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
filter.
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 protected]
Villadreef 14 B-3128 Baal BELGIUM (JO20IX)
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