Isolation Enhancement in Microstrip Antenna Arrays 
Slotted Meander-Line Resonator (SMLR) is implemented by creating defect in the microstrip structure. This resonator is designed to block the surface current at the resonant frequency of the two patch antennas coupled along H-plane operating at 3.6 GHz. The proposed configuration provides an improvement in isolation by 10 dB (measured value) with a reduced edge-to-edge spacing of 7 mm (λ0/9). This resonator has been designed, simulated, and validated experimentally. KeywordsAntenna array, isolation enhancement, meander lines, mutual coupling reduction, resonators.
Mutual coupling, gain and directivity of an array of two identical antennas
It is shown that there is an optimum nonzero value of mutual coupling and passive reflection coefficient that is a function of only the embedded element pattern and element spacing. Optimum is defined as yielding maximum array gain. For closely spaced arrays, the optimum mutual coupling and passive reflection coefficient are quite large, but tend to cancel in the active mode to produce a low active reflection coefficient. A smaller value of mutual coupling would give lower gain than the optimum case. This again demonstrates that mutual coupling is a natural and desirable effect in array behavior. It is well known that it is possible to achieve array superdirectivity, in the sense that the directivity of an array of " N " elements is greater than " N " times the embedded element directivity; however, in this case the mutual coupling and passive reflection produce an active reflection coefficient that results in an array gain lower than " N " times the embedded element gain. In fact, higher superdirectivity always implies lower gain, relative to embedded element gain.
Long Ceramic Beam Tubes for Accelerator Magnets
The ceramic beam tubes for the fast and abort kickers and the bucker and pinger magnets used at Fermilab must meet a number of exacting requirements. The tubes must be long and sufficiently straight so as not to limit magnet design. They must have wall strength capable of withstanding the atmospheric and mechanical forces encountered during construction and operation. Vacuum tight ceramic to stainless steel transitions must be of reasonable cost; they must also withstand high temperature processing and remain vacuum tight to the 10/sup -9/ Torr range. To remove the possibility of static charge buildup each tube must have a surface coating of indium oxide applied to the inner wall. This thin coating is difficult to achieve and requires the most careful attention to detail. The methods used at Fermilab to achieve these goals are presented in this paper.