[AMRadio] 1500 KHz trap for NC-156


Larry Szendrei ne1s at neandertech.com
Fri Aug 8 20:08:33 EDT 2008


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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