RF Transformers
Solenoid RF Transformer
Neat Construction
Torroid RF Transformers
Ferrite Cores
Balun
converts between
Balanced and Unbalanced Line
Areal Coax Balun
The THEORY
Ideal transformer
A lossless transformer with an input
(primary) winding having Np turns and
a secondary winding of Ns turns.
The voltage applied to the primary
winding vp(t)
The voltage produced on the
secondary winding vs(t) is
vp (t) : vs (t) = Np : Ns = a
Here a is the turn ratio of the transformer.
(1)
Ideal transformer
The primary ip(t) and secondary is(t) currents
ip (t) : is (t) = 1 : a
In the phasor notation:
Vp / Vs = a
Ip / Is = 1/a
The phase angles of primary and secondary voltages are the same. The phase angles
of primary and secondary currents are the same also. The ideal transformer changes
magnitudes of voltages and currents but not their angles.
Impedance transformation
The impedance is defined as a following ratio of phasors:
ZL = VL / IL
The apparent impedance of the
primary circuit is:
ZL‘ = Vp / Ip
which is
ZL‘ = Vp / Ip = α2 ZL
It is possible to match magnitudes of impedances (load and a transmission line) by
selecting a transformer with the proper turn ratio.
Ideal transformer Limitations
•
Vs and Vp are always in phase (or anti - phase )
irrespective of the load.
Vp / Vs = a
•
Zero primary current for the open secondary.
Ip = (1/a) Is
The voltage ratio across a real
transformer
A portion of the flux
produced in the primary
coil passes through the
secondary coil (mutual
flux); the rest is lost
(leakage flux):
Equivalent Circuit of a Real
Transformer
Leakage flux modeled by primary and secondary inductors Xp
and Xs.
Magnetization current modeled by a reactance XM = L1 ω .
Im = Vp/ Xm
Core-loss current modeled by a resistance RC .
Transmission Line
In two adjacent wires the induced
current flows in OPPOSITE direction.
Transmission Line Transformer
Pulse travelling from left will produce a negative
voltage spike across load.
When to use Magnetic Core.
RULE:
Maximum Coupling when length is an
odd multiple of QUATER WAVE
•At HF High Permeability Core reduces Wavelength
allowing smaller size
•At UHF permeability drops so use Transmission
Line
Transmission Line Balun
For a Matched Load a transmission line
provides 1:1 Balun
1:4 Impedance Transformer
Source End
Load End
Voltages across top and bottom transmission lines ADDED:
VL = 2 Vin
Source current halved:
IL = ½ Iin
Load Impedance
= 2 Vin : ½ Iin
=
4 x Source Impedance
1:4 Impedance Balun
2:1 Current Balun