The flea jumped out,

Me mother gave a shout

And in came daddy 

With his shirt hanging out.

So goes the nursery rhyme from Ireland.  You would have noticed the total absence of an Irish accent.

You also would have noticed that the rhyme has nothing whatsoever to do with Amateur Radio. Well almost nothing. 123 Hz is a commonly used frequency for CTCSS  in radio equipment, both commercial and Amateur. Back in two thousand mumble, around 2008 I think, the WIA facilitated a meeting of Amateurs from around the Sydney Basin to discuss congestion of repeaters, mainly in the two metre band. One outcome of this meeting was that the WIA Technical Advisory Committee changed the recommended CTCSS tone for repeaters from 123 Hz to 91.5 Hz.  

Recently the subject of CTCSS tones was raised in Western Australia.  The event I just described were remembered, but nobody remembered or understood the reason for the change to 91.5 Hz. Will VK6UU contacted John Martin, the head of the Technical Advisory Committee and received the following response:

Problems had emerged with false detecting at sites where there were amateur repeaters and non-amateur services both using 123 Hz tones. 

Peter Mill VK3APO suggested the solution of using an alternative tone. The issue was discussed at a meeting of repeater licensees in Sydney, which as I understand it was attended by a number of repeater licensees from VK2, VK3 and VK4, which are the states where these problems had been occurring. 

They agreed that an alternative tone was needed, and they suggested 91.5, which had already been in use for some years in VK3 and especially in VK4. 

There was (and still is) no requirement for any repeater - in VK6 or anywhere else - that is presently using 123 Hz to change tones if it is not experiencing false detection problems.  

So there is it from the horse’s mouth. The wise men from the East issued no decrees. Western Australia  wasn’t deliberately excluded from any process.  The change to the recommended CTCSS tone is recommended and is entirely optional. 

Thanks Will for pursuing the issue and gaining clarification.