Electronic – Coplanar waveguide vs open space

The issue I see if you're trying to create a matched termination, is that except for the one at upper-right, your terminations are all short circuits, not matched terminations.

Since your frequency band is exactly one octave, it's possible that you could design the length of the CPW from your probe pads to the short-circuit to be approximately 1/8 wavelength, so that the short will appear as a match when seen from your probe point. This will work well for a narrow band around, say, 1.414 GHz, and will be a very bad approximation at the edges of your band at 1 and 2 GHz. If you have space, you could make different test structures with different lengths for testing in different portions of your band.

If you can work out how to do it, the option at upper-right would create a matched termination over a much broader band, but as you say it would require very careful design to ensure it's really a broadband 50 Ohm termination. From a geometry p.o.v., I'd suggest using a symmetric structure with 100 Ohm resistance from the central trace to the ground on each side.

An option that might be even better is to build a "through" structure instead of a stub structure. Put probe pads at both ends of your transmission line, and use two probes. Then let the VNA and its 2-port calibration math work out the errors due to the slight mismatch of the probe at the far end, instead of relying on your assumed-perfect 50-Ohm load as a reference for determining the trace impedance.

I have been using these general guidelines in transmission line design:

1. To have a continuous GND connection between planes on different layers, the distance between GND vias should be below 0.1 wavelength.
   You should calculate your wavelength carefully as it depends on the dielectric you are using.
2. Vias have parasitic inductance; multiple vias reduce parasitic inductance and create lower impedance path between GND planes on separate layers.
3. The GND planes on both sides of the co-planar waveguide should extend to at least 5x width of the line.
4. I like to use co-planar wave-guides because it is easy to add components along the line where needed. So, basically it is OK to place components on the line as long as you consider the size of the pads of the components- ideally they should be the same width as the line width. So 0201 components will require thinner line than 0402, for example. Additionally, when placing components between the line and the near GND, you would like to reduce parasitics, so I usually place 2 GND vias near each component I place.
5. It is also important to keep everything as symmetric as possible. So an ideal placement of a component will be to both sides of the line. So, for example, if I would like to place a 1pF to GND, I would place two 0.5pF in opposite directions on the line.
   Having said that, for 2.4GHz I usually don't follow the last guidelines and so far had no issues with that.