¿ªÔÆÌåÓý

ctrl + shift + ? for shortcuts
© 2025 Groups.io
  1. Nanovna-Users
  2. Topics

Screen Scaling

 

Can someone point me to a reference in the Wiki for screen scaling? I¡¯m trying w1aew¡¯s method for determining the characteristic impedance of an unknown piece of coax as an exercise. I¡¯m getting close but the Smith chart trace is a tight little knot in the center. W1aew¡¯s video shows a nice ¡°ram¡¯s horn¡± trace on the Smith chart. I think the scaling will be useful in other areas too. Thanks in advance.
 

Display, scale... Set to 0.2 (to multiply resolution by 5)...
Dg9bfc sigi

Am 07.11.2023 21:58 schrieb "Russell C. Nixon" <rcnixon@...>:




Can someone point me to a reference in the Wiki for screen scaling? I¡¯m
trying w1aew¡¯s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I¡¯m getting close but the Smith
chart trace is a tight little knot in the center. W1aew¡¯s video shows a
nice ¡°ram¡¯s horn¡± trace on the Smith chart. I think the scaling will be
useful in other areas too. Thanks in advance.
 

1) Be sure your coax is less than 1/4-wavelength long in the coax
(considering Vp). Conversely, be sure your measurement frequency of
measurement is less than the electrical length of the cable.

2) Open terminate the far end of the cable.

3) On the Smith chart more-or-less in the center of the locus, note the
capacitance.

4) Short terminate the far end of the cable.

Do not change frequency or move your measurement frequency.

5) On the Smith Chart note the inductance.

6) Calculate the impedance using:

Zo = SQRT [ L / C ]

Dave - W?LEV

On Tue, Nov 7, 2023 at 8:58?PM Russell C. Nixon <rcnixon@...>
wrote:

Can someone point me to a reference in the Wiki for screen scaling? I¡¯m
trying w1aew¡¯s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I¡¯m getting close but the Smith chart
trace is a tight little knot in the center. W1aew¡¯s video shows a nice
¡°ram¡¯s horn¡± trace on the Smith chart. I think the scaling will be useful
in other areas too. Thanks in advance.





--

*Dave - W?LEV*


--
Dave - W?LEV
 

Zo = SQRT(L/C) is exact when R and G (primary constants) are zero. An exact expression when they cannot be ignored is:

Zo = SQRT[(R + jWL) / (G + jWC)] = SQRT(L/C) * SQRT[(1 - j(R/WL)) / (1 - (G/WC)]

where W is the radian frequency, 2 * pi * f.

Note that when R and G are zero, the exact formula reduces to the approximation SQRT(L/C).

The difference here may be important when precise measurements are desired and the attenuation per wavelength of the line is not negligible.

73,

Maynard
W6PAP

On 11/7/23 13:48, W0LEV wrote:
1) Be sure your coax is less than 1/4-wavelength long in the coax
(considering Vp). Conversely, be sure your measurement frequency of
measurement is less than the electrical length of the cable.
2) Open terminate the far end of the cable.
3) On the Smith chart more-or-less in the center of the locus, note the
capacitance.
4) Short terminate the far end of the cable.
Do not change frequency or move your measurement frequency.
5) On the Smith Chart note the inductance.
6) Calculate the impedance using:
Zo = SQRT [ L / C ]
Dave - W?LEV
On Tue, Nov 7, 2023 at 8:58?PM Russell C. Nixon <rcnixon@...>
wrote:

Can someone point me to a reference in the Wiki for screen scaling? I¡¯m
trying w1aew¡¯s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I¡¯m getting close but the Smith chart
trace is a tight little knot in the center. W1aew¡¯s video shows a nice
¡°ram¡¯s horn¡± trace on the Smith chart. I think the scaling will be useful
in other areas too. Thanks in advance.
 

We had discussed at length the measurement of characteristic impedance. See this production

/g/nanovna-users/message/32568

Which shows how, with a nanoVNA, to calculate the characteristic impedance of a cable as a function of frequency

With Excel, it is easy to manipulate complex numbers. Here we take the square root of the product.
--
F1AMM
Fran?ois

-----Message d'origine-----
De la part de Maynard Wright, P. E., W6PAP
Envoy¨¦ : mercredi 8 novembre 2023 16:40
 

Where you can measure the input impedance of a cable pair, a good way to determine Zo is:

Zo = SQRT(Zoc * Zsc)

where Zoc is measured with the distant end open circuited and Zsc is measured with the distant end short circuited. Note that Zoc and Zsc are complex values when the attenuation of the line is not negligible and are functions of frequency so they need to be measured accurately.

73,

Maynard
W6PAP

On 11/7/23 13:48, W0LEV wrote:
1) Be sure your coax is less than 1/4-wavelength long in the coax
(considering Vp). Conversely, be sure your measurement frequency of
measurement is less than the electrical length of the cable.
2) Open terminate the far end of the cable.
3) On the Smith chart more-or-less in the center of the locus, note the
capacitance.
4) Short terminate the far end of the cable.
Do not change frequency or move your measurement frequency.
5) On the Smith Chart note the inductance.
6) Calculate the impedance using:
Zo = SQRT [ L / C ]
Dave - W?LEV
On Tue, Nov 7, 2023 at 8:58?PM Russell C. Nixon <rcnixon@...>
wrote:

Can someone point me to a reference in the Wiki for screen scaling? I¡¯m
trying w1aew¡¯s method for determining the characteristic impedance of an
unknown piece of coax as an exercise. I¡¯m getting close but the Smith chart
trace is a tight little knot in the center. W1aew¡¯s video shows a nice
¡°ram¡¯s horn¡± trace on the Smith chart. I think the scaling will be useful
in other areas too. Thanks in advance.
 

Thank you, that's the ticket! I couldn't remember it despite looking right at the "Display" menu item.
 

Thank you to you and Fran?ois for the dive into Zo.
I'm always ready to listen and learn.

Russ
kf4wxd

1 - 8 of 8