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Re: Measuring Inductance.

 

Joe,

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:


Well,

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.

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.
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