The Bromley and District Amateur Radio Society

Experimental Attended MF Beacon
by Andy G4JNT

Andy G4JNT is currently testing a low power experimental beacon on 1960.000kHz +/-1Hz.

It uses a transmission format similar to that of the 5MHz beacons, GB3RAL/WES and ORK, except that the sounder sequence is replaced by a PSK31 message.

The sequence transmitted is :
T+0s CW Callsign
T+7 - T+15s Nine power steps with the output reduced by 6dB each time
for a total of -48dB
T+16 - T+24 A repeat of the power steps.
T+25 - T+30 Full power (level measurement reference period)
T+30 - T+47 PSK31 data
T+47 - T+59 Full power

The reference time T starts at one minute past the hour and repeats every 15 minutes, so transmissions occur at 01, 16, 31 and 46 minutes past the hour.

If the 'GB3RAL' monitoring software is used for logging this signal, the transmission will be shown and recorded as if it were that from GB3WES.

The transmissions will be switched on for evening and overnight runs when I'm at home. Power output is 3 Watts to my 7m high Tee antenna - esimated gain -9dBd

The beacon hardware consists of an identical DDS and PIC controller to that used on GB3VHF, although without the GPS locking; frequency is derived from a TCXO alone. A Motorola Oncore GPS module controls timing.

Completely new PIC software uses the 12 bits of amplitude control possible within the AD9852 DDS chip to generate the power steps directly rather than having to use a separate programmable attenuator as was done for the 5MHz beacons.

The PSK31 envelope is directly generated at RF by looking up 250 half-sinewave amplitude values for each 31.25 baud symbol at a sampling rate of 7812.5Hz.

Full details of both the 5MHz beacon designs and the driver for GB3VHF can be found at http://www.scrbg.org/g4jnt/

Andy G4JNT