EMRFD Message Archive 12407

Message Date From Subject
12407 2016-02-17 09:07:22 KENNETH CHASE Broadband transformer windings
Hi All

I've discussed this subject in the past about the windings on a transformer.

In particular the reactance of the secondary.

If I have a impedance ratio of 200:50, then what reactance do I need on the secondary to calculate the secondary windings?

I've been told 5 times minimum and I've been told 10 times minimum.


12411 2016-02-17 10:04:36 Mike Re: Broadband transformer windings

My understanding (and if others feel that I am wrong, please chime in)
is that the "rule of thumb" for transformer windings (and also for RF
chokes) in a broadband circuit is to have the windings of the
transformers each have an inductive reactance at the operating frequency
which is not less than (about) 4 or 5 times the nominal impedance of the
circuit to which the winding is connected. For a transformer that is
providing an impedance transformation from 200 ohms to 50 ohms, the 200
ohm winding should have an inductive reactance of not less than (about)
800 or 1000 ohms, and the 50 ohm winding should have an inductive
reactance of not less than (about) 200 or 250 ohms. Higher reactance
than these values is OK, although at RF if we try to go too much higher
than the "4 to 5 times" value (depending on the frequency, the size and
type of the ferrite or iron powder, the shape of the core form, etc)
then we can easily get into trouble with the self resonance (from
distributed capacitance) being an issue.

Mike N4MWP

12413 2016-02-17 11:27:05 Graham / KE9H Re: Broadband transformer windings
Mike is basically right. But it is not a Black/White number or a cliff you go off.

I have made them work as low as three times the rated inductance, but in this case, it was trying to get a transmitter transformer to work from 160 to 6 meters.  This required a compromise of 3 at 160 Meters, and the transformer performance suffered because of it, but only on 160 Meters.  As the frequency rises, the winding inductance rises linearly, so three at 160 becomes six at 80 Meters and all worked great above that. (Until the transformer hits ferrite resonance around 20 MHz, and goes into transmission line mode above that.

--- Graham / KE9H


12415 2016-02-17 12:03:21 KENNETH CHASE Re: Broadband transformer windings
Thanks Silva and Graham

My transformer consists of FT37-43 with impedance from 3300 ohms down to 50 ohms. Transformer is for 7 MHZ.

If I set the reactance of the secondary (50 ohms) to 250 ohms, then secondary winding is 4 turns.

If impedance ratio is 66:1 then turns ratio is 8.12. This makes primary winding 4 turns x 8.12 = 33 turns.

Does this look right?



12417 2016-02-17 17:06:42 Graham / KE9H Re: Broadband transformer windings

Yes, for a single band transformer, that is how I would do it.

To test that you did it right, go put a 3300 Ohm carbon film resistor
across the primary, and connect the 50 Ohm secondary to an antenna
analyser, and sweep it across 40 Meters, and see if the secondary stays
where it is supposed to be.

If the VSWR stays low, you did it right.

--- Graham