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I would like some help from anyone who could let me know any flaws or improvements in my design they can find and help me reconsider any last minute design fixes before I send my new project off to be mass produced. This is the second version of the design but the changes I made were extensive.

So basically I'm building an Arduino shield where the primary use case is to be a SWR meter with an LCD display (not depicted here as that is a seperate shield). But it provides all the functions of a Vector Network Analyzer (VNA) and as such goes well beyond your typical SWR meter.

Because the primary use case here is to act as an inline meter on a transmission line with an existing transmitter its not designed as a typical VNA would be using mixers. The only thing it doesn't have that a VNA would have is a function generator, as it relies on the transmitter to do that, and the directional coupler would be external. However I have designed it in a modular way so not only can the directional coupler be swapped out for one the user prefers or more suited for their setup, but it can also be configured to work more like a traditional VNA as well. With the additional of an additional shield with an in-built low power directional coupler and a sine wave generator it would be possible to also pop on this other shield and effectively have a handheld VNA instead. Being modular there are also several other possibilities for how this device can be configured including as a remote SWR meter so you can have a meter at both the transmitter and the antenna to properly calculate feedline loss, or to understand how the complex impedance of your antenna changes with frequency.

Because of its role as an in-line meter in an existing antenna system it also provides functions a traditional VNA would not, specifically the ability to analyze properties of the transmitter such as accurately determining the true RMS under modulation or precisely determining the frequency the transmitter is transmitting on. Perhaps in the future I may add other features as well either in software of hardware.

Some specifications:

  • Operates 1 MHz to 500 MHz

  • Can measure input signals from -52dBm to 0dBm (adjust external directional coupler and attenuator to handle any power transmitter).

  • Inputs are 50 ohm matched but if building yourself you can switch out different resistors to match different impedances

Note: This is an open-source project and free for anyone else to replicate my work. At some point I may sell kits and/or the printed PCB to people to make it cheaper than needing to pay to get your own PCB printed. So I'd like to make sure if I provide PCBs they have been scrutinized and tested first.

The link to the projects source can be found here for anyone who wants to access the actual files.

https://git.qoto.org/roes/roes-hardware/

Here is a picture of the schematic:

enter image description here

Here is an older picture of the UI in demo mode. The data on the screen is intentionally bogus, and the glitches in the rendering have since been fixed.

enter image description here

This is a four layer board so youll have to see the GIT repo if you want to pick apart my layout in detail. But for now I will share the front side and back side so you can at least get a sense of my placement of the chips and my use of shielding.

Front:

enter image description here

Back:

enter image description here

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  • \$\begingroup\$ Minor note, you can right-click those global labels in KiCAD and choose between input, output, and bidirectional for added clarity. I'd suggest making 3pcs and selling to others at cost to gather some real-world usability data. \$\endgroup\$
    – rdtsc
    Sep 8 '20 at 12:36
  • \$\begingroup\$ Yes I have 3 PCB prototypes coming in already and intend to do just that. I havent selected who will get the other two yet though. \$\endgroup\$ Sep 8 '20 at 12:37
  • \$\begingroup\$ Have you built a prototype (before full run?) \$\endgroup\$
    – Andy aka
    Sep 8 '20 at 12:41
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    \$\begingroup\$ @Andyaka Well yes and no, V1 I had printed off in small batches and tested, but it was significantly different. This version now I've soldering up prototypes of each section of the schematic independently using manhatten style construction to ensure each component works, but havent ran the whole thing together yet. I did order 3 PCB though that are on their way which I will run as prototypes first before I do a production run of course. \$\endgroup\$ Sep 8 '20 at 12:43
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    \$\begingroup\$ @alex.forencich yes that is true, but the point is an SWR meter is still going to be effected my modulation in much the same way a complex-valued SWR meter would. So far all the early testing suggests its fairly accurate. \$\endgroup\$ Sep 13 at 23:17
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The fact that it only handles 0 dBm makes it basically useless. Directional couplers are very expensive and each model is a little bit different. A user would have to calibrate/characterize a directional coupler which is also not easy.

The VNA does one job really well, the SWR meter another. When you combine them, you are doing both jobs just so so. Even though my oscilloscope has a Fourier transform function I still needed to get a spectrum analyzer because each does a job really well, just not both together.

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  • \$\begingroup\$ Thats not much of an answer. For starters the circuit works with any directional coupler you need. The one I built and used to test this cost me about 10$ to make at most and can handle 1kW just fine. The directional couplers would be an extra kit for a few bucks, hardy a deterrent. The rest of your answer just talks vaguely about it not doing anything well, doesnt really give any logic or reason to it. Sorry but this seems like an exceptionally poor answer with no real time put into it. \$\endgroup\$ Sep 12 at 2:59
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    \$\begingroup\$ The V1 had the directional coupler built in already and at 1kW. The problem is with that design approach you are limited to the frequency/power of the built in directional coupler and could not switch it out. V2 is an improvement in that the directional coupler is seperate and can be interchanged, thats all. A directional coupler is literally just wire and 2 ferrit coils, nothing more. Here is a picture of V1 where the top half is the directional coil which we removed to make it separate, nothing more. tinyurl.com/ROESv1DirCoup \$\endgroup\$ Sep 13 at 20:25
  • \$\begingroup\$ well that's pretty cool i was not aware of that. maybe you can add some taps to these ferrite coils to make the directional coupler usable at different frequencies or sell them as a kit, like you said where you can interchange it. \$\endgroup\$
    – pgibbons
    Sep 14 at 13:02
  • \$\begingroup\$ indeed I will likely sell it as a kit with HF and VHF/UHF directional couplers as options. As far as I know I dont think taps would work too well, but I could be wrong, mainly because the choice of ferrits would differ between HF and UHF. I wonder if its possible to find a core that covers the whole range... doubt it but might be worth looking into. \$\endgroup\$ Sep 16 at 1:10
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I got one more answer for you. Since this is using low frequencies, I would ditch the RF shield. It will cost as much if not more than the whole board.

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    \$\begingroup\$ Good advice, though this thing can go 500 MHz and perhaps further in theory. The thing is the chips int he shield are sensitive to ghz ranges so the shielding is in fact needed. The first prototype i had didnt have a shield at all, it would pick up on static and report false information. The shield was needed to get it to work at all. Thanks though, this is useful suggestion to have tried. \$\endgroup\$ Sep 13 at 20:27

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