(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:

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)."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"

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.

## 2 comments:

"Al is irrelevant for transformers"

Huh? Al allows you to calculate the inductance of a winding. Look at the basic transformer equations and you'll see there's a direct relationship between primary and secondary inductance, just as there is with the number of turns on a winding.

Try the mini-Ring Core Calculator from DL5SWB:

http://dl5swb.de/

Look at the Help screens in mini-Ring, some good explanations there.

Thanks for the feedback. My understanding is the statement is generally true for transformers unless the turns required becomes very small. See a more recent blog entry quoting a posting by M0BMU on the RSGB LF group yesterday.

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