Hi all,
The information from Petr is very interesting, and below I discuss
aperture and noise figure considerations:
Petr Maly wrote:
Hi LowFers.
Too lazy to make quality narrow input filter I tried another thing.
I made a loop ant of a shape of square, 65cm by 65cm. Size mainly
limited by my wife's tolerance. There are two windings on it. Main is
50 turns and the latter is separate 5 turns. Used wire is insulated
copper wire diameter 0.3 mm only . The main one is tuned to
resonance by varible capacitor abt 500 pF.
Hot end is fed to gate of BF245. In collector
(drain) there is the the latter winding in series with trimmer
2200 ohm. The 'polarity' of this secondary winding must tried to
find positive feedback. By the trimmer the feedback can be
easily adjusted so that you can find a point where oscillation begins.
It you set feedback close to this point (not necessary to be
very close) the ant has following behaviour.
It tunes very sharply. It has to be retuned even within 136 kHz
band. When it is carefully tuned to a certain fq (136 kHz)
the fq 3 kHz distant (139 kHz) seems to have attenuation about
15 dB! I was surprised that the band is actually empty, full of
smooth ground noise, which is different from what I experienced before
on this band. Also, galloping horses can be clearly heard on 100 kHz.
To tell the whole story I live in a block of flats so that there is
a lot of TV sets and other appliances in vicinity.
I tried to listen several times during this wekend but I have not heard any
ham-looking signals. My QTH is Hradec Kralove which is about 100 km
east from capital Prague. Locator is JO70WE. Who is closest to me?
Is he willing to make a beacon for me for few minutes? Ones I
saw 137 kHz in DX-Cluster too, but not this weekend. Are there any
prefered times/fqs to try to listen on the band?
73! Petr, OK1FIG
In New Zealand some LowFers have used tuned (frame) loops for receive
only. The question was how small the area (loop aperture) could be to
still be dominated by external noise (atmospheric QRN) rather than noise
from within the loop and pre-amplifier. This depends winding technique,
and whether shielding is placed around the windings (a high loaded Q is
very desirable, but we also use SSB here in New Zealand, so a Q in the
range of 50 to 100 is about as high as we seek). We operate in the 181
to 182 kHz region. The experimental answer to loop area that can just
detect external QRN is about 1.5 metres per leg. Using a balanced
winding with a pushpull pre-amplifier and RF grounded centre tap seems
to be satisfactory for controlling common mode pickup (no shielding
needed, and shielding does lower the Q). I have made smaller area tuned
loops with a view to fitting them under the house (it would be very
convenient), but they have turned out to not receive weak LF signals
that I can receive via my active whip sited in my backyard.
Petr is using some positive feedback (as in Q multiplier or regenerative
techniques) to improve the apparent Q of his 0.65 x 0.65 metre loop.
While it is clear that the selectivity is greatly improved, I am
uncertain as to how the positive feedback influences the net noise
figure. I doubt if positive feedback (equivalent to placing negative
resistance in the loop) can increase the effective aperture of a loop,
and there may even be some additional noise injected by feedback. While
I wish Petr all the best with being able to receive amateur LF signals,
the technique he is trying involves some questions about noise figure of
"Q multipliers" used for LF receiving. Other readers may wish to
comment.
Regards,
Bob ZL2CA
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