OK1RR DX & Contesting Page

Ham Radio web site dedicated to CW DX operating and contesting

Reject unwanted MW AM signals from your RX Front End

A nearby broadcast AM station in the Medium Wave band can sometimes cause interference to HF receivers over a broad range of frequencies. A band-stop filter can reject the unwanted frequencies and keep them out of your receiver. This design provides a high attenuation between 457 and 1792 kHz with negligible affect to the 160 m band.


Elliptic MW Band-Stop filter schematic.
(n = 1000 pF, ie. 1,2n = 1200 pF, 2,2n = 2200 pF)

 


PCB layout, size 38 x 20 mm.
(Click to enlarge, this image can be used also for the PCB fabrication)

 


Parts placement.

 


Frequency response. Cutoff frequencies are at 457 and 1792 kHz,
minimum attenuation is 65 dB. Input and output impedance is 50 ohms.

The component values were chosen from the standard values widely available. This variant was built with SMCC axial chokes as inductors, however the Q factor is not very high (4.7 uH has Q of 70 on 8 MHz, 10 uH has Q of 80 on the same frequency and the 33 uH SMCC coil has a Q as low as 55 on 2.5 MHz). If T68-2 (red) toroids used, you can get a Q of 200 and the attenuation is better of some 10 dB.

Inductors wound on T68-2 toroids

Inductance
(uH)
Turns Wire length
cm inches
4.7 29 63.4 25.0
10 42 90.2 35.5
33 76 159.7 62.9