Topband: BC Band Energy Reject Filter

Mon Jan 17 15:18:23 EST 2005

There was some discussion awhile back on the BC band
reject filter that has been described in the ARRL
handbook for many issues. It's pretty well hidden in
at least in the 1988 issue. It's under "HF Radio
Equipment" and it doesn't show up at all in the index
under "filters". I recently built one of these out of
curiosity to see how it performed. It ended up pretty
much like the response curve in the handbook. My
measurements were taken with a calibrated HP 8753A
network analyzer and S-Parameter test set so I think
they are pretty accurate.

Insertion Loss:

@ 1.800 Mhz  =  0.52 dB
@ 1.850 Mhz  =  0.40 dB
2.0 thru 30 Mhz  =  0.1 dB or less

In the BC BAND reject region (band edges)
Insertion loss:

@ 550 Khz  =  63 dB
@ 1600 Khz  =  9.0 dB

The three deep notches in the BC band were:

@ 651 Khz  =  78 dB IL
@ 1110 Khz  =  88 dB IL
@ 1340 Khz  =  66 dB IL

These can obviously be moved around by "tuning" the
component values. Also, you can see that with an
insertion loss of only 9.0 dB at 1600 Khz, a nearby
high power AM station rear the high end of the BC band
could still cause front end overload to the receiver
even with the filter installed. 

It was an interesting project on a lousy WX weekend!

73, Stew K3ND

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