How Do I Reliably Access A 2m Repeater

By B Reay (G8OSN)

Introduction

For some time now there have been frequent, often heated, “discussions” on my local repeater (GB3KN) regarding problems with accessing and using the repeater following the changes to accommodate the new 12.5kHz channel spacing standard.   It is clear that there is some confusion over the issues involved and this article is intended to clear up this confusion- both locally and around other repeaters.

The primary issues involved are:

 

  • The audio deviation of users’ transmitters
  • The access method

 

Deviation Problems

It is clear that many users do not understand the difference between Channel Spacing and Deviation. Channel Spacing is simply how far the channels are apart. On 2m this used to be 25kHz but is now 12.5kHz. While deviation (some times known as ‘peak’ deviation) is how much the transmitter frequency varies (or deviates) when you speak into the microphone.  Thus, while the deviation should be chosen to suit the channel spacing, few radios (especially older models) automatically adjust the deviation to suit the channel spacing.

Note: Deviation and ‘microphone gain’ are not the same- the latter only sets how much audio (i.e. how loud you need to speak) to get the maximum (or peak) deviation.

If the deviation is set too high several things can happen. You may get ‘break through’ onto an adjacent channel (‘bleed over’ in CB parlance).  If the receiver (in the repeater) has a narrow filter (suitable for a lower deviation) the signal may be ‘chopped up’ with only parts of words being heard. You may also not be able to ‘access’ or ‘bring up’ the repeater (see below).

In ideal circumstances, all of our transmitters would be set for the correct deviation, 2.5kHz, which suits 12.5kHz channel spacing. However, many of us are still using equipment originally used under the 25kHz system and, unless they have been adjusted, these are still set up for 5kHz deviation- even if they can select channels in 12.5kHz steps.

Also, older equipment often has receiver filters designed for the wider deviation but this only affects us on receive. So, unless someone is using wide (5kHz) deviation on a channel 12.5kHz away, this is unlikely to be a problem in real terms.

Just to add to the joy, many new radios are delivered with their deviation set to the 25kHz channel spacing standard of 5kHz- even if they have the correct receive filters and are otherwise designed for 12.5kHz spacing!

Different repeater groups and managers have handled the migration from 25kHz to 12.5kHz channel spacing in slightly different ways.  It should be remembered that they didn’t ask for these changes but they are expected to enact guidelines defined by the RA and RSGB.

To meet the new standard they had to decrease their transmitted deviation to 2.5kHz. This has made little difference to users- at worst some will have noticed a slight decrease in volume and compensated with their receiver’s volume control.

Some repeater operators have also replaced the filters in the repeater’s receiver with ones designed for 12.5kHz. It is this change that has caused some users grief as, if their deviation exceeds 2.5kHz, at best their signal will be ‘chopped up’ by the repeater.  In some cases, users may not be able to ‘access’ or ‘bring up’ the repeater at all

 

Access Problems

It is important that users understand that the term ‘accessing’ or ‘opening’ a repeater means ‘waking it up’, not simply talking through it once it is ‘open’. Different repeaters around the country require a different approach to accessing them.

The most basic require a short (about 0.5sec) tone at 1750Hz. Once the repeater detects this tone it starts to relay the received audio to the transmitter and the repeater is said to be in ‘talk through’. Provided there are enough users to keep the repeater ‘awake’ then further tones are not normally required, until the repeater returns to ‘listen only’ and the cycle must be started over. (Some repeaters ‘time out’ if users talk too long and the repeater returns to ‘listen only’ and must be accessed afresh.)

The 1750Hz tone must be nearly ‘spot on’ in frequency, not too long, nor cause the user’s transmitter to exceed the 2.5Khz deviation, or the repeater may not accept it as valid and therefore fail to ‘open’.

Some repeaters require that the 1750Hz access tone is followed by transmitted audio- typically 2 seconds of speech. This offers some protection against ‘phantom bleepers’, who sometimes ‘bleep up’ the repeater to annoy others.

These days, most repeaters will also accept CTCSS to access them. With CTCSS a low level, low frequency (between 60 and 250Hz), and continuous tone, is super-imposed on the user’s transmitted audio.  If this tone matches that set for the repeater, then the repeater will open and enter ‘talk through mode’. The tones are generally chosen  on a county-by-county basis and are listed in the RSGB Year Book. 

CTCSS tones must be very accurate in frequency and tend to be sent at a low level of deviation. Both of these requirements are normally met by the components used to generate them and are rarely a problem.

 

So, how do I Reliably Access A Repeater?

1.       Make sure your transmitter is set up for 2.5kHz deviation on transmitted audio.

2.       If you have CTCSS, set it for correct frequency for the region (as published in the RSGB Year Book). This is the most reliable method.

3.       If you only have 1750Hz tone burst, check it is on frequency, no longer than 0.8 seconds, the level isn’t too high, and follow it with 2/3 seconds of audio. Giving your callsign twice (a good habit to get into and complies with your license) is enough.

4.       If you have a manual tone burst button, then just ‘jab’ it or your tone will be too long. Ideally use the 1750Hz tone only to ‘access’ or ‘bring up’ the repeater, not on every over.

5.       Limit your overs to less than 4 mins (or the local limit) or you may ‘time out’ and wait for the acknowledgement tone before returning to a call as this resets the time out timer.