Topband: High Power Filters

[George Cutsogeorge]

There is a very simple, inexpensive way to build high power, low pass
filters.  I described this method in the Nov/Dec 1998 NCJ in my article
titled "High Power Harmonic Filters".  Basically, you design a low pass
filter using the usual tables, Butterworth, Tchebyschev, etc., and then
substitute coaxial cables for the capacitors.  If the cutoff frequency of
the filter is moved about a bit, the coax capacitors can be set to lengths
that null the harmonics of the transmit frequency.  There is a design in my
article for a 5 element filter for 7 MHz which has 50 dB rejection on 20, 15
and 10 meters.  It consists of three coils and a few feet of RG 213.  This
method can be used for any of the amateur bands.

There is also a detailed design of this filter shown in my book, "Managing 
Interstation Interference."

If more attenuation of harmonics is required, stubs can be used.  A single
stub will give 25 to 30 dB rejection.  A double set gives 2x a single plus 6
dB, or 56 to 66 dB and triple sets approach 100 dB.

George, W2VJN