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opposition to a signal flow through a component or a line. Overall, the theory of impedance in radio systems is
beyond the scope of this user guide. All you need to know is that the transceiver output, the feedline, and the
antenna all have their own characteristic impedances, and for proper transfer of power, should all be the same.
The problem with these
characteristics is that they must all
match. If they aren't at least close
in value, a mismatch will occur.
For example, if the transmitter
and the feedline are both 50 ohms
and the antenna is 100 ohms, a
portion of the power coming from
the transmitter, which should be
radiated by the antenna, will be
"reflected" back toward the
transmitter. This radiation, which
is developing on the feedline, is
known as standing waves. The
ratio of the peak voltage going to the antenna and the measured peak voltage reflected back to the transmitter is
called the standing wave ratio (SWR).
When the SWR exceeds about 2:1, the HF transmitter will reduce power to reduce the stress on the
components. Reflected power has nowhere to go, so it is turned into heat. This heat can be dissipated by the feedline
or it can be dissipated by the components in the output circuit of the transmitter. Either way, one of the rules of HF
equipment is "heat is bad!" When older transceivers and transmitters were under high SWR
conditions, they would continue operating at the same power level and gradually (or not so
gradually!) damage or destroy the final output tubes.
You could change the characteristic impedance of the transceiver to match the antenna,
but you would have to tear apart the final stages of the transmitter. Modern transceivers have
a connector that is meant to connect coaxial cable feedline. Coaxial cable has a center
conductor that is surrounded by an insulating dielectric material. That material is surrounded by a grounded copper
braid, which is wrapped in an outer insulating sleeve. The output of modern transceivers is rated at 50 to 75 ohms.
The easiest method to get a matching impedance on the feedline is simply to use a type
of coaxial cable that has an impedance of from 50 to 75 ohms. Some of the most frequently
used types of coaxial cable for HF use are RG-8, RG-8X, and RG-58. The last ways to get
the impedance to match up properly are to use an antenna with a characteristic impedance of
50 to 75 ohms and make sure that the antenna is cut to the frequency of operations. The most
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