Re: LF: 137.780 dash's

[Markus Vester <markusvester@aol.com>]

Hi Joe,

 

your carrier was well visible as a straight line on 137777.000 Hz. If the phase had alternated every 50 seconds it should have split up into two lines 20 mHz apart, accompanied by additional weaker lines. I also fed the 1:34 UT data file to EbNaut (10 second symbols, 3 Hz), which produced an all-asterisk pure-carrier decode and a single line of dots in the phase plot.  So apparently the PSK modulation hasn't come through.

 

In general, it would not be optimal if two stations on either side of the pond simultaneously transmitted PSK on adjacent frequencies, as the strong nearby signal will cause QRM to those who listen for the distant signal. The situation is worse than for visual FFT modes, because the FEC signal is intentionally making use of a high symbol rate (a kind of spread spectrum in that sense), and also because the sidebands from the unshaped keying fall off rather slowly outside the occupied band (20 dB/decade sinc weighting). The same applies again to the receive side, with far-off spectral response due to aliasing with the rectangular symbol-integration window. Even though some interference may be tolerated by the decoder it will raise the SNR threshold for the weak signal. A few years ago, similar arguments led KK7KA to implement smooth edges for WOLF mode.

 

Unfortunately smooth phase ramping does not really improve the spectrum much, at least as long as the ramp is much shorter than the symbol duration. If on the other hand we made the ramp slow enough, we'd more or less end up with something like coherent (G)MSK - which is actually a very bandwidth-efficient modulation scheme for nonlinear transmitters.  

 

All the best,

Markus (DF6NM)

 

 

 

 

 

-----Ursprüngliche Mitteilung----- 
Von: jcraig <jcraig@mun.ca>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mi, 18 Nov 2015 2:08 pm
Betreff: Re: LF: 137.780 dash's

Hi Paul, 

 

I was on last night/this morning until about 0240, hoping to see 

some sidebands from the BPSK attempt. Will try again tonight. 

 

73 

Joe VO1NA 

 

On Wed, 18 Nov 2015, Paul Nicholson wrote: 

 

> 

>> http://136.su/index.php/topic,18.msg13738.html#msg13738 

> 

> Can I see a faint line at 137.777kHz from VO1NA on the first 

> of those two spectrograms? 

> 

> -- 

> Paul Nicholson 

> -- 

> 

> 

 

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