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Setup: I have a 40 meter dipole at 30 feet. Feeding with 100 ft of RG8X
Measurement: 7.200 Mhz, SWR at RADIO end of COAX can be measured (For the Analysis System -- coax + antenna)

Question: Can the Reference plane be "calculated" by software. It's the location of the antenna at the END of the Coax, discounting the Feedline, perhaps by entering the Feedline Impedance and Velocity Factor and Loss?

Asked a different way:

Can the NanoVNA be used to isolate the antenna at the far end of the COAX and somehow IGNORE the impact of the COAX itself ?

I realize the best way to check the antenna is AT the feedpoint, but that is not possible right now. I'm stuck at the Radio End of the COAX and trying to determine of the DIPOLE is too long or short.

One possible "solution" IF I could take a second piece of 100' RG8X and do the calibration at the far end (OPEN, SHORT, LOAD) and push the Reference plane out to the 100' mark.
But can it be done without a second piece of COAX ??

Has anyone calibrated their NanoVNA using a long piece of coax and doing the CAL at the end of the coax before attaching to an antenna to be tested ?

Jeff, W6FCC, 3-31-2020

The coax, unless it's defective, only adds loss (and rotation around the
Smith Chart) to the measurement of the antenna. It doesn't affect the
frequency response.

You could calibrate S11 with the coax connected and applying Short, Open,
Load at the far end of the coax with the antenna disconnected.

Or, just measure it as is - the Return Loss dips will be at the same
frequency as if you measured directly at the antenna. Just the depths of
the dips will be different.

Make sense?

73, Carey, WB4HXE


On Tue, Mar 31, 2020 at 1:30 PM Jeff W6FCC via Groups.Io <w6fcc=
[email protected]> wrote:

Setup: I have a 40 meter dipole at 30 feet. Feeding with 100 ft of RG8X
Measurement: 7.200 Mhz, SWR at RADIO end of COAX can be measured (For the
Analysis System -- coax + antenna)

Question: Can the Reference plane be "calculated" by software. It's the
location of the antenna at the END of the Coax, discounting the Feedline,
perhaps by entering the Feedline Impedance and Velocity Factor and Loss?

Asked a different way:

Can the NanoVNA be used to isolate the antenna at the far end of the COAX
and somehow IGNORE the impact of the COAX itself ?

I realize the best way to check the antenna is AT the feedpoint, but that
is not possible right now. I'm stuck at the Radio End of the COAX and
trying to determine of the DIPOLE is too long or short.

One possible "solution" IF I could take a second piece of 100' RG8X and do
the calibration at the far end (OPEN, SHORT, LOAD) and push the Reference
plane out to the 100' mark.
But can it be done without a second piece of COAX ??

Has anyone calibrated their NanoVNA using a long piece of coax and doing
the CAL at the end of the coax before attaching to an antenna to be tested ?

Jeff, W6FCC, 3-31-2020

--
Carey Fisher
careyfisher@...

Carey,

If the antenna is NOT 50 ohms (it's actually an offset multi-band dipole with a 4::1 balun transformer), or not resonant, I was thinking that the COAX would introduce some problems.
They do use small runs of COAX for matching, that's what I was thinking, that the SWR would not be constant across the length of the COAX if the antenna was not 50 ohms resonant.

I was looking for a way to put the imaginary connector to the antenna at the far end of the COAX but maybe that's not going to solve anything.

Jeff

You can only measure the actual antenna impedance remote from the antenna, with a 1/2 wavelenght of coax or feedline (x the velocity factor of the feedline), measured at a single frequency of interest.

If you are going to use a multi-band antenna, you particularly want to measure the impedance at the coax input point in order to know what the transmitter will be actually looking at, as a function of the antenna/feedline system.
Ray,
W4BYG

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This email has been checked for viruses by AVG.

Thanks for replying. I thought the vector network analysis would be able to adapt to the cable properties to get an accurate measure of the load at the end of the feedline.

Jeff

Knowing the coax type and length, you can use SimSmith to propagate the
measurement back to the antenna feed point.

I've offered so many emails to the SimSmith download site on this group
that I'll leave it as an exercise to the interested and motivated user to
find the link. It's easy. Just use any search engine.

Dave - W0LEV

On Tue, Mar 31, 2020 at 5:30 PM Jeff W6FCC via Groups.Io <w6fcc=
[email protected]> wrote:

Setup: I have a 40 meter dipole at 30 feet. Feeding with 100 ft of RG8X
Measurement: 7.200 Mhz, SWR at RADIO end of COAX can be measured (For the
Analysis System -- coax + antenna)

Question: Can the Reference plane be "calculated" by software. It's the
location of the antenna at the END of the Coax, discounting the Feedline,
perhaps by entering the Feedline Impedance and Velocity Factor and Loss?

Asked a different way:

Can the NanoVNA be used to isolate the antenna at the far end of the COAX
and somehow IGNORE the impact of the COAX itself ?

I realize the best way to check the antenna is AT the feedpoint, but that
is not possible right now. I'm stuck at the Radio End of the COAX and
trying to determine of the DIPOLE is too long or short.

One possible "solution" IF I could take a second piece of 100' RG8X and do
the calibration at the far end (OPEN, SHORT, LOAD) and push the Reference
plane out to the 100' mark.
But can it be done without a second piece of COAX ??

Has anyone calibrated their NanoVNA using a long piece of coax and doing
the CAL at the end of the coax before attaching to an antenna to be tested ?

Jeff, W6FCC, 3-31-2020

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

Yes, you disconnect the antenna from the cable and attach your calibration kit to the free end of the cable with the analyser at the shack end and do a single port calibration. Then reattach the antenna and make your measurements.

Hi Jeff
If you use the NanoVNA-Saver and do a proper calibration then you just measure at the TX side and save the measurement to a s1p Touchstone file. Then download the ZPlots by AC6LA and open up the s1p file- Then you can shift the measurement the 100 feet to the antenna position.
Following conditions:
The cable type and manufacturer to be selected in ZPlots must be know
The cable length must be known exactly else use the Time domain of NanoVNA-saver but VF must be know for the cable
You need to have Windows EXCEL to run ZPlots
Kind regards
Kurt

-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Jeff W6FCC via Groups.Io
Sendt: 31. marts 2020 19:31
Til: [email protected]
Emne: [nanovna-users] 100 ft Coax to Dipole Antenna - Can the NanoVNA show just the Dipole analysis?

Setup: I have a 40 meter dipole at 30 feet. Feeding with 100 ft of RG8X
Measurement: 7.200 Mhz, SWR at RADIO end of COAX can be measured (For the Analysis System -- coax + antenna)

Question: Can the Reference plane be "calculated" by software. It's the location of the antenna at the END of the Coax, discounting the Feedline, perhaps by entering the Feedline Impedance and Velocity Factor and Loss?

Asked a different way:

Can the NanoVNA be used to isolate the antenna at the far end of the COAX and somehow IGNORE the impact of the COAX itself ?

I realize the best way to check the antenna is AT the feedpoint, but that is not possible right now. I'm stuck at the Radio End of the COAX and trying to determine of the DIPOLE is too long or short.

One possible "solution" IF I could take a second piece of 100' RG8X and do the calibration at the far end (OPEN, SHORT, LOAD) and push the Reference plane out to the 100' mark.
But can it be done without a second piece of COAX ??

Has anyone calibrated their NanoVNA using a long piece of coax and doing the CAL at the end of the coax before attaching to an antenna to be tested ?

Jeff, W6FCC, 3-31-2020

Jeff,
Of course the antenna is not 50 Ohms. No antenna is 50 Ohms across the
whole band or bands. Like I said, the coax just rotates the antenna
impedance around the Smith Chart. While you can't measure the impedance
without knowing the exact length and Vf of the transmission line, the
VSWR/Return Loss is the same at both ends of the transmission line minus
twice the insertion loss of the line.
73, Carey, WB4HXE

On Tue, Mar 31, 2020 at 4:27 PM Jeff W6FCC via Groups.Io <w6fcc=
[email protected]> wrote:

Carey,

If the antenna is NOT 50 ohms (it's actually an offset multi-band dipole
with a 4::1 balun transformer), or not resonant, I was thinking that the
COAX would introduce some problems.
They do use small runs of COAX for matching, that's what I was thinking,
that the SWR would not be constant across the length of the COAX if the
antenna was not 50 ohms resonant.

I was looking for a way to put the imaginary connector to the antenna at
the far end of the COAX but maybe that's not going to solve anything.

Jeff

--
Carey Fisher
careyfisher@...

On Tue, Mar 31, 2020 at 01:30 PM, Jeff W6FCC wrote:

RG8X

RG8X (foam dielectric) has Vp of 0.79 and loss at 7.2 MHz of 0.75 dB per 100 feet.

Electrical wavelength length of RG8X @ 7.2 MHz is 107.9 feet. One trip around the Smith chart is half wavelength so 100' would be 1.85 rotations around the chart. The round trip loss would be 1.5 dB which is 0.84 reduction in Rho vector that spirals inward as length of coax gets longer and loss increases. So you should push the value point on Smith chart outward (increase number) by 1/0.84 or 1.19 times to correct of loss of coax.

It would be best if you can put a short at the end of the coax to measure the electrical length and loss. If coax is RG8X and 100' in length, and VNA is calibrated without coax, the left most point on Smith chart (minimum Z) should be at 7.77 MHz. At this point the round trip loss should be -1.59 dB ( rho = 0.833) inward from outer ring of Smith chart, at left-most point along the real horizontal center line. (loss at 7.77 MHz is 0.794 dB per 100')

But none of that affects the resonant frequency of the antenna. You can
measure from the end of 100 ft (or whatever) of coax connected to the
antenna and you'll be able to see whether the antenna is short or long by
what frequency the resonant dip is at.
73,Carey,WB4HXE

--
Carey Fisher
careyfisher@...

A tool that is useful for de-embedding the DUT (here the antenna) from the measurement data can be found here:



The description is here:



This is a lot easier at 7 MHz than at much higher frequencies. The accuracy is affected by the structural return loss of the cable. At really high frequencies (UHF & microwave) you can have a precision 50 ohm termination calibrated at the instrument's port and then compare to the same termination at the end of a coaxial transmission line and only get 20 dB or so. This is because the coax is not ideal and has structural imperfections. -- Anyway I hope the link is useful. An excerpt from the spreadsheet README tab is:
"Reference plane extensions are useful as a "poor man's de-embedding procedure". You can use this to subtract out known lengths of 50 ohm transmission line on input and output, it only affects the phase and group delay plots.".

Also see:

As Nigel said, no need for complications here. Just calibrate at the end of the cable and you're fine.

The only requirement for the cable is to not change its properties between calibration and measurement.
Usually the problem is phase stability, when the cable is moved or the temperature changes (Sun shine on cable...).
You can check for cable problems by attaching a load at the end (after calibration), moving the cable around a bit and checking return loss on the VNA.

So, measure immediately after calibration, and don't move the cable much between calibration and measurement.
The cable length and exact impedance is not important, when you calibrate at the end.

But of course, the greatest advantage of the nano is its extreme portability, so why not just bring it out to the antenna??

Marko Cebokli

It may not be possible to climb up the tower for calibration.
As an experiment you can try to set a NEGATIVE electrical delay (setting can be found in the DISPLAY/SCALE menu)
About -3.5nS per foot of cable, depending on the velocity factor
This should rotate the smith chart back.
If you set the purple trace (phase) to CH0 (instead of CH1) and the objective of the rotation is to have a flat purple trace.
Changing the electrical delay should impact the angle of the purple trace. try increasing/decreasing till you have a somewhat flat trace
If this is done you have removed all most of the delay from the cable
Now the impedance of the antenna will be more clearly visible as the impact of the cable is (mostly if you have good quality cable) removed

--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

Forgot to mention, do this rotation removal with a stimulus from 50kHz to twice the target frequency.
Once the rotation is removed you can set the stimulus to the target range of the antenna

--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

If you are certain of the exact physical length of your coax and its velocuty factor. You could add a length at the radio end to make it equal to a multiple of a half wavelength, which will make the NanoVNA see the same impedance as the antenna. But it must be precise to be accurate.

Not discussing unnecessary coax cutting - that would be a possibility for one frequency - if you agree to neglect
losses without knowing them. But sometimes unknown losses are not negligable. And often you want the antenna
frequency response across a more or less wide band, without measuring the response of your n x Lambda/2 coax.

So what's better?

Do a local (at NanoVNA jack) SOL cal for the frequency span of the band of interest.
Connect NanoVNA to rig side end of the coax to the antenna.
Easily measure with NanoVNA using that same frequency range. (Stay warm and dry in the shack at any wheather.)
Save the resulting .s1p file for that band using NanoVNA saver.
Import the .s1p file into freeware SimSmith.
Use one of two ways explained by SimSmith author Ward Harriman in this video:



So you get the exact antenna feedpoint impedance over frequency response without having been at the often
not reachable feedpoint - without disturbing the antenna by the influence of your body or by disconnecting it.

----

SimSmith is currently being updated, so that hopefully soon the NanoVNA can be directly used within SimSmith.
That will make the above even more easy then, as no transfer of the .s1p file will be needed then any more.

As SimSmith is extremely useful and already very well developed, The Combo NanoVNA and SimSmith can be
predicted to become something most ham operators will never want to miss.

Use EZNEC to design your dream antenna, build it, and then use the combo NanoVNA/SimSmith to verify and optimize it.

73, Hans
DJ7BA

-----Ursprüngliche Nachricht-----
Von: [email protected] <[email protected]> Im Auftrag von Neil Preston W0NRP via groups.io
Gesendet: Sonntag, 5. April 2020 17:25
An: [email protected]
Betreff: Re: [nanovna-users] 100 ft Coax to Dipole Antenna - Can the NanoVNA show just the Dipole analysis?

If you are certain of the exact physical length of your coax and its velocuty factor. You could add a length at the radio end to make it equal to a multiple of a half wavelength, which will make the NanoVNA see the same impedance as the antenna. But it must be precise to be accurate.




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Excellent advice, Hans. I might comment there is an alternative to EZNEC
which is more power for antenna design, analysis, and modeling and if free
for the download. That be 4NEC2. Being more powerful, the learning curve
is steeper than EZNEC, but is is well worth the effort. I, so to speak,
"trained" first on EZNEC and only then dug into 4NEC2. You will never be
sorry you took the time and effort to learn 4NEC2.

Either site has the download for 4NEC2:







Dave - W0LEV

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

Thanks Dave ...

... for the 4NEC2 download link.

I didn't know 4NEC2 is free. I will be pleased to download and use it.

73, Hans
DJ7BA

-----Ursprüngliche Nachricht-----
Von: [email protected] <[email protected]> Im Auftrag von David Eckhardt
Gesendet: Sonntag, 5. April 2020 19:09
An: NANO VNA <[email protected]>
Betreff: Re: [nanovna-users] 100 ft Coax to Dipole Antenna - Can the NanoVNA show just the Dipole analysis?

Excellent advice, Hans. I might comment there is an alternative to EZNEC which is more power for antenna design, analysis, and modeling and if free for the download. That be 4NEC2. Being more powerful, the learning curve is steeper than EZNEC, but is is well worth the effort. I, so to speak, "trained" first on EZNEC and only then dug into 4NEC2. You will never be sorry you took the time and effort to learn 4NEC2.

Either site has the download for 4NEC2:







Dave - W0LEV




On Sun, Apr 5, 2020 at 4:56 PM DJ7BA <dj7ba@...> wrote:

Not discussing unnecessary coax cutting - that would be a possibility
for one frequency - if you agree to neglect losses without knowing
them. But sometimes unknown losses are not negligable. And often you
want the antenna frequency response across a more or less wide band,
without measuring the response of your n x Lambda/2 coax.

So what's better?

Do a local (at NanoVNA jack) SOL cal for the frequency span of the
band of interest.
Connect NanoVNA to rig side end of the coax to the antenna.
Easily measure with NanoVNA using that same frequency range. (Stay
warm and dry in the shack at any wheather.) Save the resulting .s1p
file for that band using NanoVNA saver.
Import the .s1p file into freeware SimSmith.
Use one of two ways explained by SimSmith author Ward Harriman in this
video:



So you get the exact antenna feedpoint impedance over frequency
response without having been at the often not reachable feedpoint -
without disturbing the antenna by the influence of your body or by
disconnecting it.

----

SimSmith is currently being updated, so that hopefully soon the
NanoVNA can be directly used within SimSmith.
That will make the above even more easy then, as no transfer of the
.s1p file will be needed then any more.

As SimSmith is extremely useful and already very well developed, The
Combo NanoVNA and SimSmith can be predicted to become something most
ham operators will never want to miss.

Use EZNEC to design your dream antenna, build it, and then use the
combo NanoVNA/SimSmith to verify and optimize it.

73, Hans
DJ7BA

-----Ursprüngliche Nachricht-----
Von: [email protected] <[email protected]> Im Auftrag von
Neil Preston W0NRP via groups.io
Gesendet: Sonntag, 5. April 2020 17:25
An: [email protected]
Betreff: Re: [nanovna-users] 100 ft Coax to Dipole Antenna - Can the
NanoVNA show just the Dipole analysis?

If you are certain of the exact physical length of your coax and its
velocuty factor. You could add a length at the radio end to make it
equal to a multiple of a half wavelength, which will make the NanoVNA
see the same impedance as the antenna. But it must be precise to be accurate.




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




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