Radio system

Technical and RF Hardware Information for the GB7BS Repeater.

The GB7BS Repeater is based on the commercial standard Motorola DR3000.

 

The technical and performance details are shown below. It is important to remember that the DR3000 is a Dual Mode repeater. This means that it has the ability to operate either in Analog Mode (FM) or Digital (DMR) Mode.

 

Although the DR3000 can automatically sense and switch modes it cannot however receive in one mode and transmit on the other mode.

 

The DR3000 as used on GB7BS is only used in Digital (DMR) mode; it is disabled from operating in analog mode.

 

 

Specifications.

 

Technical RF Output 25-45W

Frequency 403-470 MHz

Voltage Requirements 100-240 V AC 47-63 Hz (13.6 V DC)

 

Current Drain: Standby 0.5 A (1 A DC typical)

Transmit 1.5 A (11 A DC typical)

Operating Temperature Range -30°C to +60°C

Max Duty Cycle 100%

 

Receiver

Frequency Stability (-30°C to +60°C) ±0.5 ppm

Analog Sensitivity 0.30 μV (12 dB SINAD)

0.22 μV (typical) (12 dB SINAD)

0.40 μV (20 dB SINAD)

Digital Sensitivity 5% BER: 0.3 μV

Intermodulation 70 dB

 

Adjacent Channel Selectivity: 60 dB @ 12.5 kHz, 70 dB @ 25 kHz

Spurious Rejection 70 dB

Audio Distortion @ Rated Audio 3% (typical)

Hum and Noise -40 dB @ 12.5 kHz

-45 dB @ 25 kHz

Audio Response +1, -3 dB

Conducted Spurious Emission -57 dBm

 

Digital Vocoder Type AMBE++

Digital Protocol ETSI-TS102 361-1

 

 

 

 

 

GB7BS is normally powered by a generator backed 240 AC supply, but also has a backup battery which is there to support the repeater between the site AC mains supply failing and the generator auto starting and taking load.

 

Of course this battery would keep GB7BS on the air for around 5-6 hours (depending on usage) should the generator fail to start. However, if the repeater is forced to use the battery backup supply, the output RF power is reduced by -3db to help extend battery life.

 

The DR3000 is virtually maintenance free. Apart from the occasional health checks such as cleaning fan filters and checking PSU volts and the backup battery, there is little upkeep.

 

All aspects of the DR3000 are also fully monitored as standard. Any faults or problems, such as low Tx Power, High VSWR or a Cooling Fan not operating correctly is reported and logged. This can all be monitored remotely by dedicated Motorola monitoring software.

Motorola DR3000

Motorola DR3000

Operation.

 

Unlike the analog repeater GB3BS, GB7BS does not have or use any Repeater Controller, in the traditional sense.

 

Apart from a few security and validation checks, the repeater repeats exactly what it receives. There are no “pips”, Speech or anything that is “controlling” the repeater. All this has now been passed out to the Terminal, which is you the user and your radio!

 

It now depends entirely on how you program and set up your radio as to if it “beeps” when someone stops transmitting for example.

 

However, there are still some things that do not change. One of these is that as part of the license conditions (NoV) we must send a Repeater Identification.

 

This is done as a traditional CW Beacon every 20min. To do this the repeater will automatically switch its transmitter to analogue mode (FM) and broadcast a CW ID. Once completed the transmitter switches back to digital (DMR) mode.

 

This beacon is only transmitted when there is no digital traffic or if both Time Slots are not in use. It should be noted that if a analogue beacon is being broadcast then no digital (DMR) traffic will be allowed.

 

Besides the analogue CW ID, there is also a Digital beacon or ID that gets transmitted every 5 min when the repeater is not in use. However, if the repeater is busy then this digital ID is broadcast as part of the bit stream.

 

This digital beacon or ID is used for radio’s that use the Roam function to determine which repeater is the best (if its in range of more than one) and automatically change channel to make use of it. This Roam function is primarily for when using the South West Cluster Network. Not all radio’s support Roaming.

 

With this goes more dependency on the user correctly programming the radio.

Failure to correctly program your radio can, in some circumstances, cause

interference or blocking to other stations.

 

Before programming any radio you should ensure that you firstly have a valid

DMR Marc ID registered against your callsign. DO NOT just make one up as this

will cause problems and also prevents duplication. Each radio MUST have its own

and unique identification number

 

Also, make sure that you have the correct frequencies for the repeater, colour code

and Tx Admit Criteria settings selected correctly.

 

Make sure that you have the channel set up to transmit and receive on the correct Time Slot and Talk Group. Please keep to the recognised Talk Groups. For GB7BS that is Talk Group 9 on Time Slot 1 and Talk Group 950 for Time Slot 2 which will give you access to the South West Cluster Network.

 

If you are unsure of the correct settings for your radio then we will be please to clarify or answer any questions you may have. Please use our Contact Us page to get in touch.

 

Finally, there is still a Timeout function. This is currently set to 5 min. As soon as this timer is reached the repeater will simply drop the current Time Slot or Talk Group without any warning.

All content is Copyright of The Bristol 70cms Repeater Group