Topband: TRANSFORMER QUESTION

[Tom Rauch]

> If you are using the autoformer (single winding with a
tap) design
> from Tom's web site, I'm not surprised to hear about the
limited high
> frequency response. Autoformers are not particularly
broadband (see
> Fig. 3-12 in W2FMI's Transmission Line Transformers book).
And they
> can be sensitive to winding technique, but more so on a
toroid than a
> binocular core.

I'm not sure exactly was Sevik is talking about, but if he
said that nothing is further from the truth. It makes no
sense in theory either.

CATV splitters normally use autotransformers, and they cover
extreme frequency ranges. I've never found one using
conventional transformers.

In measurements I have made, auto transformers using the
same core and general winding style have substantially
greater BW than primary secondary transformers. I'd bet
money on that, since I have data from dozens of cores and
winding styles!!!

I just pulled the data on a 73 mix binocular core using a 5
turn primary and 7 turn secondary both tightly packed, with
sweep starting at 300kHz:

1.5 VSWR = 8.22 MHz
2.0 VSWR = 15.49 MHz

Using an autotransformer winding with the same wire size:

1.5 VSWR = 33.45 MHz
2.0 VSWR = 60.94 MHz.

With compensation, care in winding, and proper wire and core
selection I can wind a 2: 1 autotransformer that covers
300kHz to 150 MHz with under 1.5:1 SWR.

There is no way a conventional transformer would increase
BW. I'm absolutely positive of that.

73 Tom