First steps on the 504kHz band

Noticing some WSPR activity on 504 Khz band and having 0 experience of bands
below 1.8Mhz I just tuned-in to see. Just using my untuned vertical and
despite my very noisy location I received the SM6BHZ beacon immediately.
Reminded me when I first heard W5UN on 2m EME (just using 4x9 home-made yagis
made from Al recovered from the bin...)... magic !

Knowing the improvement brought by the RX-loop on 160m, I decided to try
quickly something to see if the 160m loop could be tuned down to 504kHz.
http://f6irf.blogspot.com/2008/05/multi-turns-magnetic-loop-for-top-band...
A quick calculation, indicated that I needed around 1nF capa to get the loop
to resonnate at 504. in fact 1nF was not enough, the final value was found by
experimentation (just using the band-noise to find the resonnant freq) to be
around 1390pF (2x470pF + 2x220pF silver mica caps + a small 40pF adjustable
cap), which means that my 6 turns loop has an inductance of approx 72uH (and
not 100 as first estimated !). At this frequency, the bandwidth is extremely
limited, just approx +/- 1.5kHz at -3dB.

I then discovered, that the IC756pro2, did not have sufficient sensitivity to
use the loop on 504kHz (as the internal preamp does not work below 1.6Mhz).
Took my IC7000, and tried... yes ! the 20dB internal preamp works at 504kHz;
The gain is sufficient to get about S1 to S6 noise with the 2.4kHz filter,
depending on the loop orientation.

The rest is here: http://tinyurl.com/504khz-31aug09
As you can see the signal variation is important along the night. M0BMU
signal peaks at -4dB at 00:26 roughly 1 hour after the middle of the night... Assume that
it is the normal propagation behavior (you have to wait for 6 hours after sunset
(local SS at 18:16Z)to get the D-layer to reach the minimum of ionization). The signal
completly disappears just a few minutes before sunrise (local SR at 04:56Z).
Taking into account the distance (800kms), the loop orientation (90deg. away
fm M0MDU - that's the min of my power-line noise) and the ERP, groundwave can
totaly be excluded...

Just a question for the specialists:
What is the continuous carrier on 504.000 - I would guess that it is the H2
of a LW 252Khz transmitter ? There are 2 on this frequency: RTE (Ireland) and
Radio-Alger (Algeria). Which one ? (or maybe both...)
(the signal is not a local birdie, as the signal completly fades out after
sunrise)

Once again, a lot of fun ! My admiration goes to the few "long-waves freaks"
who are investing time, money and energy to put up beacons on a band where no
commercial equipement is available and "efficient enough" TX-antennas not
that obvious to setup...

Patrick