Half Duplex operation via AO-7

Operating Half Duplex via AO-7 and other linear transponder satellites.

This procedure was developed so that we had a system whereby portable stations that are limited to using half duplex radios such as the FT-817, FT-857 and FT-897 are able to operate on the linear transponder satellites, without having to rely on the use of CAT control and still enabling the use of SSB and or CW.

The procedure has been used on a several occasions and has enabled us to work some rare DX entities via AO-7. The system will work with any satellite with a linear transponder, but the frequencies will obviously have to be adapted to suit.

The procedure:

·    The half duplex station must transmit on a fixed up link frequency of 432.145 MHz at all times (don’t change the up link frequency). Any frequency within the pass band could be used, but 432.145 MHz happens to be in the center of the pass band.

·    It is imperative that half duplex station, call’s “CQ” often and repeatedly, pausing only to tune the down link, listening for replies.

·    The portable station will have to tune from 145.940 to 145.960 listening for replies. The portable half duplex station must actively search for stations replying to his CQ, after contact has been made with the calling station only slight corrections of the down link frequency will be required to correct for Doppler.

·    It might be an idea to announce during the CQ that the station is “half duplex, fixed up link” to alert the other users to the fact that they must track his down link and not the last station as we normally do.

·    All the other stations must find the calling half duplex stations down link and net on that frequency and thereafter remain locked to the half duplex stations down link. The first station to respond to the CQ will also have to give fairly long calls until contact has established thus giving enough time for the half duplex station to find the correct down link, thereafter it should be easy, all stations must follow the half duplex stations down link.

·   “Round Robin” QSO’s are not a good idea, after each exchange, the over must go back to the half duplex station so that he (the half duplex station) controls the frequency.

·    The above procedure can only work if we understand that that all stations must follow and net on the half duplex stations down link frequency.

VERY IMPORTANT

·    The normal full duplex stations must not make blind calls to raise the half duplex station as this will only create chaos and should be avoided at all times. The half duplex station is relying on a response to his CQ and in turn this will allow all the users to net on the correct frequency (i.e. the down link frequency of the half duplex station). If anyone chooses to ignore this request they will spoil it for all the other stations wishing to work the half duplex station.

We are all very unfamiliar working half duplex. I have no doubt we can improve on the system and identify new pitfalls.

Your comments and feedback will be appreciated.

73, Pierre ZS6A