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Trace impedance calculations

I am attempting to layout a PCB based off an existing "reference design". I have a "Bill of Materials", physical copy of the board, and a few design documents etc..

I'm stumpped on one section: I have an 50 ohm "single-ended RF port" coming off a Balun going to an SMA connector with antenna attached.

The SMA connector and antenna are both "50 ohm", so I am assuming that the trace needs have 50 ohm impedance.

All the trace impedance calculators I am using are basically telling me to use 100 mil width! That is huge!

The reference PCB I am copying has roughly an 8 mil wide trace and is nowhere near 100mils wide!

What am I missing here?

I am actively reading a few different trasmission-line / PCB design books and I only have a very basic understanding of these concepts.

Am I just crazy to even attempt this? Yes, of course, but that's not stopping me.

I'm basically going send my PCB off and try it anyway...but if anybody can point me in the right direction, I'd really appreciate it.

Reply to
blarggstar
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At certain microstrip lengths and signal rise times, a microstrip would NOT need to be considered a transmission line right?

Possibly the reference board trace is short enough?

(I'll admit that I'm pretty embarrassed for even asking these questions. This is not my field, so I must look like a bumbling idiot! )

Reply to
blarggstar

Hello, and as a rule of thumb you don't have to consider EM wave propagation for conductors whose length is no greater than about 1/10 of a wavelength. An EM modeling program like the Numerical Electromagnetics Code (NEC) considers RF currents through no-loss conductors of length 1/20 wavelength to be constant in amplitude and phase (no time delay). Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337
Reply to
J. B. Wood

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