Trace to antenna?

[elcaron]

Hi, I am laying out the PCB with a trace from the RFM69 to a throughhole pad for a spring antenna, 868MHz.
The trace is very short, the Antenna is as close to the RFM69 as possible, so not much more than half the antenna diameter plus half the pad width. I strongly assume that about 3mm trace do not matter with lambda/4 = 8.6 cm, but THEORETICALLY, how would I best layout that trace? Impedance as low as possible, so uninterrupted ground plane behind it? Angthing else?
I am just trying to learn a few RF basics here, so answers "yeah, that doesn't matter" do not help at all.

[lafleur]

These are a good starting point...

http://pdfserv.maximintegrated.com/en/an/AN2093.pdf
http://www.analog.com/media/en/training-seminars/tutorials/MT-094.pdf

Yes, it all maters, having 50 ohms from the radio to the antenna will lower the VSWR and make transfer of more power to the antenna.

[elcaron]

That looks interesting. Do you think I should take that into account for this project? I planned to send the PCB to production next weekend and I am not sure if I will have time to understand that by then. Trace in question is top right.

[DavidZH]

There are also some online calculators. But beware, there are two different ways to calculate, depending on whether there is a ground plane underneath or not.

I have tried to implement it, but the gaps are very small and I'm not sure how to use it properly in KiCAD.

These are the calculators:

• with groundplane on bottom layer
• without groundplane

My previous PCB's have been made without these calculations and work admirably (but not very efficient probably...).

[elcaron]

Awesome, understood. I had a little misconception about impedance, assuming it should contain the length, but thinking about it, I remember an exercise in university about that.
Changed the impedance to 50Ohm by adjusting the trace width and the clearance of the ground plane in that area.

[elcaron]

Quick followup: After I started looking for them, i saw quite a few traces like this. A lot of them put rows of vias in the groundplane along the trace. I see that this would change the impedance taken from the calculator if they are close (since there is more ground around), but should there be any closeby connection between top and bottom ground? At the beginning and/or end, e.g.?