July/August 2018 21
lines and antenna systems will explain how these work.
• Waveguides: Sections of waveguides with internal
baffles or tuning screws can be very effective filters at
• Antennas: Not only can antennas act as frequency-dependent filters with preferred frequencies, but they
can act as spatial filters too!
Designing Filters with ELSIE
In this column, I’ve presented some filter basics to give
you a better understanding of how filters behave and are
specified. It wouldn’t be very nice to leave you hanging
without some practice, however. So, in the next column,
we’ll experiment with a very capable suite of filter design
ELSIE (a play on “LC”) is provided by Tonne Software
(Jim Tonne W4ENE is a long-time contributor to the ARRL
Handbook and other ham radio endeavors), along with
several filter design utilities.
You’ll find that filters are well within your ability to
design and build! NV
Why are Analog Filters Still Necessary?
While you’ll find digital filters taking over more and more filtering chores, analog filters are still the only
practical choice for many roles. Obviously, at high power levels — such as for transmitters and duplexers — low-loss inductors (L) and capacitors (C) are required. The selection of appropriate components is an art in itself for
Less obviously, analog filters are also required at very low signal levels where a digital circuit may not have
enough resolution in bits to represent the signal, and at very high frequencies where the samples can’t be taken
fast enough. Digital filters also require a fair amount of supporting electronics and might be too expensive.
Analog filters will be with us for quite a while!
PRACTICAL TECHNOLOGY FROM THE HAM WORLD