Living approx. 120 km SE of a LORAN-C transmitter (island of SYLT, North Germany, Danish border) LF receiving here is troubled seriously by rattling sidebands from that site. Main frequency of LORAN-C is centred at 100 kHz. The signal strength at 100 kHz here is -60 dBm at 100 Hz of bandwidth measured with a commercial selective level meter (MV61). At 135 kHz the sideband levels are at -105 dBm in a 100 Hz window on my tuned 17m MARCONI antenna.

The following overview FFT graphic shows the whole region from 135.0 to 137.8 kHz. It was made with a Drake R4c (SSB filter) behind my homebrewed LF/VLF converter. It can be observed that the noise background may have some "structure", it does not look like white noise. Indeed, this "noise" is part of the LORAN-C sidebands which can be detected up to approx. 145 kHz.

(Recorded 9th of October 1999 at 09:09 UTC)

To demonstrate how dense those lines appear I made 14 high resolution spectral scans with a modified version of SPECGRAM-2 for UNIX workstations. Running under LINUX on an i486-PC with Soundblaster card the parameters of the FFT were chosen as follows:
- 4000 samplings per second
- 8192 samplings per FFT window
- No integration or running average
- Spectral resolution of approx. 0.49 Hz.

Receiver: Homemade converter in front of my old Drake R4b.
Antenna: T-Maconi.
Date: July, 31st 1999, 08:00-09:00 UTC.

Checking the results with a selective level meter (MV61) I could not observe any differences. The time spans of each recording were approx. 60 seconds.

Gluing these 14 single graphics together the accuracy can be estimated within 5 Hz.  The graphic can be used for comparison, theoretical purposes or in practice for those hams who do not observe any LORAN-C sidebands but want to have Visible Slow CW contacts with stations at North Germany, South Denmark or the region around South England and NW of French.

(No, this worm is not a preview of my planned new tie!)