Sanity check on what width for 50 Ohm PCB trace

Hi all, I would like to check I am doing something right. I am routing a PCB that will use a GPS antenna, and I need to use a 50 Ohm trace to connect the antenna. I am going to get it made as a two layer, 1.6mm, 1Oz copper board from JLC PCB. I have used an online calculator and it says I need a 2.7mm trace width to get an impedance of 50 Ohms?! Seems way to wide to me, am I doing something wrong? Here are some links:

Mantaro Impedance Calulator

JLC PCB layer stackup info

OSH Park 2 Layer detailed info

I used the OSHpark 2 layer board info page to find the "PCB Substrate dielectric constant". It says it is 4.6 at 1MHz. I know it changes with frequency and JLC PCB will probably use a different substarte with a different value, but I just want a rough idea, doesn't have to be prefect. For the dielectric thickness, I used 1.5mm since that is the distance between the top and bottom copper for a 1.6mm board from JLC PCB.


Does 2.7mm seem correct for this? Or am I doing something stupid? Thanks in advance!

I used a similar calculator when I wanted a 50ohm trace for an 868MHz LoRa circuit. The trace came out surprisingly wide, so I suspect you have got it right. Remember that the exact width of the gap/clearance around the trace is also important. I was also recommended to place lines of vias to connect the ground planes on both sides of the clearance gap. You are trying to make something similar to a coaxial cable out of pcb traces.

Ah, cool. Yeah I have put quite a lot of vias to make sure both ground planes are well connected. I will have to take a look and see if I can figure out what clearance to use from the trace to the ground plane. It is going to be interesting routing a 2.7mm trace, but ill figure it out. :sweat_smile:

I also got very wide traces when trying to use one of those calculators for some RF boards. I think you're supposed to use a 4-layer board with a much smaller distance between the trace and the ground plane :-(

My gut feeling says that connecting a 2.7mm trace to a pin that's only 1mm (or whatever) from its neighbors, is not going to go well, perhaps even to the extent that the transitions are going to be more of a problem than using a non-50ohm trace in the first place. But I flunked RF, so what do I know?

There's a reason that people pay big bucks for modules instead of working with the much-cheaper chips...

@westfw Yeah, I am going to use a 4 layer board for the final design, but wanted to make a cheap 2 layer test board to make sure I can properly talk to the GPS module. I am actually using a Ublox Cam-M8 which has an integrated chip antenna, but I wanted to have the option for a patch antenna as well :). I think I will just make the trace as wide as I can and see how it goes. It is just a test board anyway so it's ok if it doesn't work. Also, at least you flunked RF, my entire knowledge comes from a few YouTube videos about it