EI2GCD Multi-mode Digital Gateway Back on Air

The original EI2GCD 2 metre Multimode Digital Gateway was placed on air on the 13th of November 2017 and a year later it was placed on a high site overlooking Galway City alongside the DMR Repeater EI7RHD. The Gateway never really saw a huge amount of activity so it was put up for sale as a fully functional item which really only required a change of frequency and the new owner's parameters to be programmed into it. 

We were delighted when a member of the Limerick Radio Club took an interest in the project. This would bring all three popular modes of Digital Radio to the area which was currently being served by a 70cm Yaesu Fusion Repeater.

The intention was to replace the Gateway with a 2 metre D-Star Gateway but a new Repeater Builder STM32 MMDVM modem board was purchased. This board could be mounted on the Raspberry Pi and would allow DMR and C4FM to be switched in if required.

The previous Gateway project required a Spectrum analyser to set up the DMR but this board was simpler to set up.

A Tait TM 8105 Radio was acquired and programmed with just one channel 144.850 MHz. A lead was made to connect input and output from the Tait radio to the Modem which was mounted on the Raspberry Pi. The alignment procedure was simple.

The RX IN pot, top centre of the board above, was turned anti-clockwise to minimum. the small blue multi-turn pot on the top left of the board was backed down to a minimum setting.  A DMR transmission was made on the appropriate channel and the small multi-turn pot at the top left of the board was increased until the "Clip" LED just flashed on. The level was then backed off until the "Clip" LED no longer lit. This was the level now set for DMR.

The temporary set-up is shown on my twiddling and fiddling bench. The cooling fan is essential as the radio can get quite warm with continuous activity. The Tait TM8105 is in the middle and sittng on the blue tray is the Raspberry Pi and modem placed on top. It is necessary to have a flow of air over the chip-set on the Raspberry Pi as it does get quite hot with continuous activity. If no cooling was present the Raspberry Pi would undoubtedly overheat and expire. 

After the basic settings are entered into the Pi-Star Program it is necessary to set the levels on modem. 

 

Once the DMR level has been set it should result in the received signals being heard clealy. If there is activity showing on the modem but nothing heard on the radio try adjusting the TX invert from 1 to Zero. that should cure the problem. If not, it may be necessary to back off the DMR TX level slightly. Do this in steps of 5 until clear audio is received. 

 

Try C4FM next. This should be perfect but try backing the YSF level off in stepds of 5 until the audio just disappears. Set the level approxinamtely half way between the upper and lower level. If you press the "DX" buttion on the C4FM radio the radio should connect to the node immediately if this level is correct. 

 

D-star is aligned in the same way the sweet spot is between the point at which the the audio is received to the point where it disappears. Set he radio half way between the two points.

That completes the alignment of the audio levels. 

 

The activity should build up on the dashboard very quickly.

 

It is possible to select the modes of operation in the configuration file. If all three mode are selected, bear in mind that if one of the modes is connected to a busy Talk Group or Reflector, then that mode will dominate the use of the Dashboard and prevent the other modes coming through. Obviously a happy balance will have to be reached. The alternative is to just switch on the modes required.

Once all is working perfectly it is time to set up on a high site:

 

Everything fits snugly into the repeater housing and both the Repeater and Gateway are connected into a 3-Mobile Router. 

The Gateway is very reliable and should need very little attention. Our Previous Gateway ran continously for 2 years until it was removed from the site.

As the Galway is very well served by four 70 cm DMR Repeaters and a Yaesu Fusion DRX-1 repeater on 2 metres, it is possible that this Gateway will be permanently set on D-Star. The option is always there to include the other modes if necessary. This can be done remotely.