[RTTY] 60 meter digital

[Kok Chen]

On Feb 25, 2012, at 7:23 AM, Bill Turner wrote:

> Apparently the ARRL is unaware of 23 Hz RTTY (Baudot), which also meets 
> the requirement, I think.  Does it?

23 Hz shift MSK (Minimum Shift Keyed) will likely fit into the FCC passband.

But check your output spectrum before you transmit 23 Hz shift FSK (Frequency Shift Keyed) on 60m.  Your FSK keying sidebands might make your bandwidth greater than what the FCC permits.  If you are using AFSK, *and* the AFSK waveform is waveshaped, you *might* be able to get away with it.  See the last two figures here 

http://www.w7ay.net/site/Technical/K3/Content/digital.html

to see what kind of keying sidebands are produced by a typical rig's FSK generator and what a waveshaped AFSK looks like.  Just eyeball the horizontal axis to match 23 Hz shift to get an idea.  Remember that the decay of the sidelobes remain the same since we are still talking 45.45 baud keying sidebands.  I.e., don't just squish the horizontal axes; instead, cut out a 147 Hz chunk at the center of the signal.

Remember your Terman?  The bandwidth of an FM signal comes from two components.

You can interoperate between MSK and 23 Hz shift RTTY by setting up the FSK transmitter for 23 Hz shift, but a true MSK modulator has the correct waveshaping to keep the signal narrow and the FSK modulator does not.  The MSK symbol baud rate is also related synchronized to the shift -- in addition to proper waveshaping of the amplitudes, that also keeps the transmitter's keying sidebands in check (that is an aspect of MSK that is shared with PSK31).

Practically speaking, if you look at the aforementioned figures, placing the center of RTTY tones tones at exactly the middle of a 2.8 kHz channel on 60m should present very little QRM to the next channel, even if we use standard 170 Hz RTTY shifts.  But we are dealing with legal rules and regulations and not with mathematics.  Further, the FCC might be looking out for the primary users of the 60m band -- imagine if we spread out RTTY signals on the band like what happens during a heavy split pileup -- there is no way for the primary user to tell us to clam up when they need the spectrum, except to illegally interfere with the DX by transmitting commands over the DX station.

Speaking of which, I did not come across any verbiage in the new rules prohibiting cross channel ("split") communications -- can we transmit on a PSK31 channel while receiving on a different channel? Other than being uncivil, that is.

All that said, during selective fading conditions (read:NVIS), you will not be able to take advantage of an ATC anyway when you use a 23 Hz shift.  So there is less reason to use 60m.

Now, if we use a very wide shift, and place the Mark and Space tones at the center of individual, adjacent 60m channels, you will get very nice frequency diversity to counter selective fading :-P :-P  (Disclaimer: don't do this, I am joking!)

73
Chen, W7AY