Near Field Notes

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0:47 · With sound

Trace or U.FL jumper

When does a U.FL jumper beat a trace to the antenna?

Read Note 25

Transcript

Trace or coax jumper?

Some boards send RF through a tiny coax cable instead of a trace.

Let's see when that pays off.

A trace loses a little every centimeter.

Most of it heats the FR-4 under it.

10 cm at 5.5 GHz: 39% gone.

The jumper pays a toll at its two connectors,

then the coax loses a fifth as much per cm.

So the lines cross.

Past 2 cm, the jumper wins.

At 2.4 GHz, past 3.

A thick 2-layer board gets a wide trace that loses less.

At 915 MHz, it wins up to 14 cm.

The coax is shielded too, so it can hop over noisy parts.

The catch: a hand-plugged cable, 30 mating cycles, and $1 or more in parts.

Short run or low band: trace. Long run at 5 GHz: jumper.