[AMRadio] 1500 KHz trap for NC-156


John coleman wa5bxo at gmail.com
Sat Aug 9 12:00:07 EDT 2008


Hi Larry
	You are correct in the thinking of a higher Q (more C less L)
however it is very important the lower L factor be made of larger wire with
proper spacing (a high Q type coil).  There are special dimensions and wire
spacing to be considered for a high "Q" coil assemblies.  I won't go into
that here as there are probably more expert coil winders available here on
the list than I am.  But I do know that it makes a difference.  The whole
thing will need to be put in a shielded box else coil assembly may act as an
antenna for the 1500KHZ

As I recall, 100pf and 100uh resonates at around 1590KC but you may want
higher C yet?

Another thing to consider:
You might use an antenna tuner but not the pi-net type with the series coil
as these are low pass circuits.  Instead the seldom seen pi-net with two
coils to ground and a series capacitor making a hi-pass network would be
real good but doing two adjustable coils would be a pain. So the old
fashioned link coupled parallel tank is best for this application.  Perhaps
some of the modern "T" nets might be good also I don't know.  

Perhaps the trap and a tuner together would work well.
Good Luck
John, WA5BXO


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Larry Szendrei
Sent: Friday, August 08, 2008 7:09 PM
To: Glowbugs; AM Reflector
Subject: [AMRadio] 1500 KHz trap for NC-156

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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