I too have had very good success with ferrite transformers. A word of
caution from experience, power ratings are typical under ideal conditions.
Add some mis-match or reactance and it seems that the real numbers no long
apply. To that end, I've thus found that using very high power rated
devices do work well. Running legal limit power on the ham bands, the
smallest transformer I use is rated at 5KW, is a 1:1 device (unun) and is
about the size of a cigar box. It is wound with #8 solid copper wire with
Teflon insulation. I've built some low power 16:1 transformers to match the
output of the solid state exciter to the input of the BC transmitter
oscillator circuit for 160M and found that they work very well. Here in
this application I needed some voltage gain. Again very low powers
involved.
I believe that most bad experiences are where 4:1 baluns are used and thus
are typically under rated. I've found that 1:1 designs seem to be more
successful in most applications. Even with my commercial (ham band) tuner,
I do not use the internal balun even thought it is said to be rated at
legal limit. A high power external 1:1 device works better, no SWR change
with use, no heating observed while with the internal balun, leave the TX in
tune for say 1 min or so and watch the SWR creep up. A true sign of a
problem with something operating at more than a safe power level.
73
Bob, K4TAX
----- Original Message -----
From: "John T M Lyles" <jtml@lanl.gov>
To: <amps@contesting.com>
Sent: Wednesday, May 28, 2003 9:10 AM
Subject: [Amps] ferrite transformers
> The designs in Sevick's book and prior transmission line transformer
> design papers and books work well, as long as you pay attention to
> details such as winding the turns as tightly coupled transmission
> lines. Magnetic flux density in the core can be low, and the core
> itself extends the LF range. I built a 1:4 unun for a test setup at
> 0.3 - 5 MHz at work, using some commonly available Fair Rite
> material, where I needed to drive 200 Ohms with a kW of CW power.
> Using a calibrated Werlatone directional coupler with Hp/Agilent 437
> power meters on the 50 ohm side, and capacitive divider and Pearson
> current transformer on the 200 Ohm side, i measured the loss in the
> transformer while under full drive. It was similar to what was
> measured with a network analyzer at a milliwatt. The transformer runs
> cool and does the matching that i built it for.
>
> BTW, Sevick wasn't the first to describe these types of transformers,
> as there are papers from the 1950s from Phillips, from Herb Krauss at
> Va Tech, in Solid State Radio Engineering by Krauss, Bostian and
> Raab, and in many other references. These people didn't just 'dry
> lab' data.
>
> 73
> K5PRO
> John
>
>
> >From: 2 <2@vc.net>
> >To: "Dan" <dhearn@ix.netcom.com>, "Yury VE3XB" <ve3xb@sympatico.ca>,
> > "AMPS" <Amps@contesting.com>
> >Subject: Re: [Amps] 2:1 Transformer
> >
> >>Yury: You need a copy of Jerry Sevicks book "Transmission Line
> >>Transformers". He shows many designs of transformers which have ratios
near
> >>2:1 as well as dozens of others. These amazing xfmrs have bandwidths of
> >>160-10m or more and losses as low as 0.1 db.
> >
> >? However, as I understand it, Sevick measures the performance of
> >his designs at low power.
> >Rich, AG6K, 805-386-3734, www.vcnet.com/measures
>
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