I don't get it.? Sure, the PI network is a LP filter.? BUT when tweaked for? 1:1? swr at the middle of a band, the swr rises when u go below / above the band edges.?
I'm gonna re-check this in the next 2 days...and on a bunch of bands.??
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On a side note, per JI's? tuned input writeup, the function of the PI filter is to kill harmonics at the cathode from going BACKWARDS towards the xcvr.? In that case, the PI filter has to have LOW Z on all the harmonic freqs.?
"An RF power amplifier's tuned input circuit is more than just impedance matching. While reasonable impedance matching can be important for proper exciter operation, the tuned input circuit of a grounded-grid or cathode driven radio frequency power amplifier provides multiple important functions for the amplifier:
- The tuned input system PREVENTS? harmonics GENERATED in the PA tube from PASSING BACK to the exciter
- This PREVENTS FALSE high SWR OR exciter power readings, since cathode harmonics SHOW as reflected power???????
- The tuned input circuit provides a stable low impedance to the cathode of the PA tube
- This helps amplifier linearity and reduces IMD, and can improve PA efficiency
- The tuned input circuit matches the cathode impedance to the exciter system impedance
- This lets the exciter work into a proper SWR
A tuner between the amplifier and radio,? a matching circuit some distance from the cathode, or a high pass or broadband network will NOT do all of the above."
"Harmonics
Current in the CATHODE shares the SAME path through the tube as current in the anode, with an additional current that flows in the grid-cathode path in class sub-2 amplifiers. (AB1?amplifiers have no grid current, while AB2?amplifiers have grid current.) The cathode of a vacuum tube essentially contains the same time-varying current as the anode, and if there is ANY grid current that additional current is also harmonic-rich current."
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"Input Impedance
The driving impedance of a cathode driven amplifier always varies some amount with power level and tuning. We can say the input impedance of a cathode driven stage is?dynamic, with the tube's driving impedance changing with parameters like drive power and how the amplifier is loaded or tuned.
The input impedance changes dynamically because the output system is in SERIES with the input system. The SAME plate current flows through both cathode and anode systems. Varying anode load impedance, because the same very same current flows through both, CHANGES the input impedance. In general the lower tube mu, the greater the effect of output tuning and power level on driving impedance. This is because a low-mu tube generally has a high driving impedance, making cathode impedance a larger percentage of anode impedance.?The higher cathode impedance increases negative feedback, and increased negative feedback causes the input impedance to vary more with anode load impedance changes. The tuned input's resonance or "flywheel effect" does not stabilize this impedance variation, but additional losses brought into the cathode system by tuned input circuit components helps dilute or "swamp out" dynamic changes in driving impedance. An attenuator pad would do the same of course, but would not filter harmonics or sharpen conduction point transitions.
The dynamic cathode impedance, where cathode impedance changes with drive level, PREVENTS? us from accurately adjusting the tuned input system of a typical cathode driven amplifier at very low power. This prevents accurate use of an antenna analyzer to adjust the tuned input while using the operating PA tube as a load. The input system should always be adjusted near full output power under normal operating conditions, or with a suitable dummy load replacing the tube's dynamic impedance.
A reasonable approximation of tube input impedance, for circuit adjustment purposes, would be a small non-inductive resistor from each cathode pin to a control grid pin at each tube. This resistor should be the approximate value of the tube input resistance when driven near full power with normal tuning and loading. Filament power and HV should be off and the tubes left in place, and the input driven normally as if the amplifier was running but with a low power antenna analyzer substituting for the the normal exciter.
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