Impedance Matching Transformers -- Pete's Magic Decoder Ring!

We got some questions on this and Pete was kind enough to write up some notes on the topic.  We may talk more about this during the next podcast (Saturday): 

Broad Band Impedance Matching Transformers.

Broad Band impedance matching transformers are designed to transfer power over a wide frequency range. More basic you have an amplifier that has an output impedance of 200 Ohms and you want to match that to a 50 Ohm load.

So what is the magic decoder ring so that you get a 4:1 match, ie going from 200 Ohms to 50 Ohms. Getting technical for a moment the maximum power transfer theorem says maximum power is developed when the source is matched to the load.  The Broad Band Matching transformer enables that to happen over a wide frequency range.

So how do we get from 200 Ohms (the source)  to 50 Ohms (the load)? Just as there are many airlines that fly from LA to NY so it is with the matching approach. We will cover several.

First a short discussion about broadband cores themselves. One of the most common cores for HF work is the type 43 core which is good up to about 50 MHz. For transformers up to 200 MHz then the type 61 are a better choice. Typically at HF the FT -37-43 is one of the more common ones see (3/8 inch in diameter), as is the FT-50-43 which is ½ inch in diameter. The iron powder cores are not the 1^st choice for broad band matching.  

#1 Way:

Build a transformer that has a  primary of x number of turns (and since it is large, 200 Ohms will have more turns) and the secondary will have y number of turns ( and since it is smaller, 50 Ohms, will have fewer turns.) The transformer action is based on the ratio of the Primary turns Squared to the Secondary Turns squared. Our transformation is 4:1.

Thus if we divide the primary turns squared and divide it by the secondary turns squared the result is 4. Here are some example: if we had a primary of 8 turns ( 64) and a secondary of 4 turns (16) – 64/16 = 4. So that is our transformer a primary of 8 turns and a secondary of 4 turns. When building these transformers use two different colors of wire as that makes it much easier to identify the windings. Observe the phasing, meaning the end you connect to the collector of your output transistor is the start end. That same start end for the secondary winding is the output “hot” side of the secondary.

#2 Way.

The same ratio holds in going from 200 to 50 Ohms. But this time we will use a single winding of 8 turns and at 4 turns we will have a tap for the 50 Ohm point. Since that tap will very likely have Dc on it connect a 100 NF cap at the 4th turn winding and this is the output. What you have just done is create an auto transformer.

#3 Way.

At time one may have an oddball transformation and you can cascade transformers and multiply their individual turns ratios. At one time I needed a 9:1 transformation. I built a 2.5 transformer and then hooked that to a 4:1 and the result was a 9:1 transformer.

The attached table has “worked out” some common matches that are often needed like matching a 50 Ohm amplifier to a 500 Ohm Crystal Filter which is a 10: 1 match. This is easily done with a 6 turn primary (50 ohms 6^2 = 36) and the secondary has 19 turns ( 500 Oms 19^2 = 361). 361/36 = 10.03:1. Close enough for ham radio! The 1^st way is probably more preferable for this application.

Pete N6QW

11/2014

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