Hello all,
I'm working on putting an AM station together for a friend, consisting
of a Viking II and NC-156, both quite cosmetically-challenged, but
complete.
I did a bunch of work to bring them back to operation, including
significant audio improvements in the Viking.
However, the IF in the NC-156 is 1500 KHz, and as you might expect there
is bleed-through of broadcast stations on or near that frequency,
especially on the 160M band, and especially at night, when there is
sky-wave propagation there. The undesired signals are so strong that it
makes operation on 160M totally impossible.
I first tried a series-resonant circuit from ANT to GND, and it was
totally ineffective, probably because the source impedance of the
antenna is low. Then I reconfigured my L and C as a parallel resonant
circuit in series with the ANT at the receiver, and I was able to
totally eliminate the 1500 KHz +/- interference. But, 160M signals are
highly attenuated also (although S/N, if you will, where N is the the
1500 KHz interference, is much better). Also WBZ on 1.3 MHz is very
noticibly attenuated.
My trap consists of a 140pf air trimmer at ~40% mesh (= ~56 pF) in
parallel with 200µH. I'm thinking a higher Q circuit (more C and less L)
will sharpen the peak and I'll have less attenuation 300 KHz away from
the resonant frequency of the trap.
Or is there a better way to eliminate the 1500 KHz QRM?
73 es Tnx,
-Larry/NE1S
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