Measuring S11 for a waveguide part outside the TE01 freq. range - Page 1
Forum section: electronics / rf-microwave

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: July 19, 2020, 08:29:40 pm
content: I was attempting to measure S11 on a WR284 waveguide absorptive filter above the normal frequency range (2.6-3.95 GHz), to see how much harmonic energy I would expect. I noticed that with two waveguide to coax adapters back to back, the S21 had pronounced dips beyond 4 GHz, in some cases more than 30 dB, so the VNA gives lousy results when calibrating out the adapters.I'm wondering if the adapters are optimized for TE01 launch inside the waveguide, and so when using frequencies > 3.95 GHz, other modes are also launched so we should expect the S11 to be quite bad. What would the correct approach be to make such a measurement? Thanks.

author: OwO
author_rank: Super Contributor
timestamp: July 20, 2020, 01:49:58 pm
content: S11 is defined as the ratio of forward and reflected wave E field magnitude of a single mode, so it is undefined when you have multiple propagating modes. Each mode's waves travel independently in the same way that the forward and reflected wave in a transmission line are independent. Similarly, in 3d free space there are infinitely many propagating modes, so defining a single "reflection coefficient" for a chunk of metal in space doesn't really make sense (unless the mode is also specified, for example a plane wave hitting a metal surface at a defined angle). Ideally if you can couple to each mode individually, you can measure multiple S11's in a waveguide, one for each mode. So the short answer is no you can't really measure S11 above the transition frequency unless you can design a feed that will couple to one mode only.

author: CaptDon
author_rank: Super Contributor
timestamp: July 20, 2020, 02:52:48 pm
content: Would you be interested in some X-Band radar bench test gear?I have a complete setup for marine pleasure craft (typically 2 or 4KW peak pulse) and also works for aircraft. There is a directionalcoupler, a dummy load (I think rated for 100w CW but handles easily6kw pulse) and a military test unit made by AAI in Baltimore Md whichwill measure frequency, pulse power, PRR and also generate a movingtarget and allow receiver sensitivity measurements. I also have aTS-488 Echo Box with good diodes and spares. I have a test unit fortesting Raytheon/Raymarine display heads in the R10/R20/R40 series.I'd take $200 for everything but shipping may be another $100 unlessyou want to pick it up in the Conneaut Ohio / Erie Pa area.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: July 21, 2020, 06:28:46 am
content: Quote from: OwO on July 20, 2020, 01:49:58 pmGot it, so if I were to try to measure this, I'd need waveguide to coax adapters that are optimized to launch a different (higher-order) mode, and also to suppress lower-order modes. I can see now why the filter manufacturer didn't provide broadband S11 in their test reportQuote from: CaptDon on July 20, 2020, 02:52:48 pmThanks for the offer, but I'm set for test gear.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 21, 2020, 07:38:35 am
content: Quote from: OwO on July 20, 2020, 01:49:58 pmIndeed, this is a good summary of the problem. What can sometimes work well is if you use tapers. A good taper can have a very stable transition between a waveguide where only your 'mode of interest' is supported, and an over-moded waveguide. Since waveguides (especially at low frequencies like in your case) can be made to have very good performance, you can assume pretty much 0 mode-transition once you have excited the mode (so long you don't have sharp bends and so on). There are also ways to do it with ridge waveguides.Quote from: radar_macgyver on July 21, 2020, 06:28:46 amEven then, you will be running into some issues: there is an important requirement for measuring s-parameters (which stems from their definition) that you will not be able to meet: the \$S_{XY}\$ parameter is the amount of power/e-field/however you define it that is propagating out of port \$X\$ as a result of a incoming propagating power/e-field/whatever into port \$Y\$, with all other ports of the structure terminated in their characteristic impedance.This is the bigger challenge when measuring s-parameters of multi-mode structures - you cannot properly terminate the other modes in their characteristic impedance.