of the center conductor is increased. This is the best transmission
line to use, because it has the least power loss.
BASIC FEEDER SYSTEMS
Introduction.
The transmission line transfers the RF power from the
transmitter to the antenna.
There are two general types of
transmission lines: resonant (tuned) and nonresonant (untuned).
(a) Resonant feeder line is the same length as the antenna.
It
is rarely used in tactical applications.
(b) A nonresonant transmission line is one that has an SWR of
less than 1.5.
In order to achieve this, the impedance of the
antenna and the transmission line must match. An antenna tuning unit
is used in some applications to match the transmitter to the line and
antenna.
(1) Single-wire feed.
A single wire can be used as a
nonresonant feed line.
Because the impedance of a single-wire feed
is 500 to 600 ohms, a point on the half-wave antenna must be selected
that will match the impedance of the line.
The antenna impedance
varies from 2500 at the end to about 73 ohms in the center. A point
14 percent from the center of the antenna will provide the 500 to 600
ohms required (A of figure 31). To reduce radiation or coupling make
sure the single-wire feed is at right angles to the antenna. A good
electrical ground connection is also required to provide a return
path to the transmitter.
(2) Twisted-pair feed.
WD-1 can be used in an emergency to
provide a feed line from the transmitter to the antenna (B of figure
31). The impedance requirement of a twisted pair is 70 to 80 ohms.
The center of the half wave antenna provides that impedance.
This
type of feed should be used only as a last resort because of the very
high power loss.
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