[TowerTalk] question about a matching transformer

[StellarCAT]

Hello, 

this is to those experts out there familiar with toroidal transformers. 

The common one used in boxes like the “StackMatch” is a 2.25:1 transformer. it is tri-filiar wound on a 2.4” core. So it provides, when seeing 2 50 ohm loads (2 monobanders for example) an SWR of about 1.14:1. 

So in the WX0B version, shown in the ARRL AB, there are 4 turns. In the W2FMI version there are 6 turns. In either case the transformation is clear: 3 ‘coils’ divided down to 2 coils or a turns ratio of 1.5:1 so the impedance ratio would be 2.25:1 (1.5^2). 

But there is a version of it – shown in W2FMI’s book, that provides for a 2:1 match. And my initial thoughts are – if I am using 2 monobanders and I can do 2:1 why would I not do it? 

W2FMI shows again a tri-filiar wound coil – with 6 turns and the ‘top’ or end coil, that which is going to the source, is tapped down 1 turn from the source end. Thus you have essentially (5/6 + 2)/2 = 1.416:1 or an impedance ratio of 2:1! 

That all is straight forward. I’ve never seen someone do this – but it seems again straight forward. 

So I have been TRYING to get it to work without any success. 

I start with a 240 core (tested both K and 61 material) ... and I have attached to it – short leads, 2 50 ohm resistors with a measured resistance of 25.3. And an SO239. I’ve checked it using a SARK110. This is not on a PCB – no relays, no board traces – just short leads on a transformer, 2 resistors and comp cap. 

So wound with 5 turns I can establish, using the K material, a relatively good response for the 2.25:1 version. (The 61 material is not quite as good). I have to use a compensation capacitor at the load input and have tested various values. The best ‘fit’, using the 2.25:1 ratio (trying to duplicate its results first) is to use a value of 75pf. That gives me an SWR of 1.14:1 on 20 and 15 and 1.15:1 on 20. 

So I then tap down ~1 turn (for 5 turns tapped down 1 turn it should still be, in theory, 1.96:1 or an SWR of 1.02:1) ... and I cannot for the live of me get any kind of what seems like proper results. It is quite high on 10 meters – approaching 2:1 and typically 1.2:1 or worse on 20 depending on the comp cap. I’ve tested anywhere from no comp cap to 200pf in small increments ... it just doesn’t work. [note any comment about the choice of the resistor load, which are metal film, has to be considered under the light of – it works using the 2.25:1 version!]

Can an expert perhaps write to me directly, not burdening others here, and hopefully point me in the correct direction please? This SHOULD work! 

Gary 
K9RX at arrl dot net

[ref: The SARK has been OSL calibrated with the cable in use – and the resistor load has been checked at the end of that independent of the transformer and reads 2.00:1 from 14 through 29Mhz with 0 ohms reactance[