Tuesday, July 19, 2016

I have two low pass filters. If I put them both in series ...

Q George, AD5CQ, asks, “I have two low pass filters. If I put them both in series with my transmission line, will I get more reduction of harmonics or would the input losses be excessive?”

A The answers are “yes” and “maybe” respectively.

The amount of loss would be double that of a single filter, but depending upon the filter design, it may be acceptable. I have seen filters with insertion losses as low as 0.25 dB and a 0.5-dB total loss would be quite acceptable to most folks. The best way to check is to put a power meter in line after the filter. A 1-dB loss is about 21 W out of 100.

If you don’t have an external power meter, you can check the difference in receive signal strength. Find a strong steady signal like WWV or W1AW and try switching the filters in and out of line to see how much the S-meter changes.

A 1-dB change would be just about noticeable in terms of meter movement (it’s about 1/6 of an S-unit by the old Collins standard—not that any modern rigs follow the standard, but it should be in the ballpark).

From QST June 2001