QUESTION

Question

Hi,

We will use the antenna SR4L034-L in our LTE design and at the moment I'm doing PCB cad work and PCB design of the 50ohm transmission line on the PCB up to the feed point of the antenna. I have looked in the datasheet and found the reference design (attached in this thread). 

However, the datasheet clearly states that it is important to fulfill the specified clearance to nearast copper on all layers. But in that case the GND beloing to the transmission line cannot go all the way to the feed pad of the antenna. For a transmission there should be a  GND reference a certain distance beneatch the antenna trace and also on both sides of the trace.

But when looking at your reference PCB design in the datasheet it seems like the GND for transmission line stops before the transmission line reaches the feed pad of the antenna.

Is it correct to do it like this ?
Will the antenna trace still experience a good enough impedance even if the GND does not follow all the way up to the antenna ?
Do you have any other recommendations and/or reference antenna designs ?

Best regards

Ronny

Answer 1

Hi Ronny,

You are correct that a transmission line(CPWG) requires ground on both sides and beneath the layer for it to maintain a 50 ohm impdance. Although, the ground doesnt go all the way to the antenna feed pad it will still be fine as it will just act as part of the antenna trace in the clearance area. In terms of design try follow the datasheet as close as you can and the specifications for a 50 ohm transmission line based on your board.

Best regards,

Yu Kai Yeung