GB3UHF Beacon Technical Details

The heart of the beacon is an LMX2541 Fractional-N synthesiser designed by Andy Talbot G4JNT and also used in the GB3NGI and GB3LEU beacon hardware. This module is controlled by a 16F648 PIC controller designed and programmed by John Worsnop G4BAO and Ian Burns G0AFH. The synthesiser reference is a G3RUH 10MHz GPSDO (Global Positioning System Disciplined Oscillator). The new beacon was the culmination of six months of design and construction work by a team of radio amateurs and includes some of the latest technology and modulation methods.

The synthesiser PLL generates a signal directly at 432MHz and is low-pass filtered (using a filter suplied by Sam Jewell G4DDK) and amplified by a Class-A MCL RF amplifier and then level adjusted before passing to the PA. The PLL reference source is a G3RUH 10MHz oscillator that has an oven controlled crystal oscilator (OCXO) which is disciplined by reference to GPS signals. The beacon frequency reference accuracy after warm up is better then 1010 after 30 mins with increasing improvement in accuracy. The frequency of the CW carrier (mark) being exactly on 432.430000MHz.

Extra facilities have been incorporated to ensure that the beacon does not transmit when the PLL is initially unlocked when power is first applied and the GPSDO is warming up, and both the frequencies used for CW FSK and JT65B are controlled by a PIC controller that also takes the GPS NMEA data to give the identification timings.

The LMX2541 frequency generator & PIC controller

The beacon sends its callsign and locator in Morse, and uses F1B keying (FSK).

To enable the beacon to be monitored at extreme ranges, the beacon also transmits its callsign and locator using WSJT JT65B mode.

The GPS also provides for the precise timing of the keying sequence, such that the Morse sequence will start at every odd minute past the hour and for 13 seconds duration and the JT65B sequence will start at every even minute past the hour for 48 seconds duration. The keying software has been written so that it uses the same sequence as GB3VHF.

In the event of GPS lock being lost, the beacon will continue operation but with reduced frequency and timing accuracy until such time as GPS lock is re-established. When this occurs the JT65B sequence is suspended and replaced with the Morse sequence every minute until GPS lock is re-established.

The component parts of the driver unit

The driver front panel indicators and connections

Listen to the complete keying cycle

Full details of this LMX2541 synthesiser range can be found on Andy’s website www.g4jnt.com.

The beacon’s PA unit is based on the Mitsubishi RA60H4047 power amplifier module for the PA with components supplied by Chris Bartram GW4DGU and Grant Hodgeson G8UBN. The use of extensive RF filtering throughout ensures that the beacon complies with the most stringent CEPT regulations for transmitters operating in the UHF frequency range. Both in-band and out-band spurious signals are suppressed to better than -65dBc. The PA uses only heat sink convection cooling and no extra fan cooling is provided or needed under any environmental conditions.

The output from the driver unit is 13dBm (20mW). This drives the PA unit to produce a beacon maximum final power output of 15dBW (30Watts). This is further filtered by a high power low-pass filter which was built by Sam Jewell G4DDK and then monitored with a Bird Thruline power monitoring panel that shows both forward and reflected power simultaneously. Finally the RF is routed through a circulator that is used to protect the PA from high SWR fault condition and also to prevent intermodulation occuring in the PA due to the close proximity of the 2m beacon antennas

The PA unit and PSU

The beacon’s power supplies, delivering 12 Volts, were sourced by David Bowman, G0MRF. The 12 volt PSU for the beacon driver unit uses a 2.1A RF proof medical grade switched mode PSU, and is followed by a conventional 5 volt regulator for the PLL and PIC. The 12v is used for the G3RUH GPSDO and for the RF relay. The 12v PSU for the PA is a low noise and RF proof SMPSU that can deliver up to 29A . All PSU’s use only convection cooling and no extra fan cooling is required under any environmental conditions.

Chris Whitmarsh, G0FDZ, the beacon keeper, integrated both the driver and PA units, and produced the metalwork and housing facilities for the beacon. The beacon was designed so that the driver and PA were built as separate units to ease access and maintenance. All connections are brought to the front on the units for the same reason. External connections to the beacon are simply mains power, GPS antenna and the main antenna system. Like most commercial systems, all the beacon units are built in 19 inch rack mount style, and the whole beacon shares a rack of 18 U together with the GB3VHF 144MHz beacon.

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