Bob,
thank you for clarification. I have measured a coil in this way( connected
only to CH0). Attached Smith chart and R, X graph.
The resonance can be seen easily around 12 MHz. However, in my case it is a
parallel resonance, impedance is very high.
For the plate choke we need the series resonance[s] which can cause serious
trouble in tube PA.
My question was not about the method, but about the accuracy.
Is the stray capacitance of the NanoVNA shifting the self resonance
frequency of the coil ?
73 Ady
[image: Coil - Smith.png]
[image: Coil - R & X.png]
On Sun, Dec 22, 2019 at 7:53 PM Bob Albert via Groups.Io <bob91343=
[email protected]> wrote:
The resonance of a choke is measured by connecting it to the S0 port.
You select Smith chart and, with the frequency at a low value you read the
inductance on the screen, with the plot being a dot at the left edge.
(First you select S0 reflection mode.)
Then you sweep the frequency up until the chart becomes a circle. The
point where it passes halfway around, to the right edge, is the resonant
frequency. You carefully adjust the frequency until the inductance reading
on screen switches to capacitance. That's resonance.
Bob K6DDX
On Sunday, December 22, 2019, 08:00:43 AM PST, Ady, YO2NAA <
yo2naa@...> wrote:
Will the reactance of NanoVNA's channels influence the resonant
frequencies of the choke, or the calibration process is removing this
influence?
If the influence is present, what method will minimize it the most?
Should the choke be measured mounted in the PA, to include the influence of
the neighboring objects?
73 Ady YO2NAA
On Sun, Dec 22, 2019 at 2:34 PM Jim Potter <jpotter@...> wrote:
To find a series resonance you are looking for the frequency where the
impedance is a minimum.
All three techniques you suggest will work.
1. Series between two ports, look for the frequency of maximum S21
2. Shunt, you only need 1 port. Look for the frequency of a dip in S11.
Don't connect Port 2
3. With two coupling coils the result will be like #1, The frequency for
maximum S21 is the self resonance
4. With one coupling loop the series resonance will be the minimum in
S11,
like #2. This is pretty much what you are doing with the grid dip meter,
looking for maximum energy absorption,
Try all of these techniques to see the difference. That's one of the
great
thing about the nanoVNA it's a great educational tool at an affordable
price.
73 de K9GXC, Jim
