|[AMRadio] 1500 KHz trap for NC-156|
wa5bxo at gmail.com
Sat Aug 9 12:00:07 EDT 2008
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.
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
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
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
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,
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