GPS satellites are placed on six different orbital planes, which are angled at 55 degrees from the equator. Therefore, satellites don't pass over the North or South poles and that is why no satellites are recorded by SkyView™ on those dark areas.
GNSS satellites are placed on different orbital planes, which are angled between 55° and 65° from the equator. Therefore, satellites don't pass directly over the North or South poles, which creates the dark (no satellites) areas on the north or south parts of the polar graphs. Keep in mind that the Earth is also rotating and that also affects the shape of the dark area.
The position of the "no satellites" zone depends on the location (latitude) of the antenna being tested. GLONASS satellites produce a smaller "crown" than GPS, BDS (BeiDou) and Galileo. The simplified diagrams below show a graphical explanation for the dark zones, for three different antenna locations.
SkyView™ users checking antenna reception patterns and quality on the Northern Hemisphere, will see the no satellite zone at the top of the polar graph (north). The example below is from Fremont, California USA (Latitude: 37° north).
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For GNSS antennas located near the Equator, you will see smaller No Satellite zones on the north and south parts of their polar graphs. The sample below, from Colombia (Latitude: 11° north), shows a bit of the southern "dark crown" at the bottom.
Users checking antenna reception patterns and quality on the Southern Hemisphere, will see the no satellite zone at the bottom of the polar graph (south). The example below is from Santiago, Chile (Latitude: 33° south).
Users running SkyView antenna coverage tests at higher latitudes (closer to either pole) would see a larger "no satellite" circle closer to the center of the polar graph (90°, zenith).