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Every picture I can find of the E-3 has a blue/black radar dome on top with a fairly thick white stripe down the middle but I can't figure out why. Is it related to the position of the radar unit inside perhaps? why is it marked like this?

An example E-3 aircraft, credit: wikipedia

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    $\begingroup$ I couldn't find a specific reason why they used different material colors, but this AWACS Surveillance Radar brochure from Northrop Grumman describes the features of the radar system including the rotodome. The black parts are the electromagnetically transparent radome, and the white parts house the electronics. $\endgroup$ – Greg Hewgill Oct 6 '16 at 19:54
  • $\begingroup$ @greghewgill you should put that as an answer. $\endgroup$ – Nij Oct 6 '16 at 22:19
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The white strip is the metallic part which houses the electronics and serves as support structure for the antenna. The black part is basically a composite (fiberglass) shell, which covers the antenna. According to RAF:

The radar is actually the white part of the disc, the black being there to balance and streamline the radar as it rotates

According to this page:

Each AWACS 707 plane has two fiber glass radomes, joined by a 6 ft. (1.82 m) wide metal spine to form a 30 ft. (9.14 m) diameter radome.

This can be seen in this Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome.

Rotodome

The Northrop Grumman AN/APY-1/2 AWACS multi-mode radar inside the E-3D rotodome. (Credit: Scott Carson. Twitter.). Image taken from aviation.stackexchange.com

The fiberglass shell is painted over in black as black was one color that did not require metallic particles in its paint composition (which would interfere with radar waves) as discussed in another question in aviation.stackexchange.

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    $\begingroup$ and as a beneficial side effect, you can now see when it's rotating which can be more than beneficial if you're a maintenance guy crawling around on top of the aircraft on the ground (yes, the thing can rotate without the radar active). $\endgroup$ – jwenting Oct 7 '16 at 4:32
  • $\begingroup$ awesome answer! $\endgroup$ – dalearn Oct 7 '16 at 12:09
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During aircrew briefings for operations with AWACS, it was stated that aircraft in the vicinity of an AWACS aircraft can determine that the radar is operational if the dome is rotating, easily seen with the white stripe. Not a good idea to fly too close to an AWACS with its' powerful radar emitting EMF. Any material component can be coated in any color material so the dome could easily be all white or all black; even red or blue.

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