Why do some PCB antennas have a return path to ground?

Question

I'm finding it hard to find an answer for the question, as I'm asking about the circuit's GND, where searching for an answer online or in stack-exchange is coming up with Earth GND for large Antennas such as VHF.

As an example from this application note from Cypress, one antenna has a node connected to system GND like so:

Where as another antenna in the same doc has no connection to system GND:

I don't understand how the antenna can work when it's output is just shorted to GND. Can someone help? Thanks!

Answer 1

A connection to ground is a short at DC (0 Hz), but not necessarily at AC (if the circuit driving the antenna is running at a high enough frequency and has no DC offset). That's because even small loops (causing the "short" to ground) of metal are inductors, and thus have a measurable non-zero impedance at a high enough frequency.

So one antenna system has an inductor to match the output to the antenna, and the other has an extra stub to do the match in a different way.

Answer 2

The top example looks to me like it could possibly be something like a J pole matching stub. The J pole uses a 1/2 wave end fed radiator which has a very high (ideally infinite) impedance. To match it to the feedline, it uses a 1/4 wave stub of feedline, which transforms the low impedance on its shorted end to the high impedance on its open end, and somewhere in the middle is the impedance of the feedline.

Alternatively, it could just be a stub similar to the straight element on your lower example. Again, a 1/4 wavelength transmission line transforms the impedance on one end to the inverse impedance on the other end, so whereas the lower stub would present a very low impedance at one frequency (making something like a notch filter), the stub on the upper example would present a very high impedance at one frequency (making something akin to a bandpass filter).