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What is the right way to check 1:49 UnUn with nanovna?

Likely the

Dave - W?LEVbest way is to connect two of them back-to-back.

Dave - W?LEV

Another approach is to put a 2.5K resistor across the output.

73! Mark KA6WKE

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On Sat, Mar 14, 2020 at 9:20 AM David Eckhardt <davearea51a@...>
wrote:

Likely the

Dave - W?LEVbest way is to connect two of them back-to-back.

Dave - W?LEV

On Sat, Mar 14, 2020 at 2:43 PM Praba Karan <vu3dxr@...> wrote:

What is the right way to check 1:49 UnUn with nanovna?

On Sat, Mar 14, 2020 at 09:30 AM, Mark Schoonover wrote:

Another approach is to put a 2.5K resistor across the output.
=========================================================
That was pretty much the same reply to his question on Facebook by Robert Fontana ....

"A 49:1 transformer should map 2450Ω (49x50Ω) into 50Ω. If you place a (noninductive) 2400Ω resistor across the terminals and use the NanoVNA to measure VSWR, you should see a reasonably flat and low VSWR across the frequency of operation ...

Note, this only measures its low signal response, and does not take into account core saturation at higher power levels. It will, however, tell you over what frequency range the core is good for."

I guess he is looking for a different approach.

- Herb
.

Praba Karan,

Depends what you are trying to check and over what frequency range? Are you just looking to measure VSWR vs frequency or other things maximum power handling capability, efficiency (meaning PowerOut/PowerIn) etc.

Roger

To test for insertion loss (S21) you need two identical back to back.
You may want to observe phase with frequency as well.
In those cases you ahv eto allow for two devices so the insertion
loss is 1/2 that measured.

S11 can also be measured that way with 50 ohm termination.

Or with 2450 (2500 nominal) ohm resistor.

Power handeling is realted to mismatch and that means you
need a variable reactive load and a resistive load.
Why?

If the wire is perfectly tuned for low SWR and no reactance (s11) seen then
it will be efficient. As the frequency diverges from that point high or low the
antenna wire (the load) appears as capacitive (lower frequency)
or inductive (higher frequency). In the end the transformer becomes
less efficient for non resonant cases depending on how far off.
IN those cases the ability to withstand power is diminished.

For power handling make the core as large as possible within limits.
The biggest limits is residual lead inductance and ability to achieve
tight enough coupling.

Allison
----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

I have tested the 1:49 transformer loss and found it to be 1 to 1.5 dB depending on the band. I checked this loss several ways with good agreement using the Nanovna.

I also measured the feed point impedance of a 67 ft wire at 7 MHz using the Nanovna and found it to be 3600 to 3900 ohms shunted by about 8 pF. This Z is about 1250-j1820...way out of the range of the Nanovna (S11) so I used a tuned circuit method with the Nanovna and went to the old GR-821-A (Twin T Impedance Measuring Circuit). Good agreement with both techniques.

It happens that the equivalent source Z of the transformer is a resistance in series with an inductive reactance so there is a rough conjugate match with the EFHW wire.

This wire tested against a ground stake from 12 inches above the ground to a tree placing the wire at about a 45 deg angle to the ground. I tested this same wire at 3 locations. At home, 30 miles south of home and 80 miles to the SE of home (Home is about 10 miles from Rochester, NY). I can't be sure if the "ground" conditions are similar or different:)

I did all of this stuff because I asked questions on several on-line discussion group and didn't get very good scientific answers from anyone.

On Sat, Mar 14, 2020 at 04:36 PM, WB2UAQ wrote:

I also measured the feed point impedance of a 67 ft wire at 7 MHz using the
Nanovna and found it to be 3600 to 3900 ohms shunted by about 8 pF. This Z
is about 1250-j1820...way out of the range of the Nanovna (S11)

Yes the S11 bridge method starts to get quite inaccurate after the impedance gets higher than a few hundred ohms. Fortunately the NanoVNA can do S21 measurements and we can determine complex impedance at several thousand ohms using the "Series Method" shown below. The equations are not that complicated and I was hoping Rune could add them to NanoVNA Saver. Presently I do them in a spreadsheet after an import of a S21 Touchstone file.

Roger

Hi Roger
Using the S21 methods requires that both the source (CH0) and load (CH1) impedance are pure 50 ohm, so you get also errors this way. Have you tried to read the Rp instead of Rs in NanoVNA-saver. That makes a big difference to the R values as it is often forgotten the standard impedance displayed are R+jX and then the R value is frequency dependent due to the X value present. Often the use of port extension can cure the problem if the DUT is mounted to an adaptor introducing a shunt C.
Kind regards
Kurt

-----Oprindelig meddelelse-----
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Sendt: 15. marts 2020 00:59
Til: [email protected]
Emne: Re: [nanovna-users] Checking 1:49 UnUn with nanovna #applications

On Sat, Mar 14, 2020 at 04:36 PM, WB2UAQ wrote:

I also measured the feed point impedance of a 67 ft wire at 7 MHz using the
Nanovna and found it to be 3600 to 3900 ohms shunted by about 8 pF. This Z
is about 1250-j1820...way out of the range of the Nanovna (S11)

Yes the S11 bridge method starts to get quite inaccurate after the impedance gets higher than a few hundred ohms. Fortunately the NanoVNA can do S21 measurements and we can determine complex impedance at several thousand ohms using the "Series Method" shown below. The equations are not that complicated and I was hoping Rune could add them to NanoVNA Saver. Presently I do them in a spreadsheet after an import of a S21 Touchstone file.

Roger

On Sat, Mar 14, 2020 at 05:36 PM, Kurt Poulsen wrote:

Hi Roger
Using the S21 methods requires that both the source (CH0) and load (CH1)
impedance are pure 50 ohm, so you get also errors this way. Have you tried to
read the Rp instead of Rs in NanoVNA-saver. That makes a big difference to the
R values as it is often forgotten the standard impedance displayed are R+jX
and then the R value is frequency dependent due to the X value present. Often
the use of port extension can cure the problem if the DUT is mounted to an
adaptor introducing a shunt C.
Kind regards
Kurt

Kurt,

Thanks for your comments. Yes I am familiar with the using Rp instead of Rs in NanoVNA saver.

I agree that the CH0 and CH1 impedance needs to be close to 50 ohms for accurate results using the S21 method. I have found using attenuator pad(s) and "calibrating them out" helps but of course that reduces the impedance range that can be measured.. I am only interested below 55 MHz. so that alleviates the 50 ohm impedance issue as well.

I hope to post some results showing complex impedance measurements using my S21 test jig and different parts before too long.

Roger

I recently made a video showing how to measure impedance 3 ways with a VNA including the s11, series sw1, and the shunt s21 methods.
See if interested.

Larry. W0QE

Roger,

S21 to get it right really requires a known transformer or often easier two
of the same back to back. That keep measurement in the 50 ohm
domain and also allows for reasonable accuracy for loss as it should
be twice that of a single transformer.

Its faster and less calculation intense, usually less error.

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Larry,

Impedance measurement is more indirect then measuring the transformer
directly. Both get you to the same point but the big thing is you have to
understand what your measuring and what that means. Easier for people
versed in the theory.

Also some just want to know, does it work as claimed?

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

On Sun, Mar 15, 2020 at 08:11 AM, Larry Benko wrote:

I recently made a video showing how to measure impedance 3 ways with a
VNA including the s11, series sw1, and the shunt s21 methods.
See if interested.

Larry. W0QE

Larry I just watched your video and it does a good job of explaining the three methods. If you are interested in doing a follow-on it would be interesting to see the following:

1. Comparison between SimSmith and NanoVNA using test components on the nanoVNA
2. Investigate what happens as you introduce deviation into the ideal CH0 and CH1 50 ohm impedance as Kurt Poulson discussed above.

Maybe we should move this discussion to a new post so the topic does not get buried in the discussion of 49:1 UN-UN testing.

Roger

Allison,

My previous comment to WB2UAQ was not about testing the 49:1 UN-UN wuth S21. It was about measuring the high feedpoint impedance of his 67 ft wire at 7 MHz. For high impedances like this using the Series method with S21 is another way of measuring impedance.

Roger

That might work if it were a dipole.

for a end fed wire?
The problem then is what represents the ground plane?

An Endfed half wave (or any random wire) is like any antenna
(dipoles for instance) dependent on height above ground and
the ground under it so a point measurement is exactly that.
Since a resonant half wave is also a high impedance typically
greater than 1800 ohms ( to as high as 6000 ohms) direct
attachment is likely not meaningful as the VNA is the counterpoise.
And maybe the hand holding it.

We are also talking about likely 7mhz range so less than an inch
is less significant.

This is where inference substitution is useful, build 1:49 transformer confirm
its behavior with resistors. Place that between the VNA and said wire and
measure impedance. No matter what the wire length will have to be adjusted
for resonance in the measured environment and like a jig in the form of some
many feet of COAX to simulate the antennas interaction with it.

Also how do you put the 67Ft wire that will be fed from only one end in series?
If you touch the far end at all you change the result. That said for a resistor or
cap any small part the techniques work well but a 67 foot in air is not a small
part.

I say this as I once watched an engineer try to measure a 6ft monopole for an
E-field antenna on a PNA indoors. He concluded it could not work despite
a working version outside the window. Much explanation later the light was
seen. Its how antennas behave not the instrument and measurement
technique. They must match to a practical level.

Both can be measured without a VNA using a signal generator, bridge,
RF voltmeter or receiver as null detector plus some sense of the antennas
environment so it will be realistic.

The upside is even if wrong the 2:1 circle is rather large for those trying to
measure a EFHW.

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

WB2UAQ,

Measuring antennas requires knowing what the ground is like.
as it does change the result as well as orientation and height above it.

I know the region and it does change depending on where you are
in that region.

Your trying to answer questions about EFHW antennas. Suggestion do
so at higher frequency like 20m or 10 M as scaling is handy and makes
getting numbers for .1 Lambda height (1meter at 10M) to say .5 lamda
(5M at 10M). Also for orientation and ability to replicate the identical
thing at multiple locations.

Hint it behave much like a dipole.
It as an antenna is the extreme case of an off center fed.

Things going on...

*The impedance of an antenna depends on orientation horizontal sees ground reflections
so there is a pronounced impedance change. (visible with dipoles.)
*Reflections from large bodies (for sloping or vertical) can be seen as modified
impedance as well depending if constructive or destructive.
*Vertical does not have the same interaction with the ground.
See above about large reflective surfaces.
*Also objects like a fence or trees will interact and load (interact with) the wire.
* Resonance effectively changes with height (follows reflected impedance).
* Interaction of the gear attached to the end of it. IF the gear has a cord and
cables it changes the dimension of the antenna as its part of the measurement
plane and the reflection (counterpoise) of it. A case of what you measure and
how can change the measurement.
* Measured impedance at resonance should be Rx, J0, as that changes the
results move around but if you adjust frequency you can see what R is at J0.
WHY? if above resonance it will be inductive or -Jx and below frequency it will
be +jx that colors the result.

This is why everyone has an opinion but authoritative answers are few.
The other part is pick any value and then draw the 1.5:1 or 2:1 circle and
see how much it can vary. For a 1:49 transformer that's about 1250ohms
or 3800 ohms for the 2:1 circle so small errors like accurate to two significant
digits is likely good enough and easily small enough as variation is height
or nearby objects will easily exceed that.

The side effect is someone will argue with you about it as they do not
understand measuring it. For that it must be repeatable and account
for the known variations.

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

I was hoping that the ground conductivity was different so I did measurements at three places with the wire close to the same angle WRT the ground. The results were very similar so I guess the ground is similar at these locations. In other words, the radiation resistance plus the ground loss resistance was very similar (3600 to 3900 Ohms). The measurements were done over about a 3 month period (last April-June). It might be interesting to note that the feed point Z for a wire cut to 468/7 MHz is resonant a few hundred KHz lower (the above case) BUT this is now a rough estimate because I didn't record where the impedance became only resistive.
For the case in my backyard there is another antenna but the coupling is about 30 dB. A 30 dB separation showed a 1 or 2 ohm shift in the impedance looking in (near 50 ohms) the EFHW when the other antenna was left open-circuit or short-circuit at it's transmission line input end. I did this to address the interaction between these antennas when some one questioned the influence on the measurements. Anyway, I answered the questions that I came up with and then some about 10 months ago. Last June I did ARRL FD running 2 W CW with my old Heathkit HW-8. I worked every station I called on 20 and 40 Meters from ME to S. CA with this same 67 ft EFHW from one of the three test sites. 73

WB2UAQ,

Reminder the 468/F is a rule of thumb approximation at best. That will be
altered by wire diameter, height above ground, and if the wire has insulation.
In all cases of testing over decades it always proved to be shorter than that
for dipoles.

For EFHW one end of the wire has the transformer so its also capacitively
loaded and the differs in actual compared to a dipole of the same wire at
same height is shorter still by .01 to .05 wavelengths. Since you asked
the missing length is supplied by the coax shield.

Analysis of EFHW is best followed by considering it as a extreme form of
off center fed antenna as there is always and needs to be a "other half
or part of the antenna" to complete the RF circuit.

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO