Hydrophobic feed window

1. A microwave-range antenna assembly comprising a low noise block converter feedhorn fixed within antenna dish covered by a feed window formed from a thermoplastic polymer containing a hydroscopic-effective amount of a high molecular weight polydialkylsiloxane.

2. An assembly of claim 1 in which the feed window is formed from a propylene polymer composition containing 0.1 to 3 wt. % of an ultra-high molecular weight siloxane having a viscosity of 100,000 to 200,000 cm 2 /sec.

3. An assembly of claim 2 in which the propylene polymer composition contains 0.25 to 2 wt. % siloxane having a viscosity of 150,000 to 200,000 cm 2 /sec.

4. An assembly of claim 3 in which the siloxane is added to the propylene polymer as a masterbatch of siloxane and a polyolefin.

5. A method to protect a low noise block converter feedhorn from weather comprising covering the feedhorn with a feed window formed from a thermoplastic polymer containing a hydroscopic-effective amount of a high molecular weight polydialkylsiloxane.

6. A method of claim 5 in which the feed window is formed from a propylene polymer composition containing 0.1 to 3 wt. % of an ultra-high molecular weight polydimethylsiloxane having a viscosity of 100,000 to 200,000 cm 2 /sec.

7. A method of claim 6 in which the propylene polymer is a block copolymer of propylene and ethylene.

8. A method of claim 7 in which the propylene polymer composition contains 0.25 to 2 wt. % siloxane.

9. A feed window for a low noise block converter feedhorn incorporated into a microwave-range antenna assembly, said feed window formed from a thermoplastic polymer composition containing a hydroscopic-effective amount of an ultra-high molecular weight polydialkylsiloxane.

10. A feed window of claim 9 wherein the thermoplastic polymer is a polyolefin of monomer units containing 2 to 8 carbon atoms.

11. A feed window of claim 9 wherein the thermoplastic polymer is a propylene polymer.

12. A feed window of claim 11 wherein the propylene polymer contains up to 20 wt. % copolymerized ethylene monomer.

13. A feed window of claim 9 wherein the hydroscopic-effective amount of siloxane is between 0.1 and 3 wt. % of the polymer composition.

14. A feed window of claim 9 wherein the hydroscopic-effective amount of siloxane is between 0.25 and 2 wt. % of the polymer composition.

15. A feed window of claim 11 wherein siloxane is added to the propylene polymer as a masterbatch of siloxane and a polypropylene.

16. A feed window of claim 9 wherein the polydialkylsiloxane has a viscosity of 100,000 to 200,000 cm 2 /sec.

17. A feed window of claim 9 formed from a propylene polymer composition containing an ultra-high molecular weight polydimethylsiloxane having a viscosity of 150,000 to 200,000 cm 2 /sec in which the polydialkylsiloxane is incorporated into the propylene polymer composition as a 50:50 masterbatch of polydialkylsiloxane and polypropylene and wherein such masterbatch is incorporated at a level of 0.25 to 2 wt. % in the propylene polymer.

18. A feed window of claim 17 in which the propylene polymer is a block copolymer containing up to 20 wt. % ethylene.