What do you think of my microstrip?

[ltkenbo]

I am an electrical engineering student at a university. I am designing a project with my teach which uses zigbee technology. I have designed one of our circuit boards and I don't know a lot about microstrip stuff and RF board design but I did my best. The zigbee is in the 2.4 GHz range and the microstrip from the balun to the antenna needs to have an impedance of 50 ohms (to match the balun output and antenna).

Anyways I will include 3 pictures. The first one is the original. We had this board made on a mill machine. I designed the microstrip for using thin ".031 board, it is .055" wide which with that thickness of board should be around 50 ohms. However when we had it made at the school they used thicker ".062 board. However, it seems to be able to actually transmit a good distance. All shown dimensions are in mils (thousands of an inch)


For our second board which we are having fabbed I made the microstrip way wider to correct for the ".062 board thickness. However I realized that I should have mitered the edge. So the third picture I have included shows the current design with a mitered edge.


Third image (current design):


I would like to know what you think of my design in general, and any tips/suggestions/corrections I should do to my microstrip and/or the rest of the design. I want the best range possible with this device. These boards as mentioned above will be on ".062 FR4 board with soldermask (two layer). Since this is a two layer design I have a double ground plane (top and bottom) with many vias across the whole board spaced at 0.1 inches.

 

[vfone]

You mentioned the line between balun and antenna connector is microstrip, but from the picture you sent the filled ground is very close to the line (which will induce a CPW mode).
The ground distance should be equal or greater than 1.5 x line width. Some designers use one or two substrate heights, depending on how long the trace is.

I think the miter have too small corner cut. I don't know if you calculate the cut, but here are some examples how to compensate the right angle bends
(W is the line width)

 

[volker_muehlhaus]

but from the picture you sent the filled ground is very close to the line (which will induce a CPW mode).

Agreed. Having ground below and on the side is not bad - but it means higher capacitance to ground = lower line impedance. To keep 50 ohm line impedance, the line width must be reduced in this case.

Unrelated suggestion: I would place the ground vias closer to B1.

 

[ltkenbo]

Ok so I increased the miter (trying to follow the first example on that sheet) and moved the ground plane at least twice the substrate thickness away. What do you think now?

I can decrease the width of the microstrip I'm just not sure how to calculate it. The width I have now it calculated based on the board thickness, copper weight, etc. using a microstrip calculator for a microstrip above a ground plane (112.5 mils).

 

[vfone]

Looks fine for me, and I think you can let as it is.