Mike wrote:
I'm wanting to build this active antenna, but can't find the core to
wind the output coupling transformer.

http://www.elecdesign.com/Articles/I...ArticleID=6244

This is the authors description of the transformer.

"The toroidal transformer’s primary is 36 turns of No.24 enameled wire
wound on a core from a Sony 1-421-302 line choke. Its secondary is
nine turns of No.24 telephone wire."

With that turns ratio, I would use a smaller wire for the primary,
probably No. 30 silver plates Kynar insulated wire wrap wire.

And this is his decsription of the performance of the amplifier.

The frequency response at low gain is very flat (±0.2 dB) from 200 kHz
to 35 MHz, and is only 0.4 dB down at 60 MHz. At standard high gain,
the response is very flat to 25 MHz and -3 dB at 50 MHz. The maximum
output level in all gain configurations is over 500 mV rms into a
75ohm load.

Does anyone have any idea what type of core that would work as well?

I would try a core ordinarily used as a medium frequency noise
suppression core, with enough window area to allow non overlapped
turns around the hole. Perhaps a Steward 28B0870-000, 22mm OD, 13.7mm
ID, 6.4mm thick. This is made of 850u material, flat permeability out
to about 2 MHz, and then rolling off with increasing loss as frequency
rises.
http://www.steward.com/web_part_no.a...rt=28B0870-000
There is also one twice as thick, if you want to lower the turns count
a bit. 28B0870-100.

But if the goal is to have optimum response at the low frequency end,
and to absorb the higher frequency end, a core made of the 5000u
material would be even better. Type 35 (Steward's low frequency bead
material) has flat permeability to only 300kHz. A usable example
might be LFB220140-000, 22mm OD, 14mm ID, 12.7mm thick.
http://www.steward.com/web_part_no.a...=LFB220140-000
This one could probably work with fewer turns, but the same ratio.

These cores go for less than a dollar each from Digikey.

The material curves are in the toroid manual:
http://www.steward.com/pdfs/ToroidCatalog-rev11-C.pdf