End-fed longwire antennas

These end-fed antennas are quite popular, but a 9:1 transformer is usually needed to bring the impedance down to levels that a typical rig's ATU can match. Often less than 3:1 VSWR can be obtained on all HF bands with such a transformer, so a rig's internal ATU can usually bring this down to 1:1.

See https://pa3i.jouwweb.nl/antenne

On-line LF toroid transformer design tool?

I have some 42mm diameter 3C90 toroids and want to use these in output transformers in 3 applications:

(1) in the output of a 137kHz (up to) 25W transmitter
(2) in the output of an 8.97kHz (up to) 25W transmitter
(3) as an impedance transformer for a TX loop antenna at 8.97, 137 and 500kHz.

I was looking for an on-line calculator to help me work out secondary turns needed, but could not find one.  Andy G4JNT helped with this input:

"The magic equation is Vrms = 4.44.F.N.A.B    all in SI units.   

rearranged  Nmin = V / (4.44 . F . A . B)

Al is irrelevant for transformers.

Use a Bmax of 0.1 Tesla for Ferrites, allowing a decent safety margin.

Your A  (of 25 mm^2)   = 25*10^-6    ,   F = 137000, 

25W in 50 ohms is 35V"

To aid calculations in future I have produced a small spreadsheet to work out the secondary turns from the input data (freq, cross sectional area and RF power out).

As an aside, I use http://www.66pacific.com/calculators/toroid_calc.aspx very often to work out the turns needed for the common HF toroids such as T37-x and T50-x.