TM 11-5840-281-35/1
Figure 2-13. Antenna AS-1291/TPN-8 pattern.
are shown at all of the important points. Starting at the
physically perpendicular to each other, they are in-
horn end, note that the horn is rectangular in shape
phase.
(view A1). The throat of the horn is in the form of a
(4) In a rectangular waveguide, a signal whose
square (view A2), with a further transition to a circular
axis of polarization is parallel to the short sides (view Al)
opening (view A3). Following this is a movable rotating
is closer to cutoff than a signal whose axis of
section free to rotate about its longitudinal axis The end
polarization is parallel to the long sides. Thus, if two
nearest the feedhorn throat is circular (view A3) with a
perpendicular signals are fed into a rectangular
transition to rectangular waveguide dimensions (view
waveguide, the signal whose axis is parallel to the long
A4). Both ends of this rotating section are choke joints.
side of the waveguide will travel faster than the signal
Following the movable rotating section is a rectangular,
whose axis is parallel to the short side.
1/-wave section of wave- guide (view AS), and the
stationary feedhorn-associated waveguide.
(5) Since the feedhorn used is rectangular in
shape, when two perpendicular in-phase signals are fed
(2) Views A1 through A5 illustrate energy
into it (at the throat), the signal whose axis is parallel to
traveling through all of the transitions and out of the horn
the long side travels faster than the signal whose axis is
without any shift in axis; however, if the rotating section
parallel to the short side.
is turned to 45 degrees (view B4), the signal travels with
The length of the feedhorn is such that one signal
this new orientation of axis through the circular section
arrives at the feedhorn 90 degrees ahead of the other
(view B3), and arrives at the transition into the square
signal. Thus, the two mutually perpendicular signals are
section (view B2) with the same orientation. At this point,
radiated from the mouth of the feedhorn 90 degrees out-
the signal is separated into its two components at right
of-phase with each other.
angles to each other (view B2).
The effect is exactly the same as if two signals were
radiated 90 degrees out-of-phase from two mutually
(3) To produce circular polarization, two lin-
perpendicular antennas.
early-polarized signals are radiated 90 degrees out-of-
phase with each other. A square waveguide will not
(6) Selection of linear or circular polarization is
transmit a signal which is diagonal to it; however, it will
accomplished by placing ANT POL switch S606 on the
transmit the vertical and horizontal components of the
front panel of Control-Indicator C-6988/TPN-18 to the
signal (view B2). Although the two signals at the throat
of the feedhorn are
K4019 in Remote Switching Control C-6389/TPN-18
simultane-
2-14
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