Practical_Antenna_Handbook_0071639586

C h a p t e r 2 4 : a n t e n n a T u n e r s ( A T U s ) 541
16:1
9:1
R 1
4:1
R 2
1.5:1
Figure 24.7D Commercial
tapped transformer.
Figure 24.7C Tapped multiple-Âimpedance BALUN.
dipole feedpoint impedance will be transformed to 75-ÂW unbalanced. This type of balun
is often included inside ATUs. L 1 and L 2 have equal turns.
Another multiple-Âimpedance transformer is shown in Fig. 24.7C. In this case, the operator
can select impedance transformation ratios of 1.5:1, 4:1, 9:1 or 16:1. A commercial
version of this type of transformer, shown in Fig. 24.7D, is manufactured by Palomar
Engineers; it’s intended for feeding vertical HF antennas. It will, however, work well in
other impedance-Âtransformation applications, too.
Figures 24.7B and 24.7C are specific examples of what can be accomplished using a
toroidal core of the right material and power-Âhandling capability in conjunction with an
autoformer winding, where both input and output loads are referenced to a common
ground. If ground isolation is important, there is no reason completely separate windings
can’t be used, in a configuration analogous to a link-Âcoupled coil.
Ferrite Core Inductors
The word ferrite refers to any of several examples of a class of materials that behave
similarly to powdered iron compounds and are used in radio equipment as the cores for
inductors and transformers. Although the materials originally employed were of powdered
iron (and indeed the name ferrite still implies iron), many modern materials are
composed of other compounds. According to literature from Amidon Associates, ferrites
with a relative permeability, or µ r , of 800 to 5000 are generally of the manganese-Âzinc
type of material, and cores with relative permeabilities of 20 to 800 are of nickel-Âzinc.
The latter are useful in the 0.5-Â to 100-ÂMHz range.
Toroid Cores
In electronics parlance, a toroid is a doughnut-Âshaped object made from a ferrite material
and used as the form for winding an inductor or transformer. Many different core
material formulations are available for the designer to choose from, based on the expected
frequency range and power levels of the application.
Unlike just about any other lumped component available to the experimenter, most
toroids carry little or no information about themselves on their surface. In general,
blindly grabbing an arbitrary toroid from the junk box will lead to a failed project. Toroids
must be selected carefully—with the use of a test jig, if necessary.