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Measuring Inductance.
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Hello All,
Need a question answered here. I am the one here making a new set of coils for a Heathkit SB-220. Using A NanoVNA-H4 I learned how to measure the old coil values. And what the stimulus range needed to be to measure the whole range of taps on the coil to the best resolution.. I got values that were very close to what was expected. And on the smith chart it was a single arc curve. Now I have installed the new coil. And measured the coil so I can determine where to attach the last two taps to have the correct amount of "L" for each one. BUT.... and there always is a BUT isn't there? Using the exact same test parameters, now I have a smith chart that is two turns.? Why would that be? The measured values one is close to what is expected 2.2uH the other smith loop is 1.09uH. What's happening here, and why is it different? Joe WB9SBD |
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Not identical coil.
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I needed slightly more "L" on every band. so I made the 10/15 meter coil slightly bigger. You see the coil. 3/16" tubing, 7 turns, 1.75 diameter 2.25 long. New coil, 3/16" tubing also 7 turns, BUT 2.25 diameter, and 2.25 long. So it should have slightly more "L". I don't understand why now I am getting the 2 rotations on the smith chart. Joe WB9SBD On 9/11/2021 10:25 AM, Gary O'Neil wrote:
Identical coil identical set up identical results. Not true? Something not identical. Check set up. |
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The rotations on the Smith chart generally imply of resonance. The resonant frequency is where the trace crosses the real axis. It sounds like you may have moved the SRF inband.
Don¡¯t change your setup, but remeasure the first coil. If you repeat your original result, you can conclude that your test configuration is probably correct and sound. Beyond that, trust the measurements. For collateral confirmation, enter the coil dimensions into one of the online solenoid coil calculators, and compare the predicted inductance values for ballpark agreement with your measurements. -- 73 Gary, N3GO |
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Well,
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I left the coils in place in the amp, BUT.... I lifted all the taps, so the coil is floating free but inside the amp. And all readings are normal now. Interesting. So OK we planned on leaving the taps on 80, 40, 20, as they are, just add the extra "L" using the tubing coil. So thats what I just did, I found suitable and convenient taps for 10 = 1.1uH and for 15= 1.8uH. So original was 80= 11.6 40= 5.6 20= 2.06 15= .936 10= .448 Now is, 10 = 1.1uH 15 = 1.8uH 20= 2.94uH 40= 6.46uH 80= 12.46uH The switch must be messing with the readings when all connected. So next step is now the reverse matching thing. wish me luck. Joe WB9SBD On 9/11/2021 12:15 PM, Gary O'Neil wrote:
The rotations on the Smith chart generally imply of resonance. The resonant frequency is where the trace crosses the real axis. It sounds like you may have moved the SRF inband. |
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Joe,
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Once you have measured and installed the coil and capacitors in the RF Amplifier you can tweak the settings to compensate for stray capacitance and inductance by measuring the 50 ohm output impedance in the amplifier by simulating the RF Plate resistance. BEFORE YOU DO THIS MEASUREMENT VERIFY THE AMPLIFIER IS POWERED DOWN AND PLACE A SHORT ACROSS THE POWER FILTER CAPACITORS TO DISCHARGE THE CAPACITORS! - Place the RF Tube in it's socket. This will simulate the tube capacitance. The ARRL handbook gives the following approximation formula for calculating the load resistance of a vacuum tube power amplifier: Vp Rl = ---- KIp where K is 1.5 for class AB, 1.57 for class B, and 2 for class C. Calculate and install a non-inductive resistor from tube late to ground to simulate the Plate load. - Install the NanoVNA on the RF Output connector - Set the PI Network Capacitors and Coil for each band and adjust the capacitors (and coil tap if necessary) to get the output impedance close to 50 +j0 ohms. The plate formula is an approximation but these measurements will get you into the ballpark. - Remove the resistor before operating the amplifier. BEFORE YOU DO THIS MEASUREMENT VERIFY THE AMPLIFIER IS POWERED DOWN AND PLACE A SHORT ACROSS THE POWER FILTER CAPACITORS TO DISCHARGE THE CAPACITORS! I used to repair and install Marine MF (2 to 4 Mhz) radios on boats when I was in college 45 years ago. I had to adjust the output series coil to resonate the 20 foot whip antenna. I was I had the NanoVNA back in those days! 73 Mike N2MS On 09/11/2021 1:22 PM Joe WB9SBD <nss@...> wrote: |
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If you¡¯re looking to refine your measurement repeatability, do a fresh recal, repeat the sweeps of both induntors under as near exact conditions possible, then recheck your cal to confirm measurement integrity was maintained between cals.
Switches can frustrate repeatability, so its important to have a handle on tolerances in your measurements. It¡¯s always good practice to do end of test cal checks, and to quantify the variability between duplicate measurements. Its an easy method for determining the boundaries of uncertainty that are attributable to the quality of your test equipment and processes, while also validating the integrity of your results. -- 73 Gary, N3GO |
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I took my old DX35 tank coil that has multiple taps and my Mosley traps and they pretty match your Now is values.
Here is your now is: 10 = 1.1uH 15 = 1.8uH 20= 2.94uH 40= 6.46uH 80= 12.46uH Ps I used multiple stimulus from 500khc to 32 mhz? ranges were 1-8, 10-25 etc to get better detail. Also, tried without recalibration still had good enough results De K8HTB Joe |
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