Re: LF: 74.550kHz Sep 29/30

["Markus Vester" <markusvester@aol.com>]

Hi Bob, LF,

 

last night my improvised grabber http://dl.dropboxusercontent.com/u/26404526/df6nm_74kHz.jpg indeed showed weak and slightly fuzzy traces on both your QRG's, and on all other integer Hz frequencies as well.

 

These are presumably artifacts from DCF77 which is only about 160 km from here. In addition to the well known AM timecode, it also carries  pseudorandom phase modulation, which has been proposed in the 80ies to provide higher resolution timing (albeit orders of magnitude worse than Loran or GPS). The resulting sidebands extend a couple of kHz on either side of the carrier, with pronounced minima around multiples of the chip rate 77500/120 = 645.833 Hz, see

http://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_4/4.4_zeit_und_frequenz/pdf/5_1988_Hetzel_-_Proc_EFTF_88.pdf (page 358). The same code sequence is repeated every second, so in theory the spectrum would consist of sharp 1 Hz spaced lines. However, additionally the sign of the sequence is alternated with the disseminated timecode bits, producing some widening or "fuzzyness" of the lines.

 

Attached is a spectrogram which was taken tonight on the resonant antenna. Between statics, you can still see the fourth sideband lobe which is centered near 74.6 kHz. The spectral gaps are on 

 74916.666 Hz,

 74270.833 Hz,

 73625.000 Hz,

with small and sharp central lines, presumably caused by slight inbalances or nonlinearities in the transmitter.

 

By these criteria, if you have the choice I would recommend to operate somewhere near these gaps, but not exactly in their middle, and also preferably not exactly on integer Hz frequencies ;-)

 

Best 73,

Markus (DF6NM)

 

 

From: Bob Raide

Sent: Monday, September 30, 2013 8:02 PM

To: rsgb_lf_group@blacksheep.org

Subject: RE: LF: 74.550kHz Sep 29/30

Hartmut;
You make it look easy!!! 
That station on 74.548 is WG2XRS/5, Dex in South Carolina.  I am sure you will see him when condx favor his area.
Really great capture-this time I have my transmitter running correctly.  I thought I better drop down .1 hz as I was seeing sigs on .550 and .549 on Vester's grabber but then remembered you have directional ant and the sigs come from east of you and are not a problem.  I can go to 74.549 from now on.
Thanks for all your good work! Bob  WG2XRS/4 NY
 

> Date: Mon, 30 Sep 2013 17:33.:59 +0200
> From: h-wolff@gmx.de
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: 74.550kHz Sep 29/30
>
> Here are the captures taken last night:
>
> https://dl.dropboxusercontent.com/u/50178231/74550Hz-2013-09-30-QRSS60Screen.JPG
>
> https://dl.dropboxusercontent.com/u/50178231/74550Hz-2013-09-30-QRSS120Screen.JPG
>
> XRS4 was just below 74.549 kHz and very faint traces of another station
> on 74.548. Only visible on the QRSS120Screen.
>
> --
> 73
> Hartmut
>
>

dcf77_prn_sidebands.png
Description: PNG image