This is a updated collection of easy to build projects for radio amateurs and swl's. See all my projects in the OLDER POSTS down this blog.
Have fun.
73' John

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Friday, January 20, 2017

EFHW antenna for 40-10m qrp

I had a BN43-202 core in stock and decided to try to create a small QRP EFHW transformer from 7 - 29 MHz. As you can see in the plot the swr is verry flat from 40-10m so a capacitor at the input is not needed. A 3300 Ohm resister to ground was used for the plot but at 1700 Ohm the transformation is near perfect. Don't use thin antennawire for this rises the impedance of the antenna. 1.5mm is a minimum . With just 20.3m wire this makes a nice 40-10m antenna. You can also use the 34uH coil of the endfed down this blog to create a compact 3 band atenna of just 12m long. 10 Watts PEP is the max. power here. Be carefull, each pass is 1 turn here. A test with 2 transformers back to back showed a power loss of about 18% on 20m. I used 0.7mm transformer wire .

Disclaimer:I'm not responsable for any damage due to improper reproduction of my projects.