EMRFD Message Archive 10253
Message Date From Subject 10253 2014-09-13 19:17:28 Ashhar Farhan guanella guanella is a transformer that consists of a transmission line wound around
a toroid or even an air-core. it has transformative properties that are
very broadband. we use them all the time in the broadband stages without
much thought (as baluns).
on a different mailing list, there was a discussion about guanella being
nothing more than a current transformer except at higher frequencies.
to understand if guanella works at all, i hooked up a simple circuit. the
circuit is uploaded as guanella2.png into the VU2ESE folder of the group's
I first made the guanella:
it simply consists of a 10 turns of bifilar winding of #24 gauge wire
twisted to 10 turns per inch tightly wound on FT37-43 and held down with a
spot of superglue.
Now, the winding has two wires on each end. First end has a twisted pair
named 1 and 2. The other end has a twisted pair named 5 and 6. the wire
that starts as '1' at one end comes out as '5' at the other end. the wire
that starts as '2' comes out as '6' at the other end. i hope the
nomenclature is clear.
now, i ground '6' and connect an RF source (sweeperino) between 6 and 2. i
ground '1' and connect a power detector between 5 and 1. See the exp 1 in
the circuit. i have it redrawn to make it look like a current balun. sure
enough, the RF output from almost 1 MHz to 60 MHz follows the input.
however, if you perform this experiment, you would be aware of how much
broader this bandwidth is (almost 6 octaves) compared to a regular
transformer. but this is only intuited.
now, we swap the ground and signal end. we do this by grounding 2 and
feeding the signal input from 1. effectively, both ends of the left side of
this 'transformer' are now grounded. LTSpice will predict that the
transformer core will now absorb all the power. however, what i do notice
is that almost all the power is still showing up between pins 5 and 6 (see
exp 2 in the circuit). this is not explained by lumped element abstraction.
thus, the guanella is not just a current transformer. it is behaving like a
transmission line transformer. the second experiment is not repeatable with
a conventional transformer.
this experiment is very primitive, but it is very illuminating. i highly
recommend throwing it together and playing with it over a cup of tea. i am
pressed for time, but i would also like to attach a return loss bridge and
play with terminating it with different impedances to confirm TLT behaviour.
NOTE: the RF signal and the detector MUST have 50 ohms impedance.
otherwise, the TLT (transmission line transformer) mode will not show
loss-less behaviour due to the standing waves.
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10254 2014-09-13 20:18:43 Eamon Egan Re: guanella
What you have in experiment 2 is a shorted winding (pin 2 to pin 6) closely coupled to the 1 to 5 winding which is in series with the signal path.
This short circuit reflected to winding 1-5 unsurprisingly causes it to exhibit very low inductance and impedance, so most of the power unsurprisingly transfers through to the load. It would continue to do so up to the frequency where the impedance of the leakage inductance of the 1-5 winding becomes significant compared with the 50 ohm system impedance.
You said LTSpice “will predict” the power will be absorbed by the core, but you didn’t say whether you tried this. I personally have little doubt LTSpice using its regular coupled inductor model will produce results that agree with your experiment.
My own view about TLTs is that TL effects will only come into play when the electrical length of the TL in the transformer becomes significant. Unless I’m mistaken, this is rarely the case with typical baluns at HF frequencies.
10255 2014-09-13 20:44:43 Ashhar Farhan Re: guanella eamon,this is what comes of relying on other's experiments. i was told by someone that it would appear as a shunted short. though, it doesn't of course. i thew the spice simulation in on LTSpice VI and it complained that the inductor was shorted. so, i assumed that i didnt know how to use the spice software and accepted the results reported by others.thanks so much for correcting this. it is so obvious now that you have pointed it out to me. let me search for that spatula that i keep handy in the shack to crack the egg on such occasions.- fOn Sun, Sep 14, 2014 at 8:48 AM, Eam