A special type of 2-hole ferrite core has larger holes than the normal bead and is often referred to as a "balun core or multi-aperture core". It is commonly used in this form in TV and FM receiver antenna input circuits. The balanced to unbalanced turns ratio can be any whole number (N), which gives an impedance ratio of N2. The common TV antenna balun has a turns ratio of 2:1 which gives a 300:75 impedance transformer for matching a 300 ohm balanced antenna to 75 ohm coax. cable.
A "home-made" balun core can be made by joining two ferrite sleeves (e.g FB-( )-5621). The sleeves can be stuck together side by side with epoxy cement and the windings made in the usual way.
A common form of the high power balun uses a centre-tapped single turn as the low impedance balanced winding and the correct number of turns on the other winding to achieve the required transformation ratio. Such a transformer is often used to match the low output impedances of a high power push-pull amplifier to a 50 ohm load. The single turn winding can be made from thin wall brass tubing (one through each hole) joined at the centre tap with a piece of pc board soldered to the tube ends, and at the other end with a similar pc board with a groove down the centre between the tubes to isolate the collectors from each other. Alternatively, the woven screen from coax cable can be used as the single turn winding and the other winding threaded through the screen as required. Use of ptfe coated wire is recommended if soldering of the outer screen is necessary.
Toroid cores can also be used as baluns especially where odd turns ratios (N) are required.
As usual, the ferrite material determines the useful frequency range,
and this information is listed in the data tables.
Plots showing Initial Permeability and Loss Factor vs Frequency are available for some common ferrites used in baluns.
The effective permeability of a rod depends upon the material permeability and the length/diameter ratio as shown in the diagram. The coil placement and length of winding also affect the permeability according to the following formula:
u' = ue(lr/lc)1/3
u' = corrected u
ue = effective permeability from chart
lr = length of rod
lc = length of coil
The greatest inductance and AL value are obtained when the coil is centered on the rod.
The best Q is when the winding is spread over the entire length of the rod and the spacing between turns is equal to the wire diameter.
A rod suitable for HF operation is type R-61-037-300