[Amps] Making a broadband transformer

Bill Turner dezrat1242 at yahoo.com
Sun Nov 3 21:15:09 EST 2013

I have been doing some bench testing of broadband toroid transformers with
the intention of using them in a legal-limit solid state amp. I don't have a
"real" impedance analyzer, but I find the MFJ 259B SWR analyzer gives me
readings for both resistance and reactance which seem to be reasonably
accurate and make sense.

I am finding that creating a transformer that will cover 160-10 meters is
not an easy thing. No surprise I suppose.  :-)

My best result so far has been with a pair of FT-280-43 toroids stacked on
top of each other (not binocular). For test purposes I am keeping the turns
ratio 1:1 to make it easy with the MFJ analyzer. I have a 51 ohm non
inductive resistor across one winding and the MFJ connected to the other. 

I have tried many different numbers of turns using a twisted pair and so far
just four turns is working pretty well on the lower bands but the leakage
reactance starts to become excessive above about 11 MHz. The resistive
component only varies between 50 and 60 ohms on 160 through 10, which is
acceptable, but by the time I get up to 28 MHz the leakage reactance is up
to about 80 ohms. 

So I would like some advice on what to do. I have read all the books and
papers I can find so I have a pretty good grasp of the basics, but there
must be some tips and tricks I have yet to learn.

One question I have not seed addressed anywhere is the use of multiple mixes
in the toroids. For example, a 43 mis and a 61 mix stacked. Has anyone tried
this? It would seem that would help extending the bandwidth. I have some 61
mix toroids on order so I will be finding out myself pretty soon, but I
would like to hear from anyone who has tried it. 

One thing I am really curious about is how the commercial amp makers do it.
Have they found the secret to reducing leakage reactance, or do they
compensate for it somehow? Or just ignore it and power on through? I see the
Ameritron ALS-1300 has a 470 pF capacitor from drain to ground on each of
the transistor pairs. I don't see any other compensation for the
transformers. Interesting. That's the only schematic I have for a commercial
amp. I would love to know how Icom, Tokyo Hi-Power and SPE do it. 

Just out of curiosity, I found it is quite easy to tune out the reactance
with a capacitor, but then of course, the transformer is no longer
broadbanded, but it does give a neared perfect resistive match. 

All comments welcome. 

73, Bill W6WRT

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