开云体育

Just for fun, I fed my original post into ChatGPT to see if "she" had any insights, and here was "her" response.....a fun exercise..

"What you're describing is a fascinating experiment! It sounds like you're observing the effects of an asymmetrical antenna design, and there are a few key factors to consider that could explain the differences in the SWR and Smith charts when you reversed the dipole legs.

1. **Non-Balanced Antenna and Common-Mode Currents:**
Since the antenna is a form of off-center fed dipole (OCFD), it is inherently unbalanced. An off-center fed dipole typically has a significant imbalance in the current distribution between the two legs, which results in common-mode currents on the shield of the coaxial feedline. The role of the coax shield becomes crucial, as it can have a different effect on the antenna impedance depending on the configuration of the dipole legs.

Reversing the two dipole legs changes the relative phase and current distribution between the two antenna legs. As a result, this can alter the electromagnetic field around the antenna, affecting the impedance seen by the transmitter and causing different interactions between the antenna and the coaxial feedline. The coaxial cable’s shield could be acting differently in each configuration, either radiating or picking up different signals depending on how the antenna’s currents are distributed.

2. **Impact of Asymmetry:**
In an ideal balanced antenna (like a center-fed dipole), the two legs of the antenna are of equal length and the currents in each leg are balanced. However, in your case, the legs are of different lengths and are presumably attached to the transformer in a way that creates an imbalance. When you reverse the legs, the imbalance changes, which likely affects how the antenna is coupled to the environment and the transmitter.

In terms of impedance matching, reversing the legs could change the impedance seen at the antenna terminals. This could be why you observe different SWR, return loss, and Smith chart behaviors — it's reflecting a change in the impedance that the transmitter sees. This effect could be accentuated by the unbalanced nature of the antenna and the non-ideal placement of the coaxial shield.

3. **Coaxial Shielding and Proximity Effects:**
Since your coax shield is short (4 feet) and potentially has significant current on it due to the unbalanced antenna design, the proximity of the coax to the walls (and to the antenna) could influence the impedance. Even though the nanoVNA was dangling hands-free, subtle changes in the position of the shield, or how the shield is coupled to nearby structures (walls, etc.), might affect the measurements. This is especially true when dealing with an unbalanced antenna where the coax shield may not behave symmetrically in each configuration.

4. **Transformer Characteristics:**
The transformer you're using has a 6:4 turn ratio, which means the impedance transformation may also behave differently depending on how the antenna legs are connected. If the two dipole legs are different lengths, the impedance seen by the transformer secondary might change, which could lead to significant variations in the reflected impedance. This could explain the differences you’re observing in the SWR/return loss between the two configurations.

### What Could Be Happening:
- **Reversing the dipole legs likely changes the current distribution** and the balance between the two legs of the antenna. This results in different impedance characteristics and different interactions with the feedline.
- The **coaxial shield** might be more involved in one configuration than the other, leading to different behaviors in the SWR/return loss measurements.
- **The transformer’s impedance transformation** could also be affected by the dipole leg configuration, which would further explain the differences in the measured SWR.

### Suggestions for Further Investigation:
- **Try using a balun:** A simple 1:1 or 4:1 balun could help balance the antenna, reducing the impact of common-mode currents on the coax shield and potentially stabilizing the impedance.
- **Check the positioning of the coax shield:** Experiment with different orientations or placements of the coax cable and observe how that affects the measurements. Small changes in the physical setup might reveal further insights into how the shield and antenna interact.
- **Use a longer feedline:** If possible, try a longer feedline to see if the behavior changes, as the coax shield’s interaction with the antenna may be more pronounced over a longer distance.

Your experiment is a great example of how even small changes in antenna configurations and feedlines can lead to noticeable differences in impedance measurements. The combination of antenna imbalance, coaxial shield effects, and transformer characteristics all play a role in shaping the observed results."

Are you having problems. If it works all right, why bother changing. Yes, new models sure look good. Like Pro model of But if it works, don't fix it. If you mean firmware, there are so many basic NanoVna versions that I would not dare to touch any new firmware.

Have you tried measuring components. Vna is good for that, too. If you get good results, everything works?

Ali Express should be legit. But I have purchased a NanoVNA H4, TinySA and
TinySA Ultra as well as many other products from R & L Electronics. They
are legitimate and reliable and have great service.

Zack W9SZ

On Tue, Mar 18, 2025 at 7:20?AM k6whp via groups.io <k6whp=
[email protected]> wrote:

Just joined and here goes my newbie message. Sorry.

(1) I have an H4 from 2020 with the 0.5.0 FW version. I am guessing that
an upgrade would be in order?
(2) Also looking into an "F" and saw some being sold on AliExpress for
102.93 marked down from $205.86 (50% off)
..is this a reliable source and product (link below) or, in not, what is
reliable?

Thank you in advance.
--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."

On Mon, Mar 17, 2025 at 03:13 PM, W0LEV wrote:

... If you used only the NANOVNA connected to NOTHING, not even holding the VNA, you might
not see as much difference.
...You need to decouple the feedline/NANOVNA/PC/laptop from the transformer!

Yeah, I've been thinking about that...although I will note that the laptop was NOT plugged into the AC adapter when I made the two measurements. What I'll try on my next visit will be to leave the nanoVNA dangling without ANY connection to the laptop, and then see if the patterns look more similar when reversing the dipole wires. And of course, I can also observe the impact of touching the nanoVNA SMA and see how that impacts the view...a classic test for common mode effects in the unbalanced system. But all of this points out the challenge of trying to capture what the Zachtek is actually "seeing" when connected to the antenna. It seems that with this type of setup it would be very difficult to get an accurate picture...not that it really matters much, though, given it's tolerance of any load.

I will say that based on the WSPR performance over the past day, reversing the dipole wires does seem to have had a positive effect (567 versus 504 unique spots for W9HIF), assuming the solar activity has been similar to that earlier in the week. And if that holds, that is consistent with the SWR pattern showing a lower average for the higher frequency bands...even if that means the transmission line is "participating" more to radiate some of the RF energy.

Try amazon AURSINC NanoVNA-F V2 Vector Network Analyzer 50KHz-3GHz Antenna Analyzer 4.3inch Screen

________________________________
From: [email protected] <[email protected]> on behalf of k6whp <k6whp@...>
Sent: Tuesday, March 18, 2025 3:23 AM
To: [email protected] <[email protected]>
Subject: [nanovna-users] Apologetic Intro Message

Just joined and here goes my newbie message. Sorry.

(1) I have an H4 from 2020 with the 0.5.0 FW version. I am guessing that an upgrade would be in order?
(2) Also looking into an "F" and saw some being sold on AliExpress for 102.93 marked down from $205.86 (50% off)
..is this a reliable source and product (link below) or, in not, what is reliable?

Thank you in advance.
--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."

Just joined and here goes my newbie message. Sorry.

(1) I have an H4 from 2020 with the 0.5.0 FW version. I am guessing that an upgrade would be in order?
(2) Also looking into an "F" and saw some being sold on AliExpress for 102.93 marked down from $205.86 (50% off)
..is this a reliable source and product (link below) or, in not, what is reliable?

Thank you in advance.
--
William, k6whp
--------------------
"Cheer up, things could get worse. So I cheered up and things got worse."

I would calibrate the NanoVNA again with binding posts. I would use something like 30MHz-100MHz, 900MHz is surely too much. (Like others have said.) When the Vna asks "open" I would leave the binding posts empty, "short" mean shorted binding posts and "load" means a 50 ohms resistor(or two 100ohms in parallel).
Then I would check everything with capacitors. Values around 56pF are good, like 100pF and 10pF. Have you some coils too where you know the inductance. When all these show good values (+-tolerances), you'll know that everything works.

I have used crocodile clips up to 30MHz, your binding posts have shorter leads, so they should work with 50MHz. I have noticed that with crocodile clips, test leads matter a bit, your binding posts are better.

That shows the parasitic coupling is different for each side of the bedroom HI
And as left leg lenght is different of right one, if the left side of the bedroom is ??more?? parastic than the right sine, it is logical that you don’t mesure the same thing when reversing antenna’s wires.
It would be interesting to repeat the mesure outside between two trees or two chairs HI

And yes with OCFD you need a choke just beside the transformer.

Interesting anyway. Tnx
73, Ben

Hello

To perform this type of measurement, I use two BNC sockets bolted to a printed circuit board. I solder the components. This way, I can create a shunt or a series connection. I connect the SOL to the end of the cable that plugs into the BNCs.

The NanoVN is connected to a PC running nanovna-saver.

I've noticed that I get good results with capacitors and measurements in shunt mode. I suppose this is because the pF value is indicated on the board, and that's reassuring. For coils, the results seem more random; perhaps the "coil" components are more affected by stray components than capacitors.

Try it with capacitors; that will tell you if you're on target or not.
--
F1AMM
Fran?ois

Roger,

The only thing I can think of right now, may be in relation with the fact even though I ordered my inductor / capacitor design kit for RF, the test frequency of the inductors is 100MHz. The curve from the datasheet shows it is fairly flat up to 1GHz but right above the chart it is written "typical value". Maybe I should try out component specifically tested for 800MHz as you mentioned.

Nico

Roger,

Thanks for the idea. I already tried sending them an email a while ago for that exact request ! The phone is still ringing, nobody answered yet at the other end ;( But I should throw the fishing stick once again using my email address from the office, that would look more serious... I'll let you know.

In the meantime, I've conducted one more test tonight, with interesting but a little disappointing results. arrrrgg, I'm missing the knowledge and there may be something obvious I don't get. Maybe someone has the right batteries to fit my torch !

I've cut on end of a 36" long RG316. I soldered it to the PCB. I reinstalled the pcb in its case and made the cable going through a hole on the underside. Straight out the box I wrapped the cable 3 times around a Fair-Rite 61 material that has a 0.9" inside diameter. I connected this to another 12" premade cable then to the VNA just to give me some length. That is for sure, I took the time to calibrate OSL right before mounting all that. BTW the load has been done a 50 Ohm 0603 chip resistor at 0.1% accuracy spec (at 3$ a piece !!).

Picture 1 : Test setup (the same as all my other tests)
Picture 2 : Measurement in this condition (no matching network, only a 0 ohm resitor to pass through)
Picture 3 : Data from the VNA plotted in SimSmith,
Picture 4 : Expected reading (or close) after putting a 10nH in series.
Picture 5 : Actual reading I got after installing the inductor.

Every time I tried to correct/match the antenna it gave me horrible results like that. There must be something obvious I don't see.

At the end, I do not expect to achieve the 1.5 VSWR flat as in the datasheet. It is a personal project after all. But if I could at least match it down to 2, I would be more than happy. This module will be installed in my backyard and the receiver will be at less than 50 feet. It is more a matter of learning new things. The one thing I want to avoid though, is to make it work by pure luck without knowing why.

I'm searching for a needle in the haystack, can someone lend me a metal detector please !!

Nico

The RF switch is damaged and can be restored by replacing the MXD8641.

Good point on the upper frequency limit of home brew (and some
professional) fixtures. My home brew fixture which uses alligator clips
instead of binding posts has an upper limit of 100 MHz. I seriously doubt
your fixture is much good above 100 to 150 MHz. This is why the "big boys"
charge big bucks for their test fixtures.

I hate to think what the capacitance between just the two binding posts
is! That will be a major perturbation much above 300 MHz, let alone 900
MHz. A meticulous calibration is unable to cover all fixture "sins". The
best "amateur" fixture is to solder cal. standards and test items to the
butt end of an SMA female connector - WITH ABSOLUTELY NO LEADS.

Dave - W ?LEV

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Virus-free.www.avg.com
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<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>

On Mon, Mar 17, 2025 at 9:52?PM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:

On Mon, Mar 17, 2025 at 12:29 PM, KJ5FRJ wrote:

Hello, new to this group. I've learned a lot from reading here, but i

seem to

have run into some issues and could use some advice. I'm building a low

pass

filter, and trying to learn to measure inductance using the nanovna. I've
built a test rig that I found on one of the threads here somewhere- three
pieces of double sided pcb for OSL, and I'm soldering my air core

inductors to

that. I'm trying to use the S11 shunt method for a coil that needs to be
72.4nH.

I posted about my binding post and PCB OSL a few years ago. For best
results you need to keep the slit in the middle as wide as possible and the
same for all 3 boards. You also need to connect the top side to the bottom
one with copper tape and then solder the tape to the board all the way
around. The 50 ohm load should be an 0805 SMD.

Your upper frequency is set to 900 MHz. My jig gave reliable results to
150 MHz. so I suggest you cal with this as your upper frequency.

Ive got a couple things I'm not understanding- should my coil be

adjusted for

the necessary inductance at the frequency for the filter of 50-55mhz?

And does

it need to be adjusted for impedance at that band as well? I've attached
photos- my 90deg phase and 50 ohm mark is at 112mhz, but this inductor

is for

the 6m filter.

A couple of things to mention here.
- First the S11 Phase is NOT the impedance phase. It is the phase of the
reflection coefficient (gamma). Recent versions of DiSlord firmware allow
you to select Z Phase for the trace.
- Yes you need to measure the inductance at the frequency of operation.
With an air coil the inductance will be affected slightly by the "skin
effect" and current not flowing as deep in the wire as the frequency
increases. But the biggest factor is that the NanoVNA measures the
"apparent inductance" which is different than the actual inductance. This
requires some explanation. Any inductor will have some self capacitance
due to coupling between the turns. This capacitance is in parallel with
the inductor. So the measured Xm = Xl || Xc and the NanoVNA calculates L =
2*pi*F/Xm. The end result is that the apparent inductance will increase
with frequency and at some point the self resonant frequency is reached.
After this point it looks like a capacitor. The attached plot of an
inductor I measured is attached.

Another thing I'm not understanding is the 5khz self resonance dip seen

at the

beginning of the sweep, shouldn't it be reading capacitive since it's

after

the phase reversal?

Not sure what you mean here and keep in mind you are not looking at
impedance phase in your plot.


Roger

--

*Dave - W?LEV*


--
Dave - W?LEV

On Mon, Mar 17, 2025 at 02:51 PM, Roger Need wrote:

"apparent inductance" which is different than the actual inductance

The inductance you measure at the operating frequency is the value that influences the circuit. Some people reserve the word inductance for the value an inductor exhibits at low frequencies where self-resonance effects can be ignored. Some call this actual inductance or true inductance, but since it is not the value that affects the circuit at the operating frequency, these terms can be confusing.

self capacitance due to coupling between the turns

Ever wonder why a turn doesn't short out this inter-turn capacitance? Others have, and they have come up with modern theories of inductor self-resonance:




Brian

On Mon, Mar 17, 2025 at 12:29 PM, KJ5FRJ wrote:

Hello, new to this group. I've learned a lot from reading here, but i seem to
have run into some issues and could use some advice. I'm building a low pass
filter, and trying to learn to measure inductance using the nanovna. I've
built a test rig that I found on one of the threads here somewhere- three
pieces of double sided pcb for OSL, and I'm soldering my air core inductors to
that. I'm trying to use the S11 shunt method for a coil that needs to be
72.4nH.

I posted about my binding post and PCB OSL a few years ago. For best results you need to keep the slit in the middle as wide as possible and the same for all 3 boards. You also need to connect the top side to the bottom one with copper tape and then solder the tape to the board all the way around. The 50 ohm load should be an 0805 SMD.

Your upper frequency is set to 900 MHz. My jig gave reliable results to 150 MHz. so I suggest you cal with this as your upper frequency.

Ive got a couple things I'm not understanding- should my coil be adjusted for
the necessary inductance at the frequency for the filter of 50-55mhz? And does
it need to be adjusted for impedance at that band as well? I've attached
photos- my 90deg phase and 50 ohm mark is at 112mhz, but this inductor is for
the 6m filter.

A couple of things to mention here.
- First the S11 Phase is NOT the impedance phase. It is the phase of the reflection coefficient (gamma). Recent versions of DiSlord firmware allow you to select Z Phase for the trace.
- Yes you need to measure the inductance at the frequency of operation. With an air coil the inductance will be affected slightly by the "skin effect" and current not flowing as deep in the wire as the frequency increases. But the biggest factor is that the NanoVNA measures the "apparent inductance" which is different than the actual inductance. This requires some explanation. Any inductor will have some self capacitance due to coupling between the turns. This capacitance is in parallel with the inductor. So the measured Xm = Xl || Xc and the NanoVNA calculates L = 2*pi*F/Xm. The end result is that the apparent inductance will increase with frequency and at some point the self resonant frequency is reached. After this point it looks like a capacitor. The attached plot of an inductor I measured is attached.

Another thing I'm not understanding is the 5khz self resonance dip seen at the
beginning of the sweep, shouldn't it be reading capacitive since it's after
the phase reversal?

Not sure what you mean here and keep in mind you are not looking at impedance phase in your plot.


Roger

Something is wrong. NOTHING should show outside of the Smith Chart for
S11, the green trace after a proper calibration is completed!

While the following has been mentioned many times:
1) Did you "RESET" the calibration before you started the intended cal.?
2) Once the cal was complete, did you store it in one of the registers for
convenience?
3) Once you completed the cal and before you installed your inductor, did
you go back and check for the proper position on the Smith Chart for the
open, short, and load? This is always a good practice.
Of course, all with the fixture in place and the OSL installed on the
binding posts.
***

Yes, the inductance should be measured at the frequency of application.

I presume the filter is intended for 50 to 55 MHz, and not 50 to 55
milliHertz?
***

If the filter is designed for a match at 50-ohms, it should measure close
to that when terminated with 50-ohms - both non-reactive.
***

I can not read the start frequency. But I must ask why you are sweeping to
900 MHz when the LPF design goal is a roll-off starting at 50 to 55 MHz.
Unless you are using a PC or laptop based application, you are losing
resolution as the number of measurement points, both for the cal. and the
measurement is severely limited (101 points?). As such, I do not see the 5
kHz dip you refer to.
**

In measuring the inductor (or capacitor where applicatle), I'd suggest
sweeping from 30 to 70 MHz since this brackets your intended roll-off
frequency for the LPF. Again, sweeping from "DC" to 900 MHz you will lose
a major amount of resolution where you need it.

Another caution: Most disc ceramic capacitors are pretty good below your
intended roll-off frequency. But they may or may not be good to 900 MHz.
With the VNA, verify they are still close to the intended values from your
roll-off frequency through 900 MHz.
***

Dave - W?LEV


On Mon, Mar 17, 2025 at 7:29?PM KJ5FRJ via groups.io <coreyjenkins24=
[email protected]> wrote:

Hello, new to this group. I've learned a lot from reading here, but i seem
to have run into some issues and could use some advice. I'm building a low
pass filter, and trying to learn to measure inductance using the nanovna.
I've built a test rig that I found on one of the threads here somewhere-
three pieces of double sided pcb for OSL, and I'm soldering my air core
inductors to that. I'm trying to use the S11 shunt method for a coil that
needs to be 72.4nH.

Ive got a couple things I'm not understanding- should my coil be adjusted
for the necessary inductance at the frequency for the filter of 50-55mhz?
And does it need to be adjusted for impedance at that band as well? I've
attached photos- my 90deg phase and 50 ohm mark is at 112mhz, but this
inductor is for the 6m filter.

Another thing I'm not understanding is the 5khz self resonance dip seen at
the beginning of the sweep, shouldn't it be reading capacitive since it's
after the phase reversal?

Any help is appreciated

--

*Dave - W?LEV*


--
Dave - W?LEV

for just a few nanohenry you need to make your calibration very exact ... i would not try to exactly finetune it ... i would see if i am close .. then build my filter and tune the filter with all parts installed

dg9bfc sigi

Am 17.03.2025 um 20:22 schrieb KJ5FRJ via groups.io:

you would need a choke below the transformer to decouple the shield of the coax being part of the antenna (and then it would still couple a bit to the antenna cause all couples with all in an indoor installation)

dg9bfc sigi

Am 17.03.2025 um 19:12 schrieb Bruce KX4AZ:

Hello, new to this group. I've learned a lot from reading here, but i seem to have run into some issues and could use some advice. I'm building a low pass filter, and trying to learn to measure inductance using the nanovna. I've built a test rig that I found on one of the threads here somewhere- three pieces of double sided pcb for OSL, and I'm soldering my air core inductors to that. I'm trying to use the S11 shunt method for a coil that needs to be 72.4nH.

Ive got a couple things I'm not understanding- should my coil be adjusted for the necessary inductance at the frequency for the filter of 50-55mhz? And does it need to be adjusted for impedance at that band as well? I've attached photos- my 90deg phase and 50 ohm mark is at 112mhz, but this inductor is for the 6m filter.

Another thing I'm not understanding is the 5khz self resonance dip seen at the beginning of the sweep, shouldn't it be reading capacitive since it's after the phase reversal?

Any help is appreciated

The transformer does not decouple the nanovna and PC/laptop and, therefore,
anything conductive within the house from the measurement. If you used
only the NANOVNA connected to NOTHING, not even holding the VNA, you might
not see as much difference. When you reverse the connections to the
transformer, the two sides of the OCF set of wires certainly interacts
differently with the house wiring and anything within the house/ceiling.
You need to decouple the feedline/NANOVNA/PC/laptop from the transformer!
Also realize the difference in coupling to anything in the house which is
conductive.

Dave - W?LEV

<>
Virus-free.www.avg.com
<>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>

On Mon, Mar 17, 2025 at 6:13?PM Bruce KX4AZ via groups.io <bruce=
[email protected]> wrote:

A ham friend has an unusual indoor antenna that he uses with a Zachtek
WSPR transmitter. He gets lots of spots from the 200mW transmitter (>500
unique band/spotter combinations in 24 hours), and the beauty of the
Zachtek is its tolerance of essentially any antenna load (from nothing
attached to fully shorted). His antenna is effectively an off center fed
dipole, where the two wire legs wrap around the inside bedroom walls, and
are of random/different lengths..i.e .just whatever would fit onto the four
walls. The two dipole legs are connected to a simple 6:4 turn ratio
transformer (approx. 2:25:1 impedance ratio), with the primary side of the
transformer attached to a 4 ft coax feedline line to the Zachtek
transmitter.

I decided to make some measurements of the SWR with the nanoVNA attached
to this "creative" antenna setup. The nanoVNA was OSL-calibrated, followed
by attaching the antenna feedline to collect data in nanovnasaver
software. After collecting the data, I decided to reverse the two dipole
legs connected to the transformer secondary, just to make sure I was
getting reproducible data. What confused me were significant differences
in the SWR/return loss/Smith charts for the two configurations. I am
wondering if this simply reflects how non-balanced the dipole is, such that
the coax feedline shield (about 4 ft long) from the transformer plays a
different role in each of the two configurations. While making the
measurements the nanoVNA was dangling hands free while connected to the USB
port on my laptop...so the coax shield side would be the same in both
cases...and the only electrical difference was in reversing the dipole legs
(of differing lengths) connected to the transformer secondary.

--

*Dave - W?LEV*


--
Dave - W?LEV