Located in South Dorset, GB3DR is a 2 metre amateur radio repeater and is co-sited with GB3SD a 70cm repeater.

TX frequency is 145.7375 and RX is 145.1375MHz, Sub Audio (CTCSS) is 71.9Hz

It is based on a 19 inch rack mountable Nokia branded - Kyodo mid band  (136 - 155MHz) FM base station, type BSR150L.

GB3DR is located  about 25 metres from a busy commercial site which carries high power paging transmitters among other things. The Kyodo base station has 2 x 3 pole helical filters in the front end and a diode ring first mixer, which makes it particularly suitable for this site.

This is a view of the base station with the top lid removed.

The RX unit is in the middle, the TX exciter unit on the left and the power amplifier at the rear.

This is a view from underneath the base station showing the interface and control board.

Provision is made on the right for sub audio and Selcall modules to be fitted, however these are not needed.

This board carries an audio amplifier to run the monitor speaker that can be plugged into the rear of the unit, it also carries 600 ohm line input and output as well as a repeater talk-through path with optional Sub Audio filter.

On the rear there is a 37 way D connector carrying the line interface etc. and a 9 way D connector originally intended for a commercial CSI repeater panel. This is where we plug-in the amateur repeater control logic.

Not all the pins on the 9 way connector were connected and we added 2 additional connections to pick-up squelch and feed in the morse ident and pip tones, these are the loose green and pink wires.

The logic has been specially developed for GB3DR but could be used on many amateur repeaters.  Below there is a block diagram of the logic, RX audio is passed to an LM567 PLL tone detector used to detect the 1750Hz tone burst for access, and also fed via a low pass filter and limiter to a PIC micro which looks for the 71.9Hz sub audio tone for access.

Once accessed by either 1750Hz or sub audio the PIC checks to make sure the squelch is open and if so keys the TX output. The PIC encodes (generates) the 71.9Hz sub audio for the transmitter using an 8 step resistor based D/A converter and a low pass filter, the resulting sub audio is a sign wave.

As long as the receive squelch stays open the repeater will repeat received audio onto the transmitter (talk-through), except that a 5 minute TX timer is also implemented in the PIC to prevent interference from holding the repeater open.  At the end of 5 minutes of continuous talk-through the morse letters T O are sounded and the transmitter is turned off. If the user has sub audio on their transmission the repeater will automatically re-access with only a slight gap, if however the user does not have sub audio, any further transmission after 5 minutes will be lost.

Normally users will not talk for more than say 3 minutes per 'over' and on finishing an 'over' the PIC sences the squelch closing and waits 1 second, at the end of the second a pip is sounded and that confirms reset of the TX timer. The Transmitter is then kept on for 5 seconds awaiting another 'over'.

A morse ident of 'GB3DR' is sent every 15 minutes regardless of the repeater being in use or not, this ident is sent at about 15 WPM with a 1.8KHz tone at 750Hz system deviation.  The pip tone is sent at 900Hz with 1.5KHz system deviation.  Both the ident and pip tone are generated by the PIC via 2 resistors and another low pass filter, again resulting in a sign wave. Sub audio is not sent with the ident when the repeater is not in use.

If the repeater is first accessed with a 1750Hz (+ or - 2%) for at least 200mS then two pips are sounded on first closing of the squelch.

Picture of the prototype logic.

A PCB has now been designed and is available alone or as part of a complete kit of parts. We also have a couple of spare Nokia (Kyodo) base stations available.

Webpage and content by G6WHI

Click here for more info from the local group website.