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Hi Barry,

I agree with Arie above.

I wonder if you have seen the slides from my 4 hour presentation on the NanoVNA (some stuff is probably outdated now).


(they are also available in the Files section here)

If you can use anything from those slides, feel free to "steal" anything you want.

73,
Luc ON7DQ / KF0CR

Howard and Rudi,
I have gotten the SWR for the DIY 70watt HF Amp kit to work with a Hermes HL2. It took a 16ohm 5watt or higher resistor across the secondary of the input transformer. If you look closely at some of the minipa100 amp pictures that use the same basic circuit, you will see the 16ohm resistor. This makes sense since the input transformer is a 2:1 ratio or 4x impedance. There is some impedance from the rest of the gate input circuit, so the 16ohms makes sense.

Also, there is a webpage with details on the build and then test and optimization of the amp here:


That reference shows 15ohms as the optimum which would be a reflected 60ohms IF there were no other impedances connected.

The other alternative is to put a Pi resistor pad on the input. This is a good solution for higher power rigs like the ?BITX where you also need to cut the input power to 5 watts.

Be sure to add the low pass filter that is needed on the output of the amp. It is difficult to get the bias just right to not have some harmonics above the allowed levels.

Hope that helps.
73
Evan
AC9TU

On Thu, Feb 11, 2021 at 02:46 PM, daniebjoubert wrote:

John. Great idea.
I will try with my smart phone.
I am also going to try and do a screen p

Hi John,
Attached the screen print before it disappears.
I am grateful for the assistance.
Danie ZS1DBJ

Hi Barry,

It's getting more beautiful every new version, two thumbs up.


About the Smith Chart and losing the audience.

If I explain the Smith Chart to my children (read: listeners) I start telling them that the Smith chart shows the reflection of the sent signal. So how big is the reflection (0-100% compared to the sent signal) and under what phase (+/-?? 0-180dgs) did we measure it. Then what happens if the same load was a bit further away; what would happen to the phaseangle. This is a first eye-opener.

Next: The reflection percentage easily transforms to SWR as a next step. What SWR is 100% reflection, what is SWR at 0% reflection, and at 50% reflection? Where is that on the chart?
And then the formula about relation of SWR and reflection coefficient can be demonstrated with simple ohmic loads leading to SWR=1, 2, and 3

Next comes the part: What Impedance caused that reflection that was measured in the first place.
First introduce simple complex numbers R+jX, what they mean and show some examples. Only after understanding this, I introduce the different Smith chart impedance circles slowly.
Then the forst demo: measure a potmeter on 10MHz and change its value, same for variable inductor and variable capacitor.

This way of introcing the Smith chart is something that a normal ham can use to build on his knowledge he or she already has. Most hams are interested in the nano for SWR measurements and antenna tuning in the first place and getting an impedance of 50 ohms with a tuner.


I've given several presentations about the Smith Chart only. That takes me about 1,5 hours from the start to finish. Last part of the presentations is about? how tuning networks ( like L, pi, of T) behave in the Smith chart and why. Of course I use the nanoVNA as measuring equipment for the demo's.
I've done this live (before COVID) and also online. Everybody had a good understanding of the Chart and could make use of it afterwards.


So.... starting with a Smith Chart with impedance circles is a too big leap. Also folding the rectangular system onto a circle. Might be so, but that kind of math does not help a ham.


My two cents worth. Hope I gave you new ideas.

73 to all and stay safe.

Arie PA3A


Op 14-2-2021 om 17:07 schreef Barry Feierman:

EXAMPLE from DAYS PAST: In the TTL days of logic long past to newbies, it
was common practice to place a black, CK05 (0.1 ?F) capacitor at each end
of a row of logic chips. This was placed between Vcc and return. In those
days, we seldom considered self resonance of a passive device. The
intended use of them on the boards was to keep logic switching noise off
the DC rail. Turns out those CK05 capacitors which peppered our boards
became self resonant somewhere between 1 and 2 MHz, usually around 1.4 to
1.6 MHz. Therefore, above self resonance, they became DC-blocked
inductors. Now, ask yourself: "is an inductor good at bypassing rail noise
as the capacitor was intended to accomplish?" NO!

Several decades ago, I had the privilege of issuing a couple of new-hires
from Kent State into the real world of 'parasitic component' behavior (the
real world they would have to live in). I was given the task as they would
specify totally unrealistic component values for designs and had only an
understanding of the ideal behavior of electronic components. I asked
them to check out a small handful of those CK05 capacitors from engineering
stock. We warmed up the HP impedance meter of the time (the one that had a
tunable drum as a frequency indicator and topped out at 110 MHz). Sure
enough, *EVERY* CK05 capacitor went purely resistive between 1.4 and 1.6
MHz and inductive above that. Their eyes bugged out. They could not
understand or comprehend how a capacitor could possibly become resonant
(+jX = -jX) and ultimately become an inductor. I sent them back to their
test books and pointed them in the direction of our local building
library. It took them a week of digging, but they finally came back with
the classic capacitance to resistance to inductance curve with frequency.

Dave - W?LEV

On Sun, Feb 14, 2021 at 2:26 PM Manfred Mornhinweg <manfred@...>
wrote:

Ed,

What I am trying to
understand is how one relates the marked value of a component to what

nanovna

shows.

The value marked on a component is supposed to be valid at a very low
frequency. As soon as the frequency goes up, parasitic effects become
increasingly important, so the actual value of the part varies with
frequency. Also, of course, there is always a tolerance. With good
capacitors and air-core inductors this tolerance might be only 5%, but with
ferrite-cored inductors it can easily be 40%, and with some sorts of
ceramic capacitors it can be even larger.

So, rule #1 is to measure on the lowest frequency possible, and rule #2 is
to never forget that the marked value is subject to a tolerance.

But then the characteristics of the measuring instrument come into play.
The NanoVNA is natively a 50? instrument. It should produce the best
accuracy when measuring impedances reasonably close to 50?. When the
impedance gets close to zero, or into the kiloohm range, the accuracy of
the NanoVNA drops. So, if you are measuring small values of capacitance or
inductance, measuring at the lowest frequency the NanovNA supports might
produce poor measurement accuracy.

So, rule #3 is to measure at a frequency where the impedance of your part
is at least close to the order of magnitude of 50?.

In practice that means that you should look at what frequency the part has
a reactance of 50?, and then measure at a frequency a few times lower. If
the measured value is reasonably constant over the range between those two
frequencies, then probably you have a valid measurement. If instead it
varies all over the place, it probably means that there are too high
parasitics even in that frequency range.

Of course I'm assuming that you properly calibrated the NanoVNA, putting
the short and the load exactly at the same place where you then put the
part to be measured.

In my experience it's best to use the shortest possible connection between
the NanoVNA and the part under test. It seems that correct measurement of
difficult impedances through a long piece of coax cable is harder for the
NanoVNA, even when carefully calibrated through that long cable.

Always keep in mind that if you do all this and get a consistent result,
then you are getting the low frequency value of the part you are testing,
and that at higher frequency its actual value will change. At a high enough
frequency a capacitor becomes a short circuit, further up it becomes an
inductor. And what's an inductor at low frequency will become an open
circuit at some high frequency, and a capacitor beyond that. Generally both
inductors and capacitors will rise in value, when you start going up in
frequency starting from a low frequency. But core materials tend to
decrease their permeability beyond some frequency, and this effect can win
over the other in some cases, so you might see cored inductors whose value
goes down rather than up, when increasing the frequency. These are all real
effects, not measurement errors! In RF work you often need to measure each
part at the frequency you will be using it, rather than trusting the value
printed on it, which is valid only at low frequencies.

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

On Sun, Feb 14, 2021 at 05:04 PM, Howard Fidel wrote:

I found that the 70 watt kit on eBay

Hello Howard,
Do you have this one bought?


I could not make it to work.

Instead I build this 45W kit:


73, Rudi DL5FA

* Nano VNA overview by K3EUI Barry *

Each time I give a "talk" on the Nano VNA I learn something new. I've given five talks now, and from the questions from listeners, I know where the hurdles are.

1) Calibration of the VNA - where you do the calibrations matters. Why?

2) Impedance varies with coax feed line length... but why?
And (mostly) SWR does NOT change with small feed line changes - why?
This is a key concept, not easily seen by most of us.
The "reflection coefficient" is established right at the antenna-feed line junction.

3) SWR vs. RETURN LOSS
They are both saying the same thing, with different units (SWR has no units, RL is in dB)

4) The magic of the Smith Chart: can we get over the anxiety? Think of it like a DART BOARD
An antenna can have some RESISTANCE and some REACTANCE.
The Smith Chart just shows you R and X, for each frequency you plot.
Of course, the Smith Chart does not show you POWER output, but does show SWR indirectly by how far
your data are from the Bull's Eye (if standardized at 50 ohm).
I included a few slides to illustrate this concept - maybe lost my audience here.

5) Resonance - when measured at the END of your coax, if not 1/2 wavelength, or multiple of 1/2 wavelength
When the reactance is zero, this is resonance.
But the resistance may not be close to your coax's 50 ohm. That sounds simple enough.
But resonance is NOT (necessarily) where the SWR will be lowest as measured in your shack.
A one-wave dipole, center fed, won't match to 50 ohm coax, but it might be "resonant".

6) What happens when you add an "Antenna Tuner" in your shack? The rig sees a SWR of 1:1
but meanwhile, what is happening on your feed line to the antenna?
Is the "forward power" really greater than the power emerging from the rig? Yikes... magic?

Comments on my latest (but not last) PPT will be appreciated by me.
I now have some experience with the S21 (two port) measurements of filters.
My next project: investigate the isolation of a 2m/70cm diplexer (Comet) with nano VNA

TU for all of the feeback off list
Feel free to share this pdf,

de k3eui Barry

I use my nanoVNA to test the input swr. I found that the 70 watt kit on eBay has a high SWR at the input on some bands. I am in the process of optimizing that now.

Clyde Spencer wrote:

CAPACITANCE AND INDUCTANCE ARE SPECIFIED AT A GIVEN FREQUENCY. See the
example.

The chart says the frequency it is measured at.
Not how much inductance varies with measuring frequency.
I have carefully measured normal HF range inductors with a dip oscillator and SDR at working frequencies comparing with a meter that measures in the 100Khz range.
I noticed no significant difference.

Alan

CAPACITANCE AND INDUCTANCE ARE SPECIFIED AT A GIVEN FREQUENCY. See the
example.

[image: image.png]
*Clyde K. Spencer*



On Sun, Feb 14, 2021 at 9:51 AM Manfred Mornhinweg <manfred@...>
wrote:

Adding to my reply:

The reactance of 470nH at 50kHz is barely 0.15?. You cannot expect the
NanoVNA to measure that accurately. You need at least 5MHz or so to bring
the reactance up into a range where the NanoVNA is pretty accurate. A small
SMD inductor like that one shouldn't have much parasitic capacitance
effects yet at that frequency. 5MHz is "low frequency" in this case.

SMD inductors typically have pretty low Q. You need to make sure that you
are not mixing up resistance with reactance. Use the RLC function.

Also SMD inductors typically have ±20% tolerance, so that can explain at
least a part of the discrepancy you see.

Adding to my reply:

The reactance of 470nH at 50kHz is barely 0.15?. You cannot expect the NanoVNA to measure that accurately. You need at least 5MHz or so to bring the reactance up into a range where the NanoVNA is pretty accurate. A small SMD inductor like that one shouldn't have much parasitic capacitance effects yet at that frequency. 5MHz is "low frequency" in this case.

SMD inductors typically have pretty low Q. You need to make sure that you are not mixing up resistance with reactance. Use the RLC function.

Also SMD inductors typically have ±20% tolerance, so that can explain at least a part of the discrepancy you see.

Ed,

What I am trying to
understand is how one relates the marked value of a component to what nanovna
shows.

The value marked on a component is supposed to be valid at a very low frequency. As soon as the frequency goes up, parasitic effects become increasingly important, so the actual value of the part varies with frequency. Also, of course, there is always a tolerance. With good capacitors and air-core inductors this tolerance might be only 5%, but with ferrite-cored inductors it can easily be 40%, and with some sorts of ceramic capacitors it can be even larger.

So, rule #1 is to measure on the lowest frequency possible, and rule #2 is to never forget that the marked value is subject to a tolerance.

But then the characteristics of the measuring instrument come into play. The NanoVNA is natively a 50? instrument. It should produce the best accuracy when measuring impedances reasonably close to 50?. When the impedance gets close to zero, or into the kiloohm range, the accuracy of the NanoVNA drops. So, if you are measuring small values of capacitance or inductance, measuring at the lowest frequency the NanovNA supports might produce poor measurement accuracy.

So, rule #3 is to measure at a frequency where the impedance of your part is at least close to the order of magnitude of 50?.

In practice that means that you should look at what frequency the part has a reactance of 50?, and then measure at a frequency a few times lower. If the measured value is reasonably constant over the range between those two frequencies, then probably you have a valid measurement. If instead it varies all over the place, it probably means that there are too high parasitics even in that frequency range.

Of course I'm assuming that you properly calibrated the NanoVNA, putting the short and the load exactly at the same place where you then put the part to be measured.

In my experience it's best to use the shortest possible connection between the NanoVNA and the part under test. It seems that correct measurement of difficult impedances through a long piece of coax cable is harder for the NanoVNA, even when carefully calibrated through that long cable.

Always keep in mind that if you do all this and get a consistent result, then you are getting the low frequency value of the part you are testing, and that at higher frequency its actual value will change. At a high enough frequency a capacitor becomes a short circuit, further up it becomes an inductor. And what's an inductor at low frequency will become an open circuit at some high frequency, and a capacitor beyond that. Generally both inductors and capacitors will rise in value, when you start going up in frequency starting from a low frequency. But core materials tend to decrease their permeability beyond some frequency, and this effect can win over the other in some cases, so you might see cored inductors whose value goes down rather than up, when increasing the frequency. These are all real effects, not measurement errors! In RF work you often need to measure each part at the frequency you will be using it, rather than trusting the value printed on it, which is valid only at low frequencies.

On Sat, Feb 13, 2021 at 07:48 PM, Cierra wrote:

use a nanoVNA to test a transmitter

To over simplify a bit, the nanovna is a transmitter. It also has a sensitive meter circuit inside, which would probably be destroyed by power from an external transmitter such as your kit. A poor, rough comparison would be like using an ohmmeter to check your house power wiring while the power is still on.

But you CAN use it to check parts and circuits inside the transmitter while the the transmitter is not turned on (not connected to a power supply of any sort). Is that what you meant?

--
Doug, K8RFT

The short answer is NO you still need a dummy load or an attenuator that can handle the power from the transmitter. Then VNAs are not really designed to do spectral analysis of transmitter output. VNAs measure the characteristics of antennas, filters, and with the correct attenuator, amplifiers. They need to control the source frequency being used to measure the characteristic. A Spectrum Analyzer (SA) can measure the output of a transmitter for signal characteristics by scanning the frequencies that the receiver in the SA measures. An inexpensive SA would be the TinySA.

So a VNA is measuring the network being tested by adjusting the stimulus feeding the network and measuring the result. A spectrum analyzer scans the frequencies in question and measures what it sees regardless of what the device is sending.

Both have limits on how much power can be applied to the input before overload and damage occurs. That is why you need a dummy load and/or the proper attenuator.

You can look up NanoVNA and TinySA on the internet to find out more about these instruments.

73
Evan
AC9TU

This might be a dumb question, but I wanted to know if you can use a nanoVNA to test a transmitter built from a PCB kit (like instead of a dummy load). I realize VNAs are usually used for antenna applications, but I was just curious.

Hello Ed,

Curious, if you go to item13 on your card, take that reading at 50 kHz and SUBTRACT the L value from the reading at 50 kHz done at item8... Do you get 470 nH? A reading of 700 nH at 50 kHz points to a serious cal issue or a fixture parasitic that is begging to be removed as well understood.

I suspect that your L is not that far off or out of tolerance by nearly 100 %.

Alan

HI,
I use a different approach when I need to know the exact value of inductance or capacitance, I build up a simple jig on a PCB mount connector with a series 50 ohm chip resistor then put the component in series to ground.? With the standard Xc or Xl formula determine the frequency where the value is between 25 to 100 ohms.? I use 1 or 10 MHz and using the formula in reverse extract the true value of the component.? Works very well.? It tells you the net value with all parasitic errors normalize in. So that the VNA reads Z50+/-j.
Mel, K6KBE

Not exactly like this, but see:

and

On 2/13/21 6:39 AM, Ed Krome wrote:

Thank you for the responses. I think I was unclear. What I am trying to understand is how one relates the marked value of a component to what nanovna shows. Using DL5FA’s item 8 inductor, my devices show 700 nH @50khz. But at 100mhz, the reading is 1uH. But the component is speced as 470nH, a value I can’t see anywhere, regardless fir frequency. How should I be reading this thing?
Thanks!

There is always some parasitic C around, which usually "reduces" (or cancels) some of the inductance - sort of the opposite of what you're seeing - but in any case, the amount of cancellation you get is frequency dependent (since the VNA is measuring X, and then converting that to nH)

Are you allowing for the inductance of the leads going to the component.? A handy thing is "about 1 nH/mm or 1 uH/meter" for a single wire.

Alan, thank you - that fixed the issue!

Dave