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Not just a quarter wave of coax in a mismatch. Any additional length of
coax or transmission line with SWR (mismatch) will produce modification of
impedance. Play with SimSmith using nothing other than additional series
lines in a mismatched condition. It's a good educational tool as well as a
design and evaluation tool.

Installation of ferrite beads and/or choke balun at the feedpoint of a
balanced load will affect the impedance as, without those, the outside of
the coax shield is part of the radiating structure.

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

There is not much mystery here. Transmission lines that are mismatched at all create impedance transformations at every odd multiple of a quarter wavelength. They become transparent, meaning the impedance at the input is exactly the impedance at the output, at even multiples of a half wave.

Calibration at the input allows you to see the same load your transmitter sees. Calibration at the output will show the actual impedance of the radiator.

Ferrite beads and baluns do not affect either measurement unless they introduce loss or an impedance transformation. Baluns especially are misused as matching devices because, when not properly deployed, they introduce loss. Loss reduces reflected power by a 2X factor and lead to grossly misleading VSWR readings at the input side of the coax.

The nanoVNA is a wonderful tool for both.

Warren Allgyer - WA8TOD

I also found them difficult to connect. You must be "right-on" and give a straight push and you'll get a "snap" into place. They certainly are not connectors that should be connected and disconnected repeatedly! I tried several sections of the card. Some were easier than others (nearly impossible). I chose to return the item as being almost useless!

Looks as NoMachine would allow access to RPI desktop , if I can get it installed.... wants to install package but does not tell you
have to change folder....
However, I do not want to overstate my welcome here , I made a reference to RPi as BTW there is another way to skin a cat.
Let's not turn this into RPi discussion.

On Sat, Jul 24, 2021 at 11:35 AM, Andrea IK0RNT wrote:

What could be the resaon for this nightmare?

Hello Andrea,
The first U-FL connection is very stiff.
So, you must center very good the connector.

I have made some documentatio about that RD Demo Kit, see:

I hope it helps.

73, Rudi DL5FA

I've got the "RF Demo Kit for NanoVNA-F" by Sysjoint. Probably I never succeded so far in inserting any of the two U-FL connectors supplied. I also broke one socket on the board, fortunately it is the one of the "open circuit". It seems the connector just does not fit the socket, no matter how much special care I use in aligning the parts. What could be the resaon for this nightmare?

Thanks in advance!


Andrea

On 7/23/2021 6:11 PM, Anne Ranch wrote:

IMHO this is the most intelligent , no nonsense article about "how to Bluetooth" .
Do not let "raspberry " distract you and SKIP all the GUI stuff , go down to command line.



PS
It is little vague describing "pairing " and its result as "connected" - then it goes on to how to connect.
( My attempt to "connect" failed - assuming because to do not have no serial I/O hooked up.
I wonder if I physically connect the TX /RX between the HC-05's - sort of "loopback " ? )
Somebody in this thread mention term "coupled " as a result of pairing. Seems more fitting.

Anne, you can have a remote desktop experience if you prefer using either RealVNC or NoMachine

<>

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RealVNC works with Raspberry Pi if you enable it using Raspberry Pi Configuration app then you can just connect to it via IP Address, so long as the Pi is on the same subnet. You can also use it outside of your LAN via creating an account and registering the Pi on your account.

NoMachine has a Raspberry Pi downloads on there website. I've not tried using NoMachine outside of my LAN, so not sure how simple it is to use that way.



--
All the Best & 7 3s
Dale Miller, KC2CBD
Cookeville, Tennessee
Putnam County TN ARES
Emergency Coordinator
Vice President Cookeville Repeater Association
Ham Operator since 1997 (Extra)

stpatrick2@...
Registered Linux User: #317401
Linux since June 2003
Registered Ubuntu User #26423

--

IMHO this is the most intelligent , no nonsense article about "how to Bluetooth" .
Do not let "raspberry " distract you and SKIP all the GUI stuff , go down to command line.



PS
It is little vague describing "pairing " and its result as "connected" - then it goes on to how to connect.
( My attempt to "connect" failed - assuming because to do not have no serial I/O hooked up.
I wonder if I physically connect the TX /RX between the HC-05's - sort of "loopback " ? )
Somebody in this thread mention term "coupled " as a result of pairing. Seems more fitting.

After few detours I can SSH to my RPi.
Now I have to re-learn now to navigate RPi using commands. Interesting how one forgets stuff after not using RPi for over a year.
The Bluetooth "manager" on Ubuntu is pretty flaky, hope upgraded / updated OS on RPi will work better.
I am sorry I took this detour discussing my RPi bluez - pun not intended.

There are videos on YT on how to measure characteristic impedances of stripline and coaxial cable.One I watched used a 50 ohm pulse generator on 75 ohm coax.I was wondering whether making the generator 75 ohm instead of 50 makes a difference

Dave - Thanks for the additional analysis and plots which are all very interesting. I have noticed the sensitivity of everything involved in these antenna measurements.

I had thought about the SO-239 impedance bump. The 6 inch SMA to SO-239 adapter at the NanoVNA Port 0 connector would introduce that bump when calibrating at Port 0. It should not be a factor when calibrating at the end of the coax. However, the short SO-239 to SMA adapter used for calibration at the end of the coax would add a small discontinuity as you mentioned. Note also that the dipole is built on a SO-239 connector. One element is soldered to the center pin. The other element is soldered to a small ring terminal and attached with a small nut and bolt.

I took Jim's suggestion and measured the phase shift with no connection at the end of the RG-58A/U coax. The frequency was varied to determine where the transmission line is a half wave. The result was approximately 22.65 MHz. I also saw that lower frequency difference between peaks after reducing the width of the sweep.

The 24 MHz I mentioned earlier was the result of having a 300 MHz sweep and only 101 data points. The resolution is 3 MHz per data point.

Your transmission line may be a half wave at a somewhat different frequency which would affect the comparison.

My NanoVNA-H4 does provide 401 data points at lower frequencies but for some reason it only provides 101 data points for the sweep above 200 MHz.

73, Kent
AA6P

Indeed Larry. Yes, hearty thanks is also due to those who set up and maintain this user group, and to all those, like yourself, who patiently contribute their time and expertise helping others with problems and questions. This group is a great information resource and necessary support for new users to successfully use the hardware and software.

Kent - Based on the additional information you've provided, it's possible that the SWR wobble in your 1st plot is due to the impedance bump in the UHF-SMA adapter with additional contribution from your RG-58A/U.

I created another SimSmith model with load data from the ENZEC antenna mentioned earlier (Inv vee, 50+j0 at 340 MHz, 200-500 MHz sweep). I added three sections of transmission line per your descriptions:

14.83 ft of Belden 8219
0.6 inch section to represent the UHF-SMA adapter
6 inches of generic 50-ohm coax.

Belden 8219 uses foam dielectric (velocity factor 0.73) and has a characteristic impedance of 54 ohms. This may or may not be the same as your Berk-Tek cable which you said has a foam dielectric. The difference between 50 and 54 ohms is enough to put a noticeable wobble in the SWR chart.

Why? If the load impedance and transmission line impedance do not match, the transmission line will transform the load impedance. If you view transmission line behavior on a Smith chart, the impedance transformation path rotates in a circle centered on the characteristic impedance of the transmission line. The degree of rotation depends on length. One full rotation occurs every half wavelength. (Loss also makes the path spiral inward, but let's hold that thought.) If the coax impedance is 50 ohms, the impedance transformation will rotate around that value, and the transformed result will stay the same distance from the 50-ohm Smith chart center reference. For example, start with a 25+j0 ohm load and add 50-ohm transmission line in 1/8 wavelength increments. First we get 40+j30, then 100+j0, then 40-j30, then back to 25+j0, then it repeats. All these impedance values have the same SWR because they are all the same distance from 50+j0. Now do the same thing with 54-ohm coax. The rotation will be centered on 54 ohms, but somewhat eccentric relative to the 50-ohm chart center. One side of the rotation will be a little further from the center. The other side will be a little closer. The SWR value will wobble up and down and repeat every half wavelength. In general, the more the transmission line impedance differs from 50 ohms, the greater the eccentricity, and therefore the greater the wobble amplitude.

So maybe your your coax is 50 ohms on the dot? Maybe it's 54? If it's 54, it contributes some SWR wobble.

UHF connectors are known to create an impedance "bump" of about 35 ohms. Inserting a 0.6-inch section of 35-ohm coax in the SimSmith model produces an SWR plot very similar to what you saw. It's remarkable that such a short section could have this much effect, but at these frequencies it makes a difference. It's clearly enough in this case to nudge the eccentricity of the impedance rotation off-center. ?

Plots are attached. There are numerous caveats and nuances to the description above. Hopefully it still gets the main points across.

73, Dave
NU8A
P.S.
The velocity factor doesn't cause the SWR wobble, but it does determine the period .

The "Talking BT to a Pi" thing is out of my experience, so I can't help you there. Never heard of anyone doing "ssh to Pi over BT", and I'm not even sure that is a thing, so I'll stop. 8*)

Finding the Pi on the LAN is pretty easy, as I believe you have Linux.

nmap -sn 192.168.0.* | grep Raspberry -B2

[Use your LAN numbers above. You might have to install nmap, but that appears to be widely available.]

Alternately, if it's on the LAN and you have a Linux or macOS machine, and (I think you said) your device is named "ARM", you should be able to just:

ssh pi@...
though depending on how you changed the name, the default:
ssh pi@...
might (still) work.

73, Willie N1JBJ

It does identify my RPi with name, but I do not know the passcode .

I am still learning about Bluetooth - especially how it is initialized on power-up.
Now I am comparing behavior of HC-05 , got two of them powered up, and RPi as far as OS goes accessing them.

That is the key - how OS responds to available commands.
Long time ago I started with Bluetooth by studding "BlueZ" library and using some software examples.
Then I learn that Linux uses "BlueZ" as standard to work with Bluetooth technology.
From there I know , for sure, that Bluetooth device has an option "to be discoverable" which means the device will
allow / respond to inquiry passed over the RF, NOT via physical I/O connection .
Now this all sounds to convoluted , but if one keeps in mind that communication is BETWEEN two points , it is not that hard to visualize.

Ok, end of sermon.
These are the facts
1. OS , irregardless what mechanism is used , "finds" all nearby Bluetooth devices and identifies them by name
2. I have "boom box", two HC-05 , my FT857 CAT and ONE "ARM" , and which is the name I have given to my RPi
when I had setup to work with it directly.
3. My RPi is now powered only, no mouse , no keyboard , no monitor. But I forgot the IP and having a heck of a time
identified it so I can SSH to it when I need it.

From all this - I am having a hard time accepting ( because how it is setup internally as client /server ) that I cannot access the RPi same as the other Bluetooth devices and pass a code to the it - if I know what it is.

I guess I need to find the IP address and SSH to the RPi and work on the Bluetooth from RPi direction .

73 Vaclav AA7EJ

My new NanoVNA-H is working very well - it is an impressively capable, wonderful, little device!

I send along very big Thank-You's to edy555, HuGen and those who provided earlier designs for the hardware, to DisLord for new firmware, to Rune Broberg for the great NanoVNA-saver software and OneOfEleven, Cathy for the NanoVNA-App software that makes the hardware more practical and a delight to use and to all those others who have and continue to support this project. You have created a remarkable package that provides utility and pleasure to many people.

Best wishes to all of you.
Thanks to everyone who posted their work here and on other sites
Gyula HA3HZ
--
*** If you are not part of the solution, then you are the problem. ( ) ***

I just could not get it to work right on manual calibration. The main issue is the sweep for 401 points is very particular and "fiddly", maybe more so on the wide sweeps. In any case, I just kept banging on it to sweep 401 points and once I avoided the errors, proceeded with the calibration helper feature. Sometimes it would fail one of the calibrations, but I just kept retrying and eventually got it to progress.

Also during verification of each calibration I got some intermittent wild screen behavior running ths S11 & S21 LogMag chart.

If anyone is interested I have about two full days of testing, so if I have any helpful data I would be happy to elaborate.

Seems to be working pretty well at this point. Windows 10 64 bit OS.

Thanks to all,

Don

If its silver and has a pin, it's your 50ohm load.

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On Thu, Jul 22, 2021 at 10:56 PM Syd via groups.io <nhuq1=
[email protected]> wrote:

I have 2 bronze colored and 1 silver colored terminators. None of them
measure 50 ohms! One of the bronze colored terminators has no center pin,
so I am assuming that it is the open circuit terminator. A 50 ohm SMA
terminator costs $4.45 on EBay.
wt1v

I still think complaining to the seller is the right way to go.
Bob

I have 2 bronze colored and 1 silver colored terminators. None of them measure 50 ohms! One of the bronze colored terminators has no center pin, so I am assuming that it is the open circuit terminator. A 50 ohm SMA terminator costs $4.45 on EBay.
wt1v