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Can you combine same 2 RF power amps with 2sc1946a?

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radium98

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rf power

hi
is there anyway to combine same 2 rf power amplifier with 2sc1946a given each one 25w in parrallel with coaxial cable

i need to know is there a way to make a digital potentiometer schematic is neede i mean i can press a buttom and the value change from ohm to kohm
thanks for ur help
 

WimRFP

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rf power

Hello,

If the amplifiers are really equal, yes it can be done. You need to make a 2 output power splitter and a 2 input combiner (wilkinson power combiner/divider). This circuit provides isolation between the two amplifiers, avoiding problems.
 

radium98

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Re: rf power

yes WimRFP the two amplifier are equal and use variable capacitor so i can tune them ,but the problem i dont need wil;enson combiner because i dont have the 100 resistor so why i need to combine them by coaxial cable ,i know that i can build one amplifier with more power transistor but i need combining ,i saw 1 amplifier that use 4 poweramplifiers units each given 100w mrf317 and combining with coaxialno wilkenson but i need 2 amplifiers any details and help thanks
 

WimRFP

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rf power

Hello,

If you are able to tune the amplifiers so that they are really equal (gain and phase, you probably need an oscilloscope for that), you can use a reactive power combiner (Air line combiner).

In fact the outputs of the amplifier are connected in parallel so you have to load with 25 Ohms. The 25 Ohms impedance is transformed to 50 Ohms (with a quarter wave section In case of narrow band).

To use a quarter wave transformer you need sqrt(50*25) = 35.4 Ohms line impedance. You can put in parallel two 75 Ohms cable pieces, this gives 37.5 Ohms. The small mismatch (VSWR=1.13) is acceptable. Don't forget the velocity factor for the coaxial cable.

The disadvantage of these types of combiners/splitter is that there is virtually no isolation between the ports, so when there is some unbalance in phase or gain, you get uneven power distribution. When one amplifier fails, the other may fail also. The Wilkinson combiner has isolation. When one amplifier fails, the other one will continue to function, but the 100 Ohms resistor will run very hot.

You can make the quarter wave section with a CLC pi circuit also, but 75 Ohms cable is relatively easy to get.
 

radium98

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Re: rf power

thanks a lot for help :) i know that it is an old way to do this combining and nowadays there are pallets at high power 600w and more with ds philips rf transistor but i want to make this for hobby maybe i will burn transistor output ,but i have final question the input combining must be the same way as output i saw something strange in an old 500w or 600w elenos that use 4 no tune amplifier equiped with coaxial cable of corse teflon semi rigid .
 

WimRFP

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rf power

Hello,

You are right, you need the same thing at the input.

From the source you go to the 2, 75 Ohms quarter wave transmission lines (that are in parallel, ground to ground, center to center on source side). From the 75 Ohms lines you go with same length 50 Ohm to the inputs of the amplifier. For the input, where the power level is less, you may add the 100 Ohm resistor between the 75 Ohms lines (Wilkinson divider).

As you have two 112.5 Ohms inputs in parallel, the source will see 112.5/2 = 56.25 Ohms (VSWR=1.13). Same procedure you can use for the output also.

Regarding your last sentence (the 500..600W stages), best is to find the article and post a link to it, as I don't understand what you mean.

Combining isn't old. When combining several Power Amplifiers, the up time will be better, because when one amplifier fails, the otherswill continue service.
 

radium98

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Re: rf power

realy i thank u a lot for ur help the link is that what do i have made 2 same tuned amplifier

http://www.3-mtr.info/shareware/Amplifier%2028Watt%20MicroStripline%20(2SC1946A)/

so i have 2 coaxial cable from 2 output amplifier with 50ohm each one i put them in parrallel gnd to gnd and inner to inner then 2 1/4 wave coaxial cable of 75ohm and then 1 piece of 50 ohm and the same for input but better to add 100ohm carbon resitor
50 ohm 75ohms 50ohm
from each ampli =========+==========+----------------rf output
can u help with schematic or any design IF u have

https://remco.tk/handig/amp300w-144MHz.pdf
 

WimRFP

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rf power

Hello,

When you look to the front page photo of the 300W 2m band amplifier: The S-curve coaxial cable and image PCB trace (also curved) is the balun to go from unbalanced to balanced. The actual length of the coaxial cable and PCB trace is almost 0.25 lambda (if you have sufficient space).

I checked the document. In this amplifier the length is less and he compensates the inductive common mode behavior with two capacitors.

He uses a discrete device with two fets on it and use them as a push-pull (balanced) amplifier. The fets do not operate in parallel.

He therefore needs baluns in both the input and output. They are not Wilkinson combiners. I can read German, I hope you are able to read it also, it is a very good documented amplifier.

When you go to page 11 of the 300W amplifier document, you see the Wilkinson divider. My solution with the 75 Ohms cables is the same, but you leave out the resistor. This means you lose the isolation, so you need equal amplifiers (gain and phase). When you going to build something, don't forget the velocity factor for the 75 Ohms cable that you use.

If you still have doubts, I will make a simple drawing.
 

radium98

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Re: rf power

hi i only grabb this document to show what do i need but really i made same 2 2sc easy tune amplifier with this link and here want ur help in combining wit full detail if u nee the testing frequency is 109.90 mhz
:D
http://www.3-mtr.info/shareware/Amplifier%2028Watt%20MicroStripline%20(2SC1946A)/
 

WimRFP

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rf power

Hello,

What are you going to do with the amplifier, as you are operating in the VOR ILS frequency band (navigation for airplanes)?
 

radium98

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Re: rf power

its for FM band 88-108 mhz low powr station
 

markken

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Hello. This splitting and combining with the wilkinson does work well. I have modified some old TV PAs from 50MHz to the FM band. Each PCB gives 200 watts out into 50 Ohms so I now get an easy 400-500 watts out.
I have spent many hours on this project and you need to be precise in your construction. I could not have done this without a spectrum anilizer and a good high power current limiting PSU. A good kit is the MRF141g/151g power kits from CCI. I have two of these producing 700 watts on the 6m 50MHz ham band.
 

radium98

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marken thanks for help do u mind to help me first i need to know the impednce of the brown coax cable in this figure
https://images.elektroda.net/57_1276892070.jpg
second some help to combine with details with coxial cables because dont have 100 ohm resistor
http://www.3-mtr.info/shareware/Amplifier%2028Watt%20MicroStripline%20(2SC1946A)/
2 or 4 amplifier only with this transistors
thanks
 

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