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67 GHz dipole antenna simulation problem

Question asked by lee@utdallas on Mar 2, 2016
Latest reply on May 11, 2016 by WaelAbdullah
Hi Guys,
I am a grad student in UTD, currently, I am using Momentum in ADS2013 to simulate 67 GHz dipole antenna, it is on a substrate with Er = 3.75, loss tangent = 0.018, thickness = 127 um, and top and bottom layer is 1/3 oz copper.
I started it with a really simple structure, with two arms on the top layer, NO copper on the bottom layer, and each arm length = 780 um, width = 150 um, center gap = 100 um, edge, differential, TML and 50 ohm port. 
My problem is that Gain is too LOW, it is less than -1 dB, and Directivity is a little greater than 3 dB, S11 @ 67 GHz is about -18 dB.
What's more, I found two interesting tings:
1. If I change my 12 um copper to sheet, the gain will be increased to 1 dB.
2. And with 1's setting, if changing the substrate to air, then the gain and directivity will match. 

Need help !!

Thank you

Edited by: Lee@utdallas on Mar 3, 2016 12:59 AM