6
\$\begingroup\$

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:

anntenna_gnd

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

antenna_no_gnd

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

\$\endgroup\$
3
  • 3
    \$\begingroup\$ antenna-theory.com/antennas/patches/pifa.php \$\endgroup\$ Mar 30, 2018 at 14:35
  • \$\begingroup\$ If I state that ground is not a place of zero energy, would that surprise you? \$\endgroup\$ Mar 30, 2018 at 15:21
  • 4
    \$\begingroup\$ The second antenna has a stub shorted to the ground plane as well. These stubs are most likely for impedance matching. \$\endgroup\$ Aug 26, 2019 at 16:37

2 Answers 2

2
\$\begingroup\$

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.

\$\endgroup\$
1
\$\begingroup\$

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).

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.