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How does one measure the series resonant frequency of a RF plate choke using a nannovna.
1. Do you put the choke in series between the 2 ports
2. Do you place the choke in parralel to the 2 ports
3. Do you use it as a dip meter with 2 x loose coupling coils on a shorted chokes.
When I used a dip meter i used to short the choke out and couple the dip meter to end of the choke
Thanks
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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
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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
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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
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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
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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
|
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
|
Thanks for all the replies. I have a box of chokes that I will set up and sweep. The plan is to build a test jig which will be a square tube with about 33mm of clearance on each side. This will ensure repeatability. I have been struggling to wind a good plate choke that covers all the ham bands including the warc bands. I just wanted to be sure that i was not wasting wire and using the wrong technique.
Henry
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You don't need a test jig to measure plate chokes.
They are influenced by the layout in the amplifier.
You need to measure the choke in it's operating position.
--Shane
KD6VXI
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Yes, you need to place the choke in its position ON THE INTENDED AMPLIFIER chassis. In this manner the exact distributive affects of shunt C are properly noted.
And with all do respects to the VNA, I would much rather do this measurement with a GDO.
Or... convert your vna into the equivalent of a GDO by exciting the ch 0 port into a small link coil which would utilize the measurement system as a coupled transformer. Not unlike the GDO, coupling must be kept to a minimum so that reduction in Q affect of the choke under test is minimized.
Note, you will need to measure both the anti resonate as well series resonate points as you described. In the series mode, the choke is shorted. Again, a direct connect of this arrangement to ch 0 would seem problematic. Consider the vna system is placing 50 ohms in series with the choke under test!
I would encourage you to correlate the measurements with the vna and with the GDO technique before applying 4000 volts to that nice tube!
I would also consider if you have one... the classic chokes like the old National Radio R175 which had strong series resonate problems in the 15 meter band. They would burst into flames! See if the vna points out the alarm. You should be able to pick up both the series resonate and parallel resonate signature on the vna-Smith plot. I know the GDO does pick up these signatures and I have modified those chokes based on the GDO results to fix their problems.
Alan
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I agree Alan, GDO or equivalent should be the best method IMO also.
@Henry: If you use NanoVNA, it needs to be calibrated on a narrow freq
interval, it has only 101 points, easy to miss a deep resonance. Better use
NanoVNA-saver, which can calibrate with many more points (ie 2020)
*Still, nobody answered my question: is the stray capacitance (or impedance
in general) of the NanoVNA channels influencing the measured resonance
frequency of the choke, or the calibration is able to remove this
influence? *
I understand so far the choke is shunted by the 50 Ohm impedance (which is
not purely resistive) of the NanoVNA channels, which reduce Q but the
resonances should still be visible.
W8JI is describing a different method of measuring the choke:
It would be interesting to compare the various methods.
73 Ady YO2NAA
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On Mon, Dec 23, 2019 at 2:56 AM alan victor <avictor73@...> wrote: Yes, you need to place the choke in its position ON THE INTENDED AMPLIFIER
chassis. In this manner the exact distributive affects of shunt C are
properly noted.
And with all do respects to the VNA, I would much rather do this
measurement with a GDO.
Or... convert your vna into the equivalent of a GDO by exciting the ch 0
port into a small link coil which would utilize the measurement system as a
coupled transformer. Not unlike the GDO, coupling must be kept to a minimum
so that reduction in Q affect of the choke under test is minimized.
Note, you will need to measure both the anti resonate as well series
resonate points as you described. In the series mode, the choke is shorted.
Again, a direct connect of this arrangement to ch 0 would seem problematic.
Consider the vna system is placing 50 ohms in series with the choke under
test!
I would encourage you to correlate the measurements with the vna and with
the GDO technique before applying 4000 volts to that nice tube!
I would also consider if you have one... the classic chokes like the old
National Radio R175 which had strong series resonate problems in the 15
meter band. They would burst into flames! See if the vna points out the
alarm. You should be able to pick up both the series resonate and parallel
resonate signature on the vna-Smith plot. I know the GDO does pick up these
signatures and I have modified those chokes based on the GDO results to fix
their problems.
Alan
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Thanks for the feedback and link Ady.
Once the cal is complete, the measurement plane is 50 ohms. Assume the cal is accurate and complete, the fixture will actually be a larger error contributor than the vna itself. It cannot be over emphasized that these large chokes must be measured in-situation as they are used. The parasitic present will be altered dependent on their placement.
I had this vna running for some other work and took a look at one of these larger unmodified plate chokes. The first clockwise crossing is the standard SRF of the parallel resonate point. Sorry for the blur photo but you can see the 2- series resonance loops, one at ~ 15 MHz, the larger, and a second smaller one at 21 MHz. This second small loop is problematic. This measurement is taken with 401 date points.
Use of the vna will require more than 101 data points to properly resolve the loops. Use of the feature in PC saver routine is required. Cal over several 100 points is necessary to achieve the loop signature clearly.
Alan
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