[BC] FeedHorn Bug Covers

[Mark Humphrey]

A potential problem with a styrofoam plug might be its tendency to
absorb water over time.

In the early days of AT&T microwave, some repeater stations used
"delay lens" antennas made of a large styrofoam block with inlaid
metal strips in front of a horn.   The composite material was thicker
in the center than at the edges, causing it to focus the RF energy
into a beam, like a convex glass lens focuses light.   The "Bell
System Practices" sheet for the delay lens antenna mentions that a
heater-blower was necessary in humid areas to keep moisture from
building up in the foam, thus reducing gain:

http://long-lines.net/tech-equip/radio/BSP402420100/1.html

This antenna design (developed by Bell Labs, of course) is interesting
in theory, but it apparently proved problematic in the field, as most
were later replaced with conventional parabolic antennas or
horn-reflectors.

Mark