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STRIPLINE FILTERS

At UHF and above, normal filter components like capacitors and inductors would
become too small physically to be stable. From UHF and beyond, a common filter
used inside communications equipment is the stripline. I am not going to go into the
operation of these filters here, as their principle of operation requires some
knowledge of transmission line theory, as this is how these filters operate. Suffice to
know for now that ordinary capacitors and inductors cause problems in filter
construction for UHF and above and an alternative is the stripline filter, with its
operation to be covered in the chapter on transmission lines.

See chapter 38 (Transmission Lines) for more information on how stripline filters
work. Striplines are covered in Transmission Lines because they use transmission
line principles for their operation.

RADIO & TV BROADCAST FILTERS

Figure 32-17 A highpass filter for a television receiver

A highpass filter suitable for a television receiver will cure the majority of interference
problems between an HF (0-30MHz) radio station and a TV set which is being
overloaded. Good filters of this type can be expensive- read later how a very cheap
transmission line notch will often quickly, simply and cheaply do the same job.

In the chapter on transmission lines, we will learn how to use a simple length of
ribbon or coaxial cable to make a quarter wave transmission line notch (bandstop).
Such a notch can be used to reduce the level of a nearby transmitter at a receiver
by easily 100 times (20dB). The vast majority of overload problems, from radio
frequencies to domestic RF receivers, can be cured using such a notch filter.

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