Easy Broadband RF
Written 07 October 2008
In working on an active antenna and preamplifier design, I
needed a broadband 1:1 transformer. I've wound many of these and written an
article for 73 Magazine about the subject as well, but there's always interest
in broadband transformers.
My favorite design uses a bifilar winding, consisting of
two no. 26 AWG wires twisted together around 4 to 5 turns per inch,
wound with 10 turns on a Steward 35T0501-H10 ferrite core.
I've used Fair-Rite 43 material toroid cores in the past,
but where low frequency performance is necessary, I've found
Steward's 35 material useful.
It has a relative permeability at low frequencies of 5000, compared with 800 for
Fair-Rite's 43 material, and it's available in small quantities from DigiKey.
Fair-Rite's Type 75 material is quite
similar to Steward's 35 and may be used if desired.
One advantage of Steward is that DigiKey stocks the H10
version of the part, which is tumbled to remove sharp edges and
coated with Parylene. Parylene is a smooth insulating coating which, when
combined with tumbling makes it much easier to wind without scraping the enamel
insulation from the magnet wire. Fair-Rite, I believe, offers a similar coating
but the usual amateur radio suppliers provide only the uncoated cores.
The core used in this transformer is DigiKey stock number
240-2524-ND, priced at 23 cents each in quantities 1-9, Steward part number
35T0501-10H. The dimensions are 12.70mm O.D. x 7.14mm I.D. x 6.35mm H. It's
quite similar in size to what is known as an "FT50" to the amateur radio
community, except that the Steward core is thicker (FT50 thickness is 4.90
mm). This gives you a bit more inductance per turn.
The photo below shows the completed transformer connected
to two floating shell BNC connectors. The spacing between the two connectors is
greater than it should be because I use the same enclosure for the Z10010
bandpass filter and happened to have a drilled cover on hand.
Although bifilar wound, I connected the windings as
in a conventional transformer to provide DC and low frequency isolation, not
like a balun connection that provides DC continuity between input and output