[Lowfer] NAVTEX 518 QRM

[Garry Hess]

Just got home and looked at my SeaTTY 518 kHz decodes, software left 
running while I was away. See a lot of messages on the QRM issue but 
nothing definitive. I looked at 1830Z and ruled out $04P, XLJ, Thunder 
Bay, ON as the interferer as I saw it come up briefly, too weak to 
decode, probably sending a "no messages" message. JB earlier ruled out 
$04H, VBR, Warton, ON as the interferer.

At 1830Z the interferer was at the mark frequency and not real strong 
because it was daylight. Now it's dark and the line is strong, but as 
Todd noted it has moved to the space frequency.

At 2358:42Z I decoded message AE36 from $04A, NMA, Miami, usually a very 
strong station and one that transmits well past it's 10-minute window 
(which starts at 0000Z). What happened next isn't entirely clear but 
something very strong sent a string of null carriers in two long bursts 
before going QRT around 0032:20. That left the strong space interferer 
active along with a weaker Navtex signal. The Navtex signal went QRT 
around 0039:10Z, leaving just the space frequency interferer. My 
schedule program (NAVTEXvw2, 2009/12/03) shows nothing for region IV at 
that time but I think it was Miami going off. It shows $04E, NME, 
Charleston, SC to come on at 0040Z and in fact I began decoding 
messages from NME at 0042:20Z because it was at times comparable in 
strength to the interferer. Then $04H, Warton came on and was even 
stronger so it captured and message HA69 decoded at 0111:35Z.

After a lot of listening and screen captures the only thing I know is 
that the interference can be at either the mark or space frequency and 
it's still there, if not 24/7 then darn close. Wish I had my 10' 
shielded loop still up. Hopefully someone with direction nulling 
capability will take a look and suggest a heading.

73, Garry, K3SIW, EN52ta, Elgin, IL