Stefan Schäfer pisze:
DCF39 is about 500 km distant to you. You should receive it some 10 dB
better as you did. This is probably caused by local QRM.
What is the gain of your amp stage?
And what is the Q of the antenna's LC circuit?
BTW this small antenna is good to find a quiet place in the local
region and help to find the location of local QRM sources. So once one
has found the quietest place in the garden, a bigger loop may be
instelled there...
I have some problems with the dynamic range of my soundcard which is
120 dB in 1 Hz. Not bad but DCF39 comes in at up to 97 dB and if i
want to stay 15 dB above the soundcards noise (in the passband) it
becomes critical.
I saw that there are big differences between different capacitor
types. Replacing a standard foil cap by a styroflex cap increased the
Q from 130 to 480! I took a cheap variable cap (100 pF) with a foil
dielectricum to resonate the antenna and saw that Q decreases with
increasing C of this small cap. The styroflex type is 2.2 nF. Then i
found a 47 pF silver plated varable cap with air dielectricum which
even has a 6mm axis. Now i think about using this cap and some
switches and fixed caps (47 pF, 100 pF, 220 pF) for resonating...
About the winding on my antenna: I stopped optimising after getting
that Q of 486 at 55 turns but actually it could be useful to further
increase the number of turns as long as the Q doesn't decrease to
much. Have to do some tests with various C values and see how the Q
changes as a function of frequency. More turns would result in a
higher output voltage. Furthermore i would get a wider range of
frequency variation by that small 47 pF varicap. The current Z of my
LC circuit is just about 500 Ohm...
Will you further optimise your system and try to receive some DX
signals with it or was it just a test? :-)
73, Stefan/DK7FC
Hi Stefan, LF
the rx setup mentioned in my previous mail has been built yesterday
afternoon/evening so it definitely needs improvements/rearrangements etc..
first of all i must raise the ferrite from the floor , find more
suitable place an so on.. the voltage gain of the buffer/amplifier stage
is abt 27 dB
the cap in LC consists of three parallel mica capacitors of nominaly 560
pf each + 500 pf air variable
i do not have a VNA at hand so in fact no serious measurements have been
made. Of course, i will make further experiments
and if i catch any distant signal, sure, will announce here.. :)
73, Piotr, sq7mpj
qth: Lodz/jo91rs/
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