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Testing directional coupler directivity and bandwidth
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Pierre Martel
Anyone have some idea if it is possible to test a directional coupler function. Like how much signal is coupled out on fwd or rev with is the max bandwidth it can be used on and what would be needed as connection to each port. I am pretty sure a variable load at the out port should be there to simulate miss match but would it need a 50 ohm load at the un tested port? (Fwd/rev) where should s0 and s1 plunged to? The input of the coupler or to the sampling port?
Thanks Pretty new with vna and learning a lot Pierre Ve2pf |
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Termination depends on the design of the coupler, some like "tandem match"
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or stripline require termination into their designed impedance, others like "douma bridge" and similar don't. The connections to the VNA depend on that you want to measure and that is up to you. On Tue, 28 Apr 2020 at 18:15, Pierre Martel <petem001@...> wrote:
Anyone have some idea if it is possible to test a directional coupler |
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Poke around in "RFCafe.com".
Yes, one can use the NANOVNA's to determine the parameters of directional couplers. However, describing the setup for each parameter would be a bit long and maybe a bit ambiguous in just verbiage. Diagrams are really needed to do the description justice and clarity. "Microwaves101.com" might also offer some good information. Dave - W?LEV On Tue, Apr 28, 2020 at 10:45 AM Dragan Milivojevic <d.milivojevic@...> wrote: Termination depends on the design of the coupler, some like "tandem match"-- *Dave - W?LEV* *Just Let Darwin Work* *Just Think* |
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Pierre Martel
Thanks all.
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Herb, you got me in the right direction I am sur I will be able to figure this with the link you provided. <> Garanti sans virus. www.avast.com <> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> Le mar. 28 avr. 2020 ¨¤ 13:34, hwalker <herbwalker2476@...> a ¨¦crit :
Pierre, |
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I use my Nanonvna with an HP 8721A directional coupler. As noted above attach ch0 to the input of the coupler and ch1 to the reflected port of the coupler. Attach a short to the directional coupler's output port and do a through cal with the short in place. S21 is now the return loss of what is connected to the coupler's output. Because the Nanovna also outputs S21 phase you can calculate the impedance of what is connected to the coupler's output port. The HP 8721A has a directivity spec of 44 dB from 100 kHz or so to 110 MHz. I have verified this with an HP cal kit load. This method might even be more accurate than using Ch0 making an S11 measurement. If you don't have a good commercial coupler, a very good one can be made. All it takes is some wire and a pair of ferrite toroids. Look up the Stockton coupler on the web.
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On Tue, Apr 28, 2020 at 03:24 PM, WB2UAQ wrote:
... I use my Nanonvna with an HP 8721A directional coupler. ... =============================== The HP 8721A bridge is hard to find on the secondary markets, especially the 50 ohm ones. I've had offers for mine that were hard to resist but I turned down. You should write up an application note for the group on using an external directional coupler to measure return loss and impedance, with accompanying math. I believed one of our members at one time asked Rune if he would add a module to NanoVNA-Saver for measuring return loss and impedance using an external coupler. - Herb |
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Hi Herb,
Still working for the next few months (retired but went back p/t for a year) BUT I will work on using a return loss bridge based on the Stockton design. The ARRL handbooks also have an RL bridge but it is not as good as the Stockton. I am impressed by how easy the Stockton bridge is to make and one that has good directivity. I think they were in QST articles and QEX for sure. You are right. The 8721 might be hard to find. I haven't looked at the MiniCircuits Labs site lately but I know they had some nice couplers and I think they were reasonable. |
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Just did some return loss testing with the hp 8721 and not working as expected. After calibrating with a short the RL for an VSWR of 1.5 should be 14 dB. Measure about 13 dB. If I calibrate with an open the RL is about 13.8 dB...closer. The phase angle looks OK (I see a 180 phase shift between open and short). I have a second RL bridge that is actually a Stockton design and will try it next but it needs work (it doesn't have compatible connectors, yet).
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Here is a pdf link to the return loss bridge based on the Stockton design:
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I¡¯ve been following this thread and I kept hearing about the Stockton design. Didn¡¯t see a link. Joe Joe Joncas WA7MHB Lincoln County Radio Amateur Club 541-272-3207 joe1@... On Apr 30, 2020, at 4:09 PM, WB2UAQ <pschuch@...> wrote: |
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On Fri, May 1, 2020 at 01:40 PM, WB2UAQ wrote:
Just did some return loss testing with the hp 8721 and not working as expected. After calibrating with a short the RL for an VSWR of 1.5 should be 14 dB. Measure about 13 dB. If I calibrate with an open the RL is about 13.8 dB...closer. =============================================== My hp 8721 is loaned out but I have on hand a MCL ZFDC-10-6 directional coupler that has >= 30 dB directivity between 5k - 20M. Using it, my test results were disappointing after calibrating the coupler with either a short or open. The NanoVNA was much more accurate measuring the return loss of a precision mis-match on CH0 than measuring the return loss of the same precision mis-match connected to a directional coupler and measured on CH1. I couldn't see where using the NanoVNA and an external directional coupler gained me anything over just using the NanoVNA standalone. - Herb |
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