John Wells, M5AML.  QTH: Derby,  IO92GW.

CLOSE SPACING OF SSTV SIGNALS

In Europe SSTV QSOs usually take the form of an exchange of pictures between two stations.  Station A sends a picture, then station B sends one.  It is then station A's turn to send a picture again.  All information is transmitted in the pictures for example name, QTH, weather, meaning that stations do not talk to each other except maybe a quick "73, thanks for nice pics" at the end of the QSO.

Here is a diagram showing the bandwidth of a typical SSTV signal.  As you can see when the transceiver is tuned to 14.230MHz the actual SSTV signal is carried on frequencies between 14.2312MHz and 14.2323MHz. Frequencies between 14.230MHz and 14.2312MHz carry no SSTV information. The same applies to frequencies between 14.2323MHz and 14.233MHz.  If a QSO is taking place on 14.230MHz the normal recommendation is to use 14.227MHz or 14.233MHz to start up another QSO.




Let's for the sake of argument say that 14.230MHz is in use and another station appears on 14.233MHz.  The stations would appear as in the diagram shown on the left.  As you can see, only a small amount of the spectrum between 14.230MHz and 14.236MHz carries useful SSTV data (approx. 36%).  I can only assume that the 3kHz spacing of SSTV signals is recommended so that stations can exchange verbal comments using SSB.  In Europe comments are rarely exchanged (as mentioned earlier) so why use 3kHz spacing?  Why not use 1.5kHz spacing as shown below.

To the right is a diagram of how two SSTV signals one transmitted on 14.230MHz and the other on 14.2315MHz would be situated on the band.  The sync signal of the 14.2315 QSO appears 2.7kHz (0.0027MHz) above the dial frequency of the 14.230 transmission - i.e. on the upper edge of the 14.230 QSO's SSB receive bandwidth (14.230MHz + 2.7kHz = 14.2327MHz). Another SSTV QSO can then take place with a dial frequency of 14.233.  If only SSTV is being transmitted and stations in adjacent QSOs do not cause splatter then this scheme works.  I would go as far to say that even the short "73" type phone messages would cause little harm to adjacent QSOs.  1.5kHz spacing means that approx. 73% of the spectrum carries useful SSTV data.


The accepted SSTV frequencies are always under threat from other users.  Not just phone QSOs but also digital transmissions occupy the usual SSTV frequencies.  If SSTVers use 1.5kHz spacing then more QSOs can take place without disturbing or suffering from DRM and rogue phone transmissions.

So for 80m SSTV, dial frequencies would be perhaps 3.7285, 3.730, 3.7315MHz etc.
20m would look something like 14.2255, 14.227, 14.2285, 14.230, 14.2315, 14.233, 14.2345, 14.236MHz ... etc.  Other bands don't seem to be so crowded but the same principle applies whenever the SSTV transmissions are SSB but not AM or FM.  

DRM has grown in popularity over the last few years so it is always worth listening for these signals before transmitting analogue SSTV (to avoid QRM). DRM seems to have finally settled on 3.733MHz and 3.736MHz on 80m (LSB) and 14.233MHz on 20m (USB).  Remember - an analogue SSTV signal transmitted or received on a radio with a dial frequency of 14.2315MHz will interfere with DRM on 14.233MHz.

Below is a rather complex diagram of how analogue SSTV and DRM can sit side-by-side on 20m without any problem.  In this example there are five
QSOs taking place - four analogue SSTV and one DRM.  The first SSTV QSO has a dial frequency of 14.230MHz.  Using 1.5kHz spacing the next SSTV QSO would have a dial frequency of 14.2315MHz but that would interfere with the DRM QSO on 14.233MHz.  An SSTV QSO on 14.233MHz would also interfere with DRM.  If the next 1.5kHz frequency, 14.2345MHz is used then the SSTV sync. signal would be on the upper edge of the DRM signal so 14.2346MHz is better to use as a dial frequency in this situation to nudge the sync. signal just above the DRM signal.  The next SSTV QSO has a dial frequency of 14.236MHz which is normal for 1.5kHz (and 3kHz) spacing, and the fourth SSTV QSO (picture data and sync. signal not shown) has a dial frequency of 14.2375MHz which is also normal.  So moving the second SSTV QSO up 100Hz makes no difference to where each QSO sits on the band but saves the DRM on 14.233MHz and the SSTV a lot of QRM.  ( As mentioned already, SSTV QSOs can easily occur on frequencies above and below those shown in the above diagram but have been omitted for clarity).

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