Acceptable for VNA connection?

[tehtehteh]

L2 is the output of an impedance matching network, 50ohm trace from L2 (highlighted in green) goes to antenna (highlighted in red), C12 and C13 are for tuning the antenna.

J1 is a U.FL connector that the VNA will connect to. Is this an acceptable location for connecting the VNA? And is it acceptable to disturb the 50ohm line with the connector?

[antenna]

Are you trying to determine the value of the trimmers needed to tune the antenna by analyzing the antenna without them or are you trying to tune said antenna with the correct trimmer caps pre-installed? If the former, I would temporarily cut the green trace so that the VNA only looks into the antenna, then once you have your measurement, disconnect the VNA, remove J1 and bridge the cut trace. You wouldn't want J1 connecting to both the antenna and the output of the transmitter during the test (even if, and hopefully, its powered off) ~ you will not get accurate results that way as the antenna and the circuit will be seen in parallel from the VNA's perspective.  If the latter is the case and all you are trying to do is get the antenna tuned to the circuit, perhaps skip the VNA all together. To use the VNA to make the adjustments such that the antenna feed is 50ohm assumes the output network at the transmitter is also 50ohm when the transmitter is active. Sure, it may be close, but how sure are you? What I would do then is to just monitor the signal strength of the powered-on circuit with a receiver and separate antenna and make adjustments to those capacitors such that the receiver (or spectrum analyzer) is getting the most power (keeping in mind the limits of the SA's input port and attenuating appropriately). When your adjustments make the received signal the strongest, you will know that, given the components used, you have the best match you will get. 

[tehtehteh]

Thanks very much for the reply. The plan was to leave L2 and C9 empty while tuning to disconnect the green trace from the rest of the circuit, then use the VNA to tune the antenna with the trimmer capacitors installed.

[antenna]

That should work ok, but I am going to take a total guess here and assume this device runs in the GHz. If I am right, that little bit of 50ohm trace will add some capacitance. What frequency are you running and how long is that green trace?

Assuming the left side of C9 is ground and the right pad of C9 is connected to the top pad of L2, can you temporarily attach the VNA across the pads for C9 instead? Then you would be taking the measurement without the green trace acting as an open transmission line stub and it also eliminates the possibility of the transmission line rotating any mismatch you might have at the antenna around the smith chart to a different impedance the matching network may not like.

[tehtehteh]

The green trace is about 10.8mm and the frequency is 433.92MHz.

[antenna]

You should be fine connecting where you originally planned to. I'd love to hear how she tunes up!  Post an update after you do the VNA part!

[Joel_Dunsmore]

you could consider creating a model of the little stub line (green line) and computing it's S11, then de-embedding it from the port of the connector to remove it's affect.  Then you don't have to cut it.  A model of the line should get most of the effect included.

[tehtehteh]

This sounds like a great suggestion but I already have no idea what I'm doing!

[antenna]

That green trace represents about 7 degrees of open transmission line, or about 1pF of capacitance which is like 366ohm capacitive reactance at that frequency. Again, you can eliminate the worry and attach the VNA where C9 goes if you really want it perfect, because then you are adjusting the antenna so that the input of the little green trace is 50ohm.

What are you using to connect the vna to the board? A temporary SMA connector?

[tehtehteh]

J1 is a U.FL connector that I planned to leave in place so that I didn't affect the tuning by removing it.

[antenna]

Then what about temporarily placing a 50ohm resistor where C9 goes (terminating the green 50ohm trace into a purely resistive 50ohm load), and, using the U.FL connector in its intended spot, you tune the antenna for a reading of 25ohm on the VNA. When the antenna is 50ohm, it will be in parallel with the green trace which is also terminated in 50ohm thus putting two 50ohm loads in parallel at J1 to give a VNA reading of 25ohm. Once you disconnect the VNA and remove the resistor from the C9 pads, the beginning of the green trace should present a 50ohm load for the matching network.  To test everything afterwards, once the 50ohm resistor is removed from the C9 pads, your VNA at U.FL should then read (50-j[whatever capacitative reactance the green trace adds looking north]) rather than 25ohm, which will then be correct once the green trace is connected to the radio by installing L2 and C9.

If you really want to get serious, you could also have a spare U.FL connector handy so that once you do an OSL calibration to the end of the coax, you can then clip on the free U.FL connector and do a port extension to eliminate the minor effects the connector may have on the reading at the trace itself.  However, I do believe we are getting too fussy as the matching network C9 and L2 will likely be standard value parts and not offer a perfect match to 50ohm anyhow.  Also, the size of that connector is completely insignificant at that frequency and you could remove J1 without complication if you wanted.  Remember, a 1.5:1 SWR will have no noticeable effect on performance, so I would go with simplicity over chasing perfection here. 

[tehtehteh]

It's all getting a little bit complicated for me. I haven't really grasped complex numbers and the smith chart, or even impedance fully. I haven't found any resource that explains them in a way that clicks with me yet.

I've muddled my way to a working circuit and very close tolerance carrier wave using mainly the typical application circuit from my transmitter IC's data sheet, and using this application note from microchip: http://ww1.microchip.com/downloads/en/appnotes/00868a.pdf I was hoping to complete the last piece of the puzzle with the antenna.

[antenna]

It's all getting a little bit complicated for me. I haven't really grasped complex numbers and the smith chart, or even impedance fully. I haven't found any resource that explains them in a way that clicks with me yet.

If nobody beats me to it, and I hope they don't, I am in the process of writing a document called Un-complicating Complex Numbers, 100% by me (no links to chase, no references to find), a person who has never had an electronics mentor, never had a class or curriculum teaching things in the proper order and I'm making it all without a single reference to anyone elses work. I am a person who, born in the 80's, never saw a toy survive my curiosity. I got my first soldering iron burn at the age of 7 and my first deadly oil burner transformer at 14. Give me until tomorrow and I'll post a pdf that will demystify it for anyone!  I've got about 4 hours into it already and it touches on all the stuff us self-learners tend to not catch along our journey towards RF. I look forward to having it criticized and improved by the members of this great forum and hope that it proves useful for people like me who hit a brick wall at the same spot you have!  I'll be up for a few hours, be patient. In a day, complex numbers and smith charts will make total sense!

[tehtehteh]

 

[antenna]

I got it wrote, Now I just need to do the illustrations and compile into a pdf. I'm headed to bed now but I will wrap it up first thing as soon as I wake up.

Edit: 10:45 CST, about 90% done. Still workin' on it.

[antenna]

Here it is, as promised. I hope it helps!  It took longer than I thought, sorry.

Anyhow, I typed it in microsoft wordpad (because there are no page breaks to fit images around), but had to save it as an openoffice document because *rtf doesn't want to attach. I recommend opening it in wordpad for the same reason. There is no telling where the page breaks will appear if opened in office, and if someone wants to make that into a pdf and straighten things up in that format, feel free!  Wordpad is nice because it just scrolls and scrolls with no page breaks! 


----------There is a couple errors, one happened in paint moving numbers on the resistance line, gonna have to edit. --------
-------------got it fixed, sorry---------------

[antenna]

had to fix an another error with the sin part (was half asleep when I did that).  Ill repost when I get it corrected

[tehtehteh]

Much appreciated, I feel I got several steps further on the path to understanding.

[hagster]

Another option for measuring this type of circuit in a finished board is to use a switched coax connector. These disconnect the through line when a connector is connected.

https://www.murata.com/en-eu/products/connector/switchconnector

[tehtehteh]

Yeah I've seen those, they probably would be perfect but I couldn't find much about the probe to use with it. I have U.FL test leads already.

[antenna]

Much appreciated, I feel I got several steps further on the path to understanding.

Glad it helped. Apologies for having to correct the math twice where I explained making the reactance curves. I haven't done that kind of math in a while and my memory is slowly failing me.  I had to sit down and think about that one!

[hagster]

They sell short probes also to convert to SMA.

[tehtehteh]

Do you have a link to these? If for any reason I have to do another iteration of the PCB I would change the connector to this.

[tehtehteh]

Well I got it all put together, VSWR of about 1.48, seemed ok, but it just doesn't transmit over more than a few inches.

[antenna]

What does the rest of the antenna look like?