MB7TR – SSTV in Magor, Caldicot

MB7TR is an SSTV repeater classed as a regenerative node operating on 432.525MHz NFM (narrow FM).

How To Use

MB7TR will repeat/regenerate your image in the following mode : the same as incoming

To have MB7TR repeat/regenerate your image, which you can transmit to it any SSTV mode, you need to do the following, all on 432.525MHz in NFM :

  1. TX your CW ident (good practice)
  2. TX a 1750Hz tone burst for around 2 seconds and key off
  3. Listen and wait for MB7TR to reply with its CW ident – this signifies it is ready to repeat
  4. TX your SSTV image within 10 seconds of the end of the CW ident
  5. Sit back and wait for your image to be repeated out

You can use your SSTV software of choice, MMSSTV being one often used.

Hardware

  • PSU : 13.8v 25A linear
  • Radio : Motorola GM340 UHF
  • Time out timer (ToT) : 240 seconds
  • Forward power at back of radio : 2.25w
  • Antenna System gain (less other losses) : 5.27dB
  • Maximum Deviation : +- 2.5KHz
  • Emission code : 8K00G9W
  • Max ERP permitted : 8.9 dBW ERP
  • Diamond MX72N Duplexer – 0.1dB insertion loss

Aerial

  • Diamond X300N
  • Gain : 6.85dBd
  • Height : 8m agl
  • Elements : 5 x 5/8th on 70cm
  • Max wind : 50m/s or 112mph
  • Length : 2.9m
  • Max Power : 200w
  • Nominal Impedance : 50ohm
  • SWR @ 432.525MHz : 1.27:1

Coax Feeder

  • 12m of Ultraflex7 from Messi & Paoloni
  • 1.48dB cable loss for the 12m feeder length (excluding connectors)
  • 12.3dB loss for 100m at 430MHz
  • 440watts at 430MHz handling
  • 75pF/m +- 2
  • 83% velocity factor
  • 50ohm nominal +-3
  • Terminated N-Type both ends (n-type to bnc adaptor at radio)

Logic

MMSSTV in repeater mode is used on a Windows10 PC.

The radio is in an outhouse and as such uses a USB audio dongle connected to a RaspberryPI 4. This PI4 runs VirtualHere which essentially brings that USB audio dongle device over the LAN to the PC in the house. The system also uses a homebrew Arduino Pro Mini emulating a Kenwood TS480 and associated CAT commands (code here). That Arduino is also connected to the PI4 and the ‘Kenwood TS480’ appears on the same Windows10 PC as a com port. MMSSTV issues CAT commands to that com port to PTT the Motorola GM340 UFH radio.

Supported SSTV Modes

MMSSTV supports the following SSTV modes.

  • Robot B/W 8
  • Robot B/W 12
  • Robot 24
  • Robot 36
  • Robot 72
  • AVT 90
  • Scottie 1
  • Scottie 2
  • Scottie DX
  • Martin 1
  • Martin 2
  • SC2-180
  • SC2-120
  • SC2-60
  • PD50
  • PD90
  • PD120
  • PD160
  • PD180
  • PD240
  • PD290
  • P3 203
  • P5 305
  • P7 406

Issues/Problems

If the are any issues please contact me via [mb7tr <at> grange-lane.co.uk]