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Ground wave noises arriving from a significant distance are vertically polarized. The path
along the earth "filters out" and removes any horizontally polarized signals. Horizontal
electric field components are "short circuited" by the conductive earth as they propagate and
are eliminated.
With the exception of ground wave-propagated noise, receiving antenna polarization effects are not
predictable. It is possible vertically polarized antennas may be quieter than horizontally polarized
antennas. The opposite is also true.
It may be difficult to remove noise with any device when:
Noise and desired signals come from the same direction and elevation angle
Both antennas don’t hear the same noise
The noise source is moving around, or noise sources are coming from several directions at
the same time
Removing Noise
The
NCC-1
generally works best when both antennas have similar patterns, polarization, and
Signal-to-Noise ratios. For the most effective nulling of noise, the antennas on both the A and B
inputs must hear the same unwanted noise and should have similar polarization. You may have to
experiment to find the best antenna, but successful operation more commonly occurs with similar
antennas.
Removal of distant interference:
Close element spacing is more desirable. Close spacing
produces a single null that is wider and more stable. Spacing of 1/4-wavelength or less is most
desirable when nulling distant interference or peaking distant signals. Spacing larger than 1/4-
wavelength can, at your operating frequency, cause multiple nulls in the patterns.
Removal of a local noise source:
Best performance occurs when the noise (B INPUT) antenna
"hears" the noise much louder than it hears desired signals. We want the noise antenna to pick up
the largest amount of noise possible, so it should be located as close to the noise source as possible.
In this case the polarization is unimportant; whatever polarization hears the noise best. The spacing
between antennas can be any convenient distance within one wavelength.
