At 15:44 14/04/99 +0200, DJ1ZB wrote:
If a receiver needs some input attenuation at LF on large aerials, as
reported
by ON7YD on his TS440, for instance, the better way should be to realize
this
"attenuation" by the loss of a simple coil filter using one to four resonant
circuits. But such an arrangement would need less resistive attenuation to
stop
the overload of the receiver, therefore even some additional
preamplification
(and selectivity) might be possible, with resonant circuits carefully
arranged
ahead and behind amplification so that in an empty band the S-meter hangs
around "S1". Then it should even be possible to receive the commercial LF
stations with an indication of S9+30dB if the receiver has sufficient AGC
range.
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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