Jump to content

Trace length going to SMA connector

Recommended Posts


There have been many discussions about the trace width on the PCB which leads the signal up to the SMA antenna connector.

On one of my receivers I am forced to move the SMA connector a bit further away from where I originally designed it. The width of these traces on a 0.8mm FR4 doble sided PCB board is calculated to 1.44mm. This mathces the 50 Ohm impedance required - at least theoretically. Up till now on test boards I used 1.5mm traces and it gave me quite good results.

The question is - if I need to move the SMA connector further away from the receiver (this is RX and not TX) do I have to make sure the trace is of a certain length? Like 1/2 or 1/4 wavelength?

The original tests were made with trace lenght = 8.33mm (calculated by the information Eagle gives me / trace segments - it might not be completly accurate..)

In the new position the trace length will be 13.14mm - calculated the same way as the previous.

So how long should it be?

Thanks, Ox.

Link to post
Share on other sites

it is the WIDTH that gives you the 50 ohm,

if the width is correct your loss will be MUCH lower and the length dont care that much. you dont have to care about the wavelength in a transmission line

made with coax or microstrip.

1.4mm width on 0.8mm FR4 = 50 ohm (Er 4.7)

1.5mm width on 0.8mm FR4 = 50 ohm (Er 4.2)

1.7mm width on 1.0mm FR4 = 50 ohm (Er 4.7)

1.9mm width on 1.0mm FR4 = 50 ohm (Er 4.2)

Be warned Fr4 fiber glass can have Er from 3.8 to 4.8 !

and also FR4 is full off loss

you are right an open or shorted transmission line must be of exact wavelength to give you what you like, a filter pass band or a stop band to kill unwanted jammers :-)

13mm length is really short !! even with FR4 and even at 2.4GHz

be SURE your ground is made correct all the way, and no tracks close to your stripline, make at lesst 1.5 the width, in space to parallel ground on top layer.

Edited by ThomasScherrer
Link to post
Share on other sites

on this page:


you see some examples of stripline designs made for 2.4Ghz and UP !,

the PCB material is Rogers 4000, but can also be Nelco and even Teflon I also use for lowest loss and low intermodulation.

Edited by ThomasScherrer
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...