station and your antenna, and a reflected path which causes
part of the signal to arrive slightly later in time. Such
signals usually degrade stereo reception. Typical multi-
path symptoms include poor channel separation, distortion,
noise or buzzing in the program and erratic operation
of a tuner's stereo indicator.
Multipath free reception is shown in Figure 13. The trace
is formed by a combination of signal strength and modula-
tion (or loudness of the program material). Since signal
strength raises the pattern in a vertical direction, the
higher the trace, the stronger the station. A powerful
local station can be expected to bring the trace to about
½- or ¾-inches below the top of the screen. Weak stations
produce lower heights on the screen.
F I G . 13 MULTIPATH F R E E DISPLAY
Program modulation causes the trace to expand in a
horizontal direction. The pattern, therefore, widens in
step with the audio intensity of the program material.
Figure 14 shows a display having multipath. The dips
and peaks are the result of a reflected signal arriving at
the tuner antenna slightly later than the direct signal.
F I G . 14 MULTIPATH INTERFERENCE
By combining displays of strength and modulation,
the MPI 4 will not only show multipath, but is, in addition,
an accurate tuning indicator Adjust the FM tuning dial
until the trace reaches its highest point, while also being
centered horizontally on the screen. Centering the trace
places the signal at the center of the tuner's IF response
curve, the most desirable point. An example of a detuned
station is shown in Fig. 15: note how the curve is shifted
to the left on the screen. Correct the tuning by adjusting
the tuners tuning knob to center the trace.
FIG. 15 DETUNED STATION
Multipath produces several visible symptons on the
screen. Peaks or valleys may appear on the curve, as shown
in Fig. 16. Note, too, how the entire display is shifted
FIG. 16 MULTIPATH & D E T U N E D
to the left which indicates a combination of both detuning
and multipath reception. In general, a trace that is not
smooth suffers from multipath contamination. In severe
cases, multipath causes considerable fuzziness or blurring
of the overall display.
Multipath is most often overcome by changes in antenna
direction. For outdoor antennas with a rotator, aiming
the antenna in a different direction usually helps. Indoor
dipoles or similar antenna may also be repositioned. In
all cases, best results are clearly indicated on the MPI 4
screen.
Make sure that the MPI 4 is property set up.
1. Turn the POWER LEVEL switch to the PREAMP
position.
2. Set the GAIN control fully counterclockwise (but
not to OFF), and the INTENSITY control fully clock-
wise.
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