How to feed or excite microstrip lines when using finite substrates?
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Users often encounter finger trouble when trying to feed microstrip lines when using finite substrates. Often an error message is obtained. This how-to gives options for constructing the feed.
In general in FEKO when using finite dielectrics with the Method of Moments (MoM) or more specifically the Surface Equivalence Principle (SEP) one cannot place a feed on an edge that is on the boundary of the substrate. One can, however, place the feeding edge completely inside the dielectric or completely outside the dielectric.
Consider a 50 Ohm microstrip line, one wavelength long on a substrate of height 1 mm and permittivity 2.
Feeding an edge completely inside the dielectric:
Consider the geometry shown in the cutplane view below. We have the metallic ground surface on the bottom surface of the dielectric, the metallic microstrip line on the top surface and vertical metalllic section at the two ends of the microstrip line connecting to the ground.
The microstrip line is fed with an edge source where the edge for the source is completely inside the dielectric, or more specifically here halfway between the ground plane and microstrip line on the vertical metallic section.
Also the via connecting the microstrip line with the ground (therefore the edge of the microstrip line) is some distance away from the edge of the substrate. Usually a separation distance of the order of the height of the substrate or more is sufficient.
Figure 1: Placing the feed inside the dielectric 
Feeding an edge completely outside the dielectric:
Consider the geometry shown in the cutplane view below. The microstrip line is extended over the edge of the substrate and the via is made completely in free space. The same separation distance recommendation as in the first case applies.
Figure 2: Placing the feed outside the dielectric 
Feeding a printed dipole:
We also consider the feed of a printed microstrip dipole. This case is very similar to the above two cases.
Figure 3: Feeding a printed dipole 
An edge port is used, but a small section of the dipole is elevated to ensure the feeding edge is not on the boundary of the dielectric. The elevated part can of course be submerged in the dielectric as well. Note the finer meshing used on the feed section and the surface of the dielectric just under the feed section. The height of the feeding edge above the dielectric surface is of the order of the size of a meshed triangle.
Alternatively, a small section of the dielectric under the microstrip line can be removed, should the user prefer to have a straight microstrip line (without any elevation). This is shown below with a zoom (cutplane) view on the right. The region (volume) just beneath the feeding edge is a free space region and all the surfaces on the boundary of this region are dielectric surfaces except, of course, for the microstrip line (top surfaces).
Figure 4: Printed dipole - removing a small part of the dielectric (zoom view on right) 
When making the dipole on an infinite substrate an edge port can be used again, but here it would not be necessary to elevate or submerge the feed section as the excitation can be placed on the boundary of an infinite dielectric. (It would not be necessary to draw a small feed section either, just a simple rectangle can be drawn.)
FDTD alternative
The Finite Difference Time Domain method does not have the above restrictions. For the FDTD it is perfectly acceptable to feed the dielectric on the surface.Restrictions regarding the width of the feeding edge
It must be noted that for higher frequencies (several GHz) it may be necessary to lessen the width of the feeding edge as higher order modes could start to degrade the desired impedance match. An edge width of around 1/20 to 1/30 of a free space wavelength should be sufficient.
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