Message-ID: <4E4EC28A.80708@toya.net.pl>
Date: Fri, 19 Aug 2011 22:07:38 +0200
From: Piotr Mlynarski <sq7mpj@toya.net.pl>
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Subject: Re: LF: Ferrite Receive Antennas
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Stefan Sch=E4fer pisze:
>
> DCF39 is about 500 km distant to you. You should receive it some 10 dB=20
> 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=20
> region and help to find the location of local QRM sources. So once one=20
> has found the quietest place in the garden, a bigger loop may be=20
> instelled there...
> I have some problems with the dynamic range of my soundcard which is=20
> 120 dB in 1 Hz. Not bad but DCF39 comes in at up to 97 dB and if i=20
> want to stay 15 dB above the soundcards noise (in the passband) it=20
> becomes critical.
>
> I saw that there are big differences between different capacitor=20
> types. Replacing a standard foil cap by a styroflex cap increased the=20
> Q from 130 to 480! I took a cheap variable cap (100 pF) with a foil=20
> dielectricum to resonate the antenna and saw that Q decreases with=20
> increasing C of this small cap. The styroflex type is 2.2 nF. Then i=20
> found a 47 pF silver plated varable cap with air dielectricum which=20
> even has a 6mm axis. Now i think about using this cap and some=20
> switches and fixed caps (47 pF, 100 pF, 220 pF) for resonating...
>
> About the winding on my antenna: I stopped optimising after getting=20
> that Q of 486 at 55 turns but actually it could be useful to further=20
> increase the number of turns as long as the Q doesn't decrease to=20
> much. Have to do some tests with various C values and see how the Q=20
> changes as a function of frequency. More turns would result in a=20
> higher output voltage. Furthermore i would get a wider range of=20
> frequency variation by that small 47 pF varicap. The current Z of my=20
> LC circuit is just about 500 Ohm...
>
> Will you further optimise your system and try to receive some DX=20
> 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=20
afternoon/evening  so it definitely needs improvements/rearrangements etc=
..
first of all i must raise the  ferrite from the floor , find more=20
suitable place an so on.. the voltage gain of the buffer/amplifier stage=20
is abt 27 dB
the cap in LC consists of three parallel mica capacitors of nominaly 560=20
pf  each + 500 pf  air variable
i do not have a VNA at hand so in fact no serious measurements have been=20
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/