LF: Ferrite rod antenna - on-air testing

["James Moritz" <james.moritz@btopenworld.com>]

Dear LF Group,

The QRN level on 136kHz has finally died away, and I was able to record the 
attached spectrogram, showing DK7FC calling CQ around 1755utc, using my 
prototype ferrite rod antenna. On the left of the spectrogram, the rod is 
oriented for N-S reception, and the background noise consists mainly of 
horizontal Loran spectral lines, plus some unidentified, low-level,  local 
"mush". On the right of the spectrogram, Stefan's signal level increases as 
the rod is rotated for E-W reception. Here, the noise level is mainly 
vertical streaks due to sidebands of DCF39's FSK modulation. The black area 
at the right hand edge of the screenshot is where the ferrite rod was 
replaced by a "dummy antenna" inductor, and shows that the internal noise 
level of antenna and preamp is below the band noise level (by around 10dB).
The band noise seen with the ferrite rod is practically the same as seen 
with larger loop antennas under quiet conditions, so this practical test 
does show that a reasonably compact ferrite rod can give sensitivity limited 
only by the external band noise on 136k. In fact, there is some margin for 
making the rod smaller still in this case.
BTW, Stefan was an S4 audible signal - CW would have been a bit difficult 
due to the bursts of noise from DCF39, but a QSO would certainly be 
possible, especially if a quieter frequency were chosen.
Cheers, Jim Moritz
73 de M0BMU

dk7fc_ferrite.jpg
Description: JPEG image