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[
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