September/October 2018 17
low frequencies, it doesn’t reach our
required attenuation of 40 dB until 1.25
MHz, which is well inside the BC band.
This is a consequence of selecting the
different filter family, requiring extra
components to construct. Sometimes
you just have to compromise!
Filter Wrap-up
I hope you enjoyed this short tour
through filters and filter design. Filters
are one of the most important and
widely-used circuits in all of radio at any
frequency. Understanding how they
are specified and used will make you a
better electronics designer, whether you
build your own or simply buy them from
a vendor.
You’ll find a lot more information about filters for all
frequencies in the various amateur radio publications, such
as the ARRL Handbook and for VHF/UHF/microwaves in
the excellent book from the Radio Society of Great Britain,
The International Microwave Handbook. Many hams who
have gone on to engineering careers got their start with
these two books.
Dive in. The water’s fine! NV
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
Receiving versus Transmitting Filters
Receiving filters only have to handle signal levels of a few volts at most.
Signal powers are measured in milliwatts. If the small components used have a
bit of loss, that usually doesn’t have a big effect unless the filter is intended to
Transmitting filters, on the other hand, require components that can handle
hundreds (or thousands) of volts and high currents of several (or dozens of)
amps. The skin effect of an inductor’s wire — constraining current flow to a thin
layer along a conductor’s surface — becomes a significant concern because
it raises the resistance of the component. Dielectric loss in a capacitor has the
same effect. In both cases, the extra resistance dissipates power.
Before building a filter that will be connected to a transmitter output,
study filters made for that frequency and power level to learn what types of
components are used. Also study how the filter is constructed and whether the
components require cooling or special orientation to avoid coupling between
them.
