[Fwd: LF: Receiving]
Thu, 15 Apr 1999 17:55:43 -0400
Hans-Joachim Brandt wrote:
> Hello all,
> concerning LF preselection I do accept Rik's experiences, and I have been
> wondering why he did not try this before using the simple attenuator. It is true
> that near by stations such as DBF39 and DCF39 will not be much attenuated by
> such an input filter (without special poles provided to suppress them). But the
> blocking of the receiver is not caused so much by these near-by signals but by
> the sum of all stations delivered by the aerial, especially the LF broadcast
> stations of course, and this sum can be reduced considerably by such an input
> One problem behind this is that suitable slug tuned coil forms as used in former
> LF/MF radio sets (I still have some which had been sold for bargain prices
> almost 20 years ago to clean the store) have almost completely disappeared from
> the market. These coils can be wound with litz wire to get rather high Q, and
> they can hardly be saturated due to the small mass of the iron powder core.
> Today the first attempt to design LF filter coils is to employ pot cores.
> Obviously pot cores with sufficient volume (especially for the first coil close
> to the aerial) but rather moderate AL value and sufficient air gap are necessary
> for the input filters we need for this purpose.
> 73 Ha-Jo, DJ1ZB
> Rik wrote:
> > Resonant circuits have the advantage to attenuate out-of-the-band signals
> > selective. But to my experience it can have 2 'hooks' :
> > 1. Attenuation of close to the band signals (eg. DBF39 / DCF39) is minimal
> > 2. When using unsuitable ferrites these filtere can even create
> > intermodulation-signals themselves
> > I did some experiments with bandpass filters but at the end a simple
> > attenuator did the best job (with me) and has the advantage that the
> > attenuation can easily be adjusted to optimum ratio between the wanted
> > signal and IM-products.
> > I managed to built a 3-stage 136kHz filter with about 700Hz bandwidth
> > (-3dB), but when I connected it to the big antenna it produced a lot of noise.
> > But I am sure that with the proper ferite materials and filter component
> > dimensions a usefull filter can be constructed.
> > It would be great if those who already constructed good filter would let
> > the others benefit from their experience.
> > 73, Rik
> > Rik Strobbe ON7YD
> > email@example.com
> > Villadreef 14 B-3128 Baal BELGIUM (JO20IX)