Chen,
I mostly operate CW on 160m. As you know, you can't determine selective
fading from one signal. That said, the overall fading on 160m is REALLY slow
compared to the HF bands.
Part of pulling signals out of the noise is to realize it might be 10 or 15
seconds before the signal comes up from its null to the peak. It's
frustrating when someone comes back and you realize they are on the downside
of the fade and by the time they get done sending your call and their call,
the report can't be heard at all!
Regarding multiple signals 200 Hz apart... I need to look at the rule you
referenced. My initial reaction is there are other multi-tone digital
signals on the bands so I wouldn't think this would be a problem. But that's
speculation prior to seeing what the rules say.
73 de Bob - KØRC in MN
----- Original Message -----
From: "Kok Chen" <chen@mac.com>
To: "RTTY Reflector" <RTTY@contesting.com>
Sent: Sunday, November 01, 2009 5:37 PM
Subject: [RTTY] RTTY Shifts
> In an email to me, Russ WA3FRP brought up something interesting -- of
> switching to wider shifts on the lower bands.
>
> When shifts are too narrow, the mark and space can be so close to one
> another that they no longer fade independently. When that happens,
> any ATC circuit that tries to work around selective fading no longer
> functions, since there is _no_ selective fading. When both tones
> vanish below the noise at the same time and the decoder can no longer
> decide if mark or space was transmitted.
>
> The question is therefore (1) is selective fading on the low band
> replaced mostly by flat fading, and (2) whether the use of greater
> frequency diversity (larger shift) will bring back selective fading so
> that at least one of the tones survives through a fade.
>
> If all else is equal, selective fading is preferable to flat fading
> for RTTY since that is what ATC circuits are designed to overcome.
> Flat fading is better solved with the use of temporal interleavers and
> error correction codes, such as used in MFSK16, Olivia and DominoEX --
> the modern digital modes tend to spread errors in the time dimension
> to overcome fading.
>
> It is easy enough to generate a signal that has 4 tones that are
> separated by 200 Hz each, for example, to try to objectively measure
> selective fading at different frequency diversity. The problem is
> that such a signal might not pass FCC 97.309. Short of that, it might
> be sufficient to just watch a 500 Hz wide MFSK signal on a spectrum
> analyzer. For starters, I think I will try to look for fast Olivia or
> DominoEX on 80m to watch carefully. The slower baud rates don't work
> as well since selective fades could have come and gone by the time the
> tone you want to watch is transmitted.
>
> 73
> Chen, W7AY
>
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