via stitching == lots of patch antennae? - Page 1
Forum section: electronics / general

author: jeremy
author_rank: Super Contributor
timestamp: December 16, 2014, 01:32:04 pm
content: Hi all,Reading another thread reminded me of this. I don't consider myself to have much experience with RF pcbs (I'm talking 10s of MHz to low GHz), and a colleague of mine told me that one should not use a consistent grid pattern for via stitching between planes because it will create patch antennae between all of the vias. On top of this, he generally cautioned me against using a lot of vias for stitching, instead telling me to use a few in randomly placed locations to prevent this problem.Is there any merit to this? If so/if not, why?

author: gxti
author_rank: Frequent Contributor
timestamp: December 16, 2014, 02:19:01 pm
content: No. The spacing between vias is supposed to be electrically short (1/10th wavelength?) so that no "funny stuff" happens. And in any case, all you have to do to see how silly it is is look at some high-end RF products and notice how they make extensive use of it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2014, 04:02:25 pm
content: No, a patch antenna is an isolated spot of metal, connected to a driving node or transmission line.At least, not a good one, I guess. I would think you'd have to go out of your way to make structures like that... but then again, perhaps I shouldn't underestimate the average PCB designer...Suppose you have ground fill around several intersecting trace routes: the ground gets cut by the traces, forming slots and peninsulas. If these are not stitched across, then there could be the effect where the traces couple into this structure, which behaves as a 1/4 wave stub, or something like that. Or conversely, susceptibility from outside radiation. As long as you are able to place vias near the 'point' of the peninsula, or spaced regularly along the slot, any resonant modes will be confined between ground planes, and pushed to much lower impedances (lossier?) and higher frequencies.A random via pattern is probably better than a regular grid, because a regular grid -- over wide patches of facing ground plane -- acts like a waveguide lattice, which will permit certain resonances and modes. Though they will be quite low impedances (probably in the 10-50 ohm range), and probably narrow bandwidths (I forget exactly what it is about waveguide geometry that matters, but I think this will be the case?). Interestingly, the physics of a waveguide lattice should be identical to the (quantum mechanical) phonon resonances of a 2D crystal. Maybe there's some interesting electromagnetic-opto-acoustic structures that can be made with this analogy?...Anyway, a random pattern will tend to break up resonant modes, though there likely will still be propagating modes, peaks and dips at random frequencies. If you really need those nailed down, you can run it through the top-dollar microwave simulator and push vias around until the cows come home.That stuff only matters at the bleeding edges of bandwidth, where signals don't like to stay in traces anyway. Down at low frequencies (100s of MHz and below?), it all looks like a nice low impedance, a little bit of inductance between vias, around slots, and between various connections, relative to a 100% solid fill that is.Tim