Explaining gaps in ADS-B receiver range

While waiting for the LNA4All preamp (more on that later) I want to find out what causes a gap in my ADS-B receiver range plot. The receiver is running quite consistent over the past month and the plot looks like this:

Receiver range plot form Virtual Radar

It pretty well covers the 200 nm range except on the east where a number of trees limit the view (although i would have expected that this would become better now the leaves have dropped). But i was wondering which causes the 6 to 12 degrees azimuth gap. And i think i found out…

Using the mapping and distance tool from http://www.acscdg.com/ i drew the 6 and 12 degrees azimuth lines.

The 6 and 12 degrees azimuth zoomed out

Since i live south-west of Eindhoven i suspected some tall buildings in the city at 5 km could be the cause. The great news is that the height all structures in the Netherlands have been mapped, and that this data is freely available at http://ahn.maps.arcgis.com/home/

So i had a look around for high buildinges in the suspected direction.

AHN building height map

The light areas show buildings from 40 to 60m high. The long stretched area on the left is building “Strijp-S“, some of the former Philips factories and now serving different purposes like living, culture, business. The rectangle in the middel is the PSV soccer stadium. When zooming in to the mapping and distance plot the 6 and 12 degree lines show again:

6 and 12 degrees azimuth zoomed in

This can hardly be a coincidence! Both lines run left and right from the series of buildings. Or in an aerial picture looking form the north:

Aerial view from the north

Another mystery solved!