Re: Type 2 PHSNA Transformer T1 replacement

Steven Dick
 

A similar experiment was done by another ham, Kerry VK2TIL. He posted his data in the EMRFD Yahoo group on Jan 31, 2015. I am copying his post here, as I think the info might be interesting to this group:

"RF transformers and the ferrite inside them are very interesting devices.

I think that the best reference to their design is in the Philips application notes ECO6907 & ECO7213 which cover both conventional and transmission-line types; Google will find these app notes.

The references tell us that the tx-line types are better but I like to see things for myself.

I try to emulate the scientist who was travelling through the countryside in a train; his companion looked out the window and remarked "Those sheep have just been shorn" to which the scientist replied "Yes, on this side anyway".

On this quiet Sunday morning I built a transformer on an FT37-43 core; it had two windings, one of 20 turns and one of 10 turns.

I placed the windings on opposite sides of the core; I have previously found that winding the shorter winding over the longer gives passable results but I wanted to see the effect of separating the windings.  Here it is, in a test fixture and with a 200-ohm resistor across the larger winding;

http://s18.postimg.org/6wxwzo2l5/003.jpg

 

Here is a sweep of Real Z, Imag Z and VSWR;

http://s30.postimg.org/ptns7u14x/trfr.png

It's very poor; indeed, I didn't expect it to be as bad as this so I made another one and the test results were almost identical.


I then made a tx-line version, using the same FT37-43 for consistency.


The new transformer used 10 turns bifilar connected as a 4:1 device; I didn't measure the line Zo but I wound it a little less tightly than the twist that I have found to give 50 ohms Zo so it's probably somewhere near the desirable 100 ohms.  With a pin-connector and a 200-ohm resistor installed as per the previous model, it looked like this;


http://s2.postimg.org/xjnbq5vs9/011.jpg


Here are the test results;


http://s8.postimg.org/suo0laaph/bifilar_trfmr.png


This is a very good transformer; it is wideband well beyond the 50 MHz limit of my tests.

That answers Vince's question 2; the designer, a very smart bloke, knew what he was doing!  :)

Re question 1; these devices can be tested with an antenna analyser in the same setup that I used.

Terminate the "non-50-ohm" winding with the appropriate resistance and apply the analyser to the 50-ohm winding to see how good the VSWR is.

Keep connections as short as possible; you can see that I use SIL pin-connectors which are very convenient for this kind of work.  In this case, a short SIL strip is soldered to a BNC(f); one SIL pin goes to centre, the next pin is removed and the outer two are soldered to the body.  Silver-plated BNCs ease soldering.

If, as occasionally occurs, neither winding is 50-ohms, apply the analyser to that which is closest to 50.

Kerry VK2TIL."

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