|[AMRadio] BC-610 on 160|
knjhanlon at msn.com
Tue Nov 5 13:51:19 EST 2013
If I were trying to attenuate my second harmonic, in this case from a BC-610 operating on 160 but in general for any transmitter with a second harmonic problem, I would use a "Half Wave Filter." I'm looking at my Editors and Engineers Radio Handbook, 15th Edition, page 387 where they are described. The "Half Wave Filter," which was also described in the Nov-Dec 1949 GE Ham News, is simple and "presents the same value of impedance at its input terminals as appears as load across its output terminals." This type of filter will give an attenuation of about 30 dB to the second harmonic, 48 dB to the third harmonic, 60 dB to the fourth harmonic, 67 dB to the fifth harmonic and so on, increasing at a rate of 30 dB per octave.
The schematic of the filter is two pi-sections in cascade, each section in a shielded box with a shield in between each section. The top of each pi section is an inductance, and the side legs are capacitances. For 160 meter and for a 52 ohm characteristic impedance, the capacitors are 1700 picofarads (all four of them) and the coils are 4.2 microhenry. The coil is described as 22 turns of #16 enamel wire, 1" diameter, 2" long. The coil should be checked and adjusted for resonance at the operating frequency when paralleled with one of the capacitors. Transmitting type ceramic capacitors are recommended for high power operation. Silver mica or small ceramic caps are OK for lower power.
I use Half Wave Filters made up for 80, 40, and 20 meters with my Collins 32RA8 to keep its harmonics in check. They seem to work well.
Jim Hanlon, W8KGI
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