tunable microwave load or short? - Page 1
Forum section: electronics / rf-microwave

author: pe2ben
author_rank: Contributor
timestamp: May 28, 2025, 10:21:33 am
content: Hello,Can i use a tunable microwave load also as a short?I was thinking to use a tunable load for SWR measurements (wr-22 33-50GHZ) in combination with a wr-22 directional coupler, wr-22 detectors and a HP8757d.Or have i to buy a separate tunable short?I asking this because on the screw range i see a scale from zero to 50. Is this scale the same as the impedance value (0= short and 50=load).Thanks Ben

author: CaptDon
author_rank: Super Contributor
timestamp: May 28, 2025, 11:55:38 am
content: I doubt very much that the 'scale' in question is an 'ohmic' scale. Most 'tunable shorts' would better be described as a 'variable phase plug' and have many uses similar to what I think were called 'lecher wires' for lower frequencies. You could measure the various voltage peaks and nulls to determine wavelength and standing wave ratios and position. The variable waveguide 'shorts' can be used in the same way and often have a millimeter scale and pointer or precision millimeter micrometer attached for direct readout.

author: CaptDon
author_rank: Super Contributor
timestamp: May 28, 2025, 12:08:38 pm
content: You do realize that in the waveguide world 50 ohms is not a typical impedance. There was always this rough estimation that free air was around 377 ohms and waveguides used in their efficient sweet spot were around 500 ohms. The trick of the waveguide horn was to give a good waveguide to free air impedance transition and the shape of the dish reflector to create the desired gain and radiation pattern. 50 ohms really only comes in to play where coax or hard line meet a waveguide 'transition' and by way of the typical 1/4 wave stub projecting into the waveguide creates the roughly 50 ohm characteristic impedance transition. Some the the stubs can be adjusted in protrusion length to adjust for best VSWR match or to vary local oscillator injection levels or to perfectly balance the so-called 'balanced mixer'.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 28, 2025, 12:23:59 pm
content: Quote from: pe2ben on May 28, 2025, 10:21:33 amYou make no mention or show any photos of what you have. By load, I assume then that it is indeed a load and not a short. No, that can not be used for a short.I put together a series on waveguides and go over the sliding load.https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4627228/#msg4627228

author: pe2ben
author_rank: Contributor
timestamp: June 01, 2025, 08:42:46 am
content: Here the picture from the mentioned item:

author: joeqsmith
author_rank: Super Contributor
timestamp: June 01, 2025, 03:12:14 pm
content: The original eBay link would have been more helpful.https://www.ebay.com/itm/325897002230They claim it is a sliding load with a VSWR of 1.05. This can not be used for a short, or an offset short. Attached showing a complete cal kit with sliding load.https://www.keysight.com/us/en/product/Q11644A/mechanical-calibration-kit-33-50-ghz-waveguide-wr-22.htmlIf you are looking for something low cost, try Pasternak.

author: pe2ben
author_rank: Contributor
timestamp: June 01, 2025, 04:07:18 pm
content: Many thanks Joe, it looks like very expensive for me.Could i use, as alternative, the following items in combination with my hp8757d analyzer with 2 WR22 detectors what i already have?A signal source is also available.For splitting signal a Magic Hybrid Tee => Hughes / Millitech 45392H-1200 Matched Hybrid TeeDirectional coupler => Hughes 45322H-1120 WR22 Directional couplerFor the load the mentioned Waveguide Sliding Matched Load but how could it be?That was initially the original plan because i can't find a load for wr-22 at reasonable price.Thanks Ben

author: joeqsmith
author_rank: Super Contributor
timestamp: June 01, 2025, 04:45:35 pm
content: Quote from: pe2ben on June 01, 2025, 04:07:18 pmAs long as you are fine with the mismatch of the sliding load, you could use it. The Narda sliding load I have has a VSWR of 1.353. I started to write software to support it but never finished. The Narda load I use has a VSWR of 1.015. I found a used offset short but made a short, shim and offset short as well. Not great but there's a big difference in the required tolerances of what you are after.