**The 4 rule**

The reactance of the windings should be at least four times the impedance the winding is designed to look into.

where

*Z _{i} … *input impedance

*Z*winding impedance

_{w}…thus

where

*L _{w} … *winding inductance

*input impedance*

Z

Z

_{i}…*… design frequency*

f

f

To find the number of turns the formula is

where

*N* … number of turns *L*_{w} … inductance in mH

A_{L} … core constant (find in the catalog)

To find the number of turns on the secondary, use

where

*Np* =number of turns on the primary*N*_{S}= number of turns on the secondary *Zp* =impedance of primary

*Zs*= impedance of secondary

**Example – a 1:9 transformer for a 450 ohm antenna: **

*Np/Ns*= sqrt(Zp/Zs) or

*Np/Ns*= sqrt(450/50) = 3, this is the ratio of primary to secondary turns. If you have a 450 impedance, 4 x 450=1800 ohms of reactance. Using 1.8 MHz as your minimum frequency, the inductance would need to be L = 1800 / 2 x pi x 1.8 MHz. So L= 0.000159 henrys or 0.159 mh. Thus

*N*=1000(sqrt(.159/8500))= 5.24 turns on the primary. So

*N*

_{S}= 5.24/3 = 1.75 turns on the secondary, a 6 turn primary and 2 turn secondary is ideal.

**Table 1. Binocular cores for transformers and the A _{L} constant**

Core |
A_{L} |

BN-43-202 | 2890 |

BN-43-2302 | 680 |

BN-43-2402 | 1277 |

BN-43-3312 | 5400 |

BN-43-7051 | 6000 |

BN-61-202 | 425 |

BN-61-2302 | 100 |

BN-61-2402 | 280 |

BN-61-1702 | 420 |

BN-61-1802 | 310 |

BN-73-202 | 8500 |

BN-73-2402 | 3750 |

You can also use the BN Calculator, available **here**.

_{L }has the BN-73-202 which would be probably the preferred core. W7IUV successfully used such cores for his transformers used with rotatable Flag antenna.

Anyway, the 73 mix seems to be the best. Even higher A_{L }can be obtained by stacking more binocular cores. W8JI uses 3 stacked BN-73-202 and also I tried to construct similar transformer. The only problem is the winding technique and the possibility to damage the enameled wire. Unfortunately, here is the teflon coated wire an ‘unobtainium’, so I put a length of soft silicone tubing into the holes. Anyway, you should work very carefully and a final check with inductance meter and impedance analyzer is a must.

**My 1:9 transformer**

**1 turn**is the wire passing through

**both**

**holes**(like a hairpin) as opposed to the convention for a toroid core where 1 turn is the wire passing through the core once.