Testing of variable stub lines - Page 1
Forum section: electronics / rf-microwave

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 19, 2019, 10:13:54 pm
content: So I caught a set of variable stubs. Now I have to figure out how to test them using a VNA.For the single ended ones, I could do S11 or would it make more sense to build them into a more complex setup. Then I could make consecutive measurements at various defined positions.The one with the through-connection could be terminated at one side and then S11 and S12 taken or maybe connect the VNA to both sides and take all 4 parameters?But what to watch for?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2019, 03:06:51 am
content: You'll see impedance peaks and valleys (or on the complex plot, loops) at harmonic frequencies inversely proportional to length. Equivalently, the step response (if you scoped it) will show a limited pulse width.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2019, 09:07:14 am
content: Quote from: Neomys Sapiens on February 19, 2019, 10:13:54 pmWhat are you trying to determine with the test?Whether they are functional?What the "significant" frequencies are?Whether theory matches practice?Whether the VNA works?

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 20, 2019, 09:13:04 pm
content: I simply love that you all try to make a better engineer of me!Whenever I deviate from good practice, I get swatted immediately.I intend to test whether they are functional and what their basic parameters are.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: February 20, 2019, 10:14:21 pm
content: There is advertisement from 1969 with performance charts for this kind of parts.https://archive.org/details/General_Radio_Experimenter_1969_05_06/page/n29

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2019, 11:37:15 am
content: Much like Tim says.An example with a SA and return loss bridge looking at the resonant frequency of a 315 MHz antenna:Using a VNA sweeping narrowly across the resonant frequency:

author: analogRF
author_rank: Super Contributor
timestamp: March 06, 2019, 04:37:38 pm
content: The OP is talking about transmission line stubs not resonant antennas. So it would be hard to see anything in the magnitude of S11They usually have a very high Q so you might only see a very narrow blip in S11 getting deeper if the stub is radiating from the open end or from the slot but still it will not characterize the stub and its resonances. For that you need the phase.So the best practice is to look at S11 ONLY on Smith Chart. There you can easily see the loops (depending on your sweep span) and seewhere the series or parallel resonances occur and even measure the actual Q of the stub at its resonance if you want...

author: tautech
author_rank: Super Contributor
timestamp: March 06, 2019, 07:08:42 pm
content: Quote from: analogRF on March 06, 2019, 04:37:38 pmSure, a SVA1015X gives this: (same project as images above)

author: analogRF
author_rank: Super Contributor
timestamp: March 06, 2019, 07:25:28 pm
content: yeah but for a simple short or open stub you ONLY see circle(s) very close to the perimeter of SC.Ideally they will be right on the outside circle since there would be no loss of any kind but in practice for low loss lines and very little radiation from the open end the circles will be close to the edge of smith chartand parallel or series resonances occur where the circle crosses the horizontal line (real values) on the right or left of the center, respectively.For an antenna of course it's a different story...

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: March 06, 2019, 09:51:27 pm
content: A first test of their DC behaviour did show that one of the three has contact problems (clearly intermittend).I'll take the other two to the VNA somewhere during next week. Thanks for all the answers!