[AMRadio] 1500 KHz trap for NC-156

Larry Szendrei ne1s at neandertech.com
Mon Aug 11 13:27:43 EDT 2008

Last night I had a chance to play some more with this.

Perusing the junquebox yielded a 3-section, 400pF/section broadcast band
air variable and a piece of air-wound coil stock about 2" diameter and
about 2.5" long, #16 tinned wire, with spacing between turns about equal
to the wire diameter. The coil looked about right for an 80M tank coil for
a single 807 or 6146, etc.

I wired them in parallel and put the parallel combination is series with
the antenna lead. MUCH better - I could still null out the 1500Kc
interference, but signals a few 100KHz from 1500Kc were much stronger
compare with the earlier network of 200µH in parallel with 56pF. The new
component values are 1000pF (measured) and 11.26µH (calculated), which
approximates a series reactance of 92 ohms inductive (ignoring the
capacitor's presence) at 1.2MHz and 88 ohms capacitive at 1.8 MHz
(ignoring the inductor's contribution). WBZ at 1.13MHz was, within my
ability to read the S-meter of questionable calibration and with QSB, 2
S-units down with the filter in, compared to what it is with the filter
shorted out. On the 80M band there is no noticeable attenuation from the

I can live with this; I'm calling it good and moving on.

Thanks to all the helpful suggestions and comments.


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