50 ohm on FR4

Grok reckons to make a 50 ohm tx line on FR4 I would need  a Trace
Width of approximately 2.73 mm and a Trace Spacing of at least 2.73
mm. Does that sound about right?

Here's the full response:

For a 50 ohm controlled impedance on a one-sided (also known as
single-layer or microstrip) FR4 PCB, you need to consider both the
width of the trace (as discussed in the previous response) and the
spacing between traces if you're planning parallel tracks. Here's how
you can approach this:

Trace Width (W) for 50 Ohms:
As calculated earlier, for a 50 ohm microstrip:

    Trace Width (W): Approximately 2.73 mm for:
        Board thickness (h) = 1.6 mm
        Dielectric constant (er) = 4.2
        Copper thickness (t) = 35 µm (1 oz)


Spacing Between Traces:
The spacing between traces (S) is critical when you're considering
cross-talk or if you're routing multiple traces in parallel. Here's
what affects this spacing:

    Crosstalk: The closer the traces, the higher the crosstalk.
    Impedance: Spacing affects the effective dielectric constant and
thus the impedance. For a single trace, spacing from the edge of the
board or other traces can slightly influence the impedance, but mostly
for very high frequencies or very thin traces.
    General Rule: A common rule of thumb for microstrip lines is to
keep the spacing at least equal to or greater than the width of the
trace to minimize crosstalk.


For practical purposes:

    Spacing (S): A good starting point for spacing could be around
2.73 mm or more, matching the trace width. However, if space is a
concern and you're operating at lower frequencies (like under a few
GHz), you might reduce this spacing slightly, but you should be
cautious about signal integrity.
    Simulation or Calculation: Use a PCB design tool or impedance
calculator that can simulate or calculate the interaction between
parallel traces. These tools can give you a more precise spacing for
your specific conditions, including frequency, trace length, and
signal characteristics.
    Manufacturing Considerations: Remember that manufacturing
tolerances can affect both the width of your traces and the spacing
between them. Therefore, slightly increasing the spacing can provide a
safety margin.


Summary:

    Trace Width: Approximately 2.73 mm for a 50 ohm single trace under
the given conditions.
    Trace Spacing: At least 2.73 mm, or more for less crosstalk,
depending on your design constraints and frequency of operation.


Always verify with PCB design tools or through empirical testing if
possible, as theoretical models can deviate from real-world
performance due to various factors like material inconsistencies or
manufacturing variations.