[Fwd: LF: Q-ed loop]

[Andre' Kesteloot]

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.
>
> 73! Petr, OK1FIG