Two 500 KC Low-Pass Filter Designs
W5JGV
Here are two possible designs for low-pass filters which may be used with our experimental transmitters. Construction of either design should be fairly easy, and at the proposed transmitter power output level of 100 watts, either air-core or toroid coils should probably work equally well.
DESIGN # 1 - 5-POLE CHEBYSHEV



This filter is designed to have an insertion loss of slightly less than 1 dB at 510 KHz. The insertion loss increases by an additional 0.1 dB at 550 KHz, where the response begins to roll off..
The second and third harmonics should be reduced by about 40 dB and 65 dB down, respectively.
The curve below shows the filter transmission loss in black, and the return loss is shown in red.

DESIGN # 2 - 5-POLE ELLIPTIC



This filter has more components, but is designed so that there is a deep notch close to the second and third harmonic frequencies.
This design has slightly more insertion loss, about 1.1 dB. It also starts rolling off slightly sooner, at 525 KHz instead of 550 KHz.
In this design, the second and third harmonics will be attenuated by about 67 dB and 95 dB, respectively.
The curve below shows the filter transmission loss in black, and the return loss is shown in red.



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