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PI network and RF output transformer ideas and questions




 
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Author Topic: PI network and RF output transformer ideas and questions  (Read 387 times)
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Patrick J. / KD5OEI
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« on: October 11, 2018, 11:19:47 PM »

Say for example a practical pi network is designed to match a tube to a 200 Ohm load instead of 50 Ohms and the transmitter covers 1.6 to 30MC.

Why not use a 2:1 turns ratio RF transformer in whatever form, to connect the 200 Ohm transmitter to a 50 Ohm load?
Assume the 50 ohm load is a good match and "vswr" is minimal, cause I don't want to get into what happens to 'baluns'/transformers start cooking when there's a lot of reflected power.

There are several "baluns" that seem like this type offered commercially. Those have a 50 Ohm coax connector and two terminals for the 200 Ohm side. OK use it in reverse.

Using a transformer to convert a 200 Ohm unbalanced signal to a 50 ohm would also allow a balanced-balanced tuner to be added between the transformer and a dipole or vee type antenna.

So, why not keep it simpler and use a 1:1 balun transformer designed for 200 Ohms, and go into the balanced tuner with 200 ohms?

Or, why not go way out there and use a ferrite bead choke balun, except for finding 200 Ohm coax and needing quite a bit more beads since the choking impedance would have to be 4x that which 50 ohms would require?

just some thoughts
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« Reply #1 on: October 11, 2018, 11:22:56 PM »

The tank circuit provides a flywheel effect.

You'd have to idle the tube in pure class A to get this to work.

Commercial mfg have tried.

Still need flywheel effect.

--Shane
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« Reply #2 on: October 12, 2018, 09:01:13 AM »



   The attached file takes a Push-Pull-Parallel final with PL519's into a Balanced Pi-Net terminating into a 200 ohm to 50 ohm balun. Still got "flywheel" effect.

Would the same thing work with a single ended final?

Jim
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* Class_AB_F_Linear (1).pdf (2629.51 KB - downloaded 34 times.)
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« Reply #3 on: October 12, 2018, 02:45:10 PM »

All

The use of a 200 ohm <=> 50 ohm matching transformer will work fine in this application provided the power level is not high enough to cause significant saturation of the transformer’s core material.

With the turns ratio fixed at 2:1 you can accommodate higher power (while preventing saturation) by:

Using a core with a higher cross sectional area (i.e. a larger area enclosed by each turn) to reduce the required amplitude of the B field in the core.

And/Or

Using more turns (e.g. six input turns and 3 output turns) to reduce the required amplitude of the B field in the core

I.e. the voltage across the input winding, v(t), is  N x dB(t)/dt x A, where

N is the number of turns in the input winding, A is the area of each turn of the input winding, and dB(t)/dt is the time derivative of the time varying B field passing through the input winding. For a sinusoidal field at RF frequency f, the amplitude of dB(t)/dt is: 2 x pi x f x the amplitude of B(t).

Stu
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« Reply #4 on: October 12, 2018, 03:23:15 PM »

Patric said that he uses a PI network to match the tube to 200 Ohms instead of 50 Ohms, so there is a resonance circuit, there is a "flywheeleffect". NO difference, just an other output impedance. And yes, when you want to match a high impedance line like a 450 Ohm ladderline, it is better to feed the tuner with 200 Ohms instead of 50 Ohms. Less impedance transformation will give you more bandwidth and less losses.
If you are worried about saturation of core material, you always can use a coax made balun 1:1 without ferrite
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« Reply #5 on: October 14, 2018, 06:48:44 PM »

Yes I was not too specific but there's a chance the pi network will be about 200 Ohms rather than the usual 50.
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« Reply #6 on: October 20, 2018, 08:23:49 PM »

Here's a question further down the pipe..

Whether the amplifier wants 50 or 200 Ohms for a load, could the amp be equipped with a loading capacitor that would also serve as the input capacitor of a balanced tuner?

Indeed, balanced tuners are usually shown with a switch to move the 'large variable condenser' from the input to the output side depending on the antenna load impedance to be matched.

The question arises as to whether this single loading capacitor can be used by the tuner "through the balun" so to speak.

In this case I am suggesting a 1:1 (or 2:1 max.) balun of the transformer type
In case of a 50 Ohm amplifier output, a section of coaxial line with ferrite beads is a reasonable question.

Tune into a dummy load then switch to the antenna tuner as usual.

It is certainly an easy and simple way to get the amp tuned and loaded, then apply the tuner and get that aligned up to match the antenna.

This all assumes that a dummy load is placed at the amp output to get it ready, then the tuner is connected up for its purpose, and also assumed that the VSWR is kept low during full power use. I can't see too much wrong with the idea and it saves an extra variable cap in the tuner section. 


* pi-and-tuner crop.png (9.75 KB, 786x252 - viewed 17 times.)
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