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I'm looking at a picture of an RF PCB meant to operate at a few GHz, and I'm having trouble understanding exactly what this is:

enter image description here

Obviously there are some transmission lines, and something that looks like a couple of radial stubs.

What are the little silver bits along the transmission lines, and in an arc around the radial stubs? Are they vias, plated slots, or just bits of isolated copper? (At least some of it is not round.) What function do they serve?

Also, what type of transmission line is this: microstrip, coplanar waveguide or SIW?

Note: It may be a bit hard to tell but most of the picture doesn't have copper on the top layer; the very bottom is a solid copper fill with soldermask, but the darker blue is apparently just substrate, and the silver parts are copper plated with something. Sorry, I don't have a better picture, and I've never seen anything like this.

EDIT After @Dan Mills explained that this is chicken dots, I realize I have seen these before - there are usually a lot fewer and bigger of them, for example:

enter image description here

These look really useful for hand tuning. Yay new thing learned :)

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    \$\begingroup\$ en.wikipedia.org/wiki/Distributed-element_filter \$\endgroup\$
    – V.V.T
    Feb 4, 2021 at 5:50
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    \$\begingroup\$ @V.V.T Thanks, I've seen plenty of distributed element stuff including stuff that looks just like this: commons.wikimedia.org/wiki/… - but that doesn't have little vias or whatever they are along the arc of each stub. Could you explain in a bit more detail? \$\endgroup\$
    – Alex I
    Feb 4, 2021 at 6:23
  • \$\begingroup\$ You are right, rectangle structures look like SIWs, bowtie structures may be surrounded by surface mounted varactors? \$\endgroup\$
    – V.V.T
    Feb 4, 2021 at 10:27
  • \$\begingroup\$ In @V.V.T distributed-element_filter (wikipedia) there are some gold-plated resonators with solder-blobs added onto the high-Z end (added by hand). I take it these are fine-tuning methods used to tweak a filter to conform to spec. \$\endgroup\$
    – glen_geek
    Feb 4, 2021 at 14:58

1 Answer 1

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Chicken dots. Used to allow fine tuning of a design by means of solder bridges to change the geometry.

You place them when you are not sure about the design and wish to have the ability to change lengths or widths on the bench.

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  • \$\begingroup\$ Wow, that's exactly it. I now recall seeing a few similar bits in other pcbs, but never as tiny or as many of them. This will come in handy :) One question: are they usually soldered or just tuned with adhesive copper strips? (or both) \$\endgroup\$
    – Alex I
    Feb 8, 2021 at 23:40

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