# Pie shaped stubs in RF supply PCB layout

In a recent question there is a photo of a RF PCB And it shows several pie shaped pads on the supply end of the biasing network.

How are these pads designed. Is there a rule of thumb for the two parameters (which appear different for the same line) - length and angular extent?

I presume that these should be capacitive and to reduce reflections back down the inductive line to the 50 ohm line

These are called radial stub; if you have a pair of them, it's bowtie, or butterfly stubs.

They indeed (at least here, as far as I can tell) fulfill a impedance matching role: Think you want to transform the impedance of some component – you can add an open or shorted stub.

Their main advantage is that by becoming wider, they can be shorter than a "straight" stub. Technically, the wider (the larger the angle), the shorter, but interaction with the transmission line that would occur at the gap between circle segment and line put a limit to that.

The fact that they are also a bit less sensitive to wavelength (i.e., have wider bandwidth) is another plus.

How are these pads designed. Is there a rule of thumb for the two parameters (which appear different for the same line) - length and angular extent?

• Length: depends on what you want to do, so there can't be a single rule.
• Angular extent: that will also depend on the effect that you want, but will be constrained by space.

Notice how the "tip" of the circular segment can't be infinitely thin; this is a factor that limits how much you could come up with an analytic formula for this. In the end, pick the narrowest line to connect your tip to the transmission line, then expect to simulate several options for length and angle, and pick the one that's closes to what you wanted.

• Is there likely to be a reason why (in the 2nd photo) the sides of the left-hand stub are symmetric to the signal line, but the other two stubs have one side perpendicular? Aug 26 at 13:35
• space: the one that nice and symmettric had enough space to both sides, the others were meant to be at the end of the transmission line, and hence had to share the space with the discrete components. Aug 26 at 13:49
• HP Application Note 976 Broadband Microstrip Mixer Design - The Butterfly Mixer describes these radial stubs hp.woodshot.com/hprfhelp/4_downld/lit/diodelit/an976.pdf Sep 10 at 10:51