I am 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, execpt 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 continue overnight, and will hopefully be switched on
for
subsequent evening & overnight runs. Power output is 3Watts 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 generaed 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 on the website below
Andy G4JNT
www.scrbg.org/g4jnt/
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