SMA end launch connector to microstrip without vias

[hmalissa]

Hi everybody,

I'm trying to design a layout where I use a SMA (or similar) end launch connector to couple MW at 10 GHz into a microstrip line. I'm new to designing MW circuits, but I did a bit of reading, and I came across the "End Launch Connector Series" from Southwest Microwave. These seem to be close to what I want. Fortunately Southwest MW gives some details on how to mount these connectors, and they mention that it can be used with microstrip lines "with top ground launch". I'm not 100% sure what the top ground launch means, but I assume I need to add a small patch of ground plane on both sides of the microstrip line and connect them to the bottom ground plane using vias.
My question is: is it possible to use these (or similar) connectors with a microstrip line without having to add vias? I found some information on CPW-microstrip transitions involving quarter wavelength stubs - something like a virtual ground - is this what I need to do? Are there any specific instructions on how to design such a transition - there seem to be lots of similar concepts around, but I couldn't find real design rules about these transitions.
In advance, thanks a lot for your help. I'm a beginner in this field, and I'm sorry if these questions seem stupid to you.

Hans

 

[FvM]

is it possible to use these (or similar) connectors with a microstrip line without having to add vias?

I think, you should rephrase the question. Obviously, the connector impedance is designed for microstrip with "top ground launsch" or
CPW. Of course you can use it without top ground connection. But the question is, how good will the VWSR be? Most likely not as good as
documented in manufacturers "Test Data". You have to try it yourself, or simulate the geometry with an EM tool.

 

[biff44]

I have some 24 Ghz Hittite evaluation boards with that connector on them. The answer is: no, you do not need any ground vias to get to the top side of the microstrip board.

The connector, if you are talking about the same one I have seen, has a clever wedge action mechanical mechansim on the UNDERSIDE of the microstrip board. It forces the metal frame of the SMA connector to but up against the edge and bottome side of the microstrip ground plane. Now, the board I have also has a metal plate soldered to the microstrip bround plane, making it extra thick (maybe 0.062" thick), and there is lots of metal to contact to. If it was just a bare microstrip board without an attached metal backer plate, I am not sure how well it would work.

 

[PeterMartin]

You just need a connector similar to this:


**broken link removed**

 

[Mazz]

The main difference between a typical SMA end launch and the SM one (that I used in a test fixture for measurements up to 6GHz with a very low VSWR) is that the inner conductor (and also the external gnd) are "tapered" down to smaller dimentions in order to fit the typical microstrip/CPWG pcb lines.
For you, without knowing the exact geometry of the connector (and SW DOESN'T tell you that), it will be impossible to simulate it (they told me that thay use a process of cut&try to find the optimal geometry).

I think that the best way is to find the right connector part number that fits your selected pcb strip and follow SW instructions.

I hope it can help.
And no, I DON'T WORK for SM

Mazz

 

[toonafishy]

Why do you not want to use vias? Are you making the boards by hand?