Communication Concepts AN758 - Manual (Page 9)

AR
C
HIVE INF
O
RMA
TI
O
N
PRODUCT TRANSFERRED T
O
M/A
–
COM
AN758
9 RF Application Reports
C1
R
R
R
R
50 Ω
50 Ω
50 Ω
50 Ω
e
a
c
b
d
T1 = 4:1
50 Ω INPUT
Figure 10. Four Port Power Divider
R R R
R
R
R
R R
R
R
R = 28.13
OHMS
R = 25 OHMS R = 18.75
OHMS
R
in
= 12.5
OHMS
R
in
= 12.5
OHMS
R
in
= 12.5
OHMS
50 OHMS 50 OHMS 50 OHMS
a) 1 LOAD OPEN b) 2 LOADS OPEN c) 3 LOADS OPEN
Figure 11.
Except for a two port power divider (5) , the resistor values
can be calculated for odd or even number systems as:
n + 1
R
L
– R
in
R =
Ǔ ǒ
n
where: R
L
= Impedance of the output ports, 50 Ω.
R
in
= Impedance of the input port, 12.5 Ω.
n = Number of output ports properly terminated.
Although these resistor values are not critical in the input
divider, the formula also applies to the output power
combiner, where mismatches have a larger effect in the total
power output and linearity.
The practical power divider employs large ferrite beads
(Fair-Rite Products 2673000801 or Stackpole 57-1511-24B
or equivalent) over a 1.2 inch piece of RG-196 coaxial cable.
The arrangement is shown in Figure 10. Both above ferrite
materials have a
µr of about 2500, and the inductance for
one turn is in excess of 10 µH.
The step-down transformer (T1, Figure 10) is wound on
a Stackpole 57-9322-11 toroid with 25 Ω miniature coaxial
cable. (Microdot 260-4118-000 or equivalent.) Seven turns
will give a minimum inductance of 4/16
µH, required at
2 MHz.
For the preamplifier interface, C1 could be omitted in order
to achieve the lowest input VSWR.
The structure is mounted between two phenolic terminal
strips as can be seen in the foreground of Figure 14,
providing a sufficient number of tie points for the coaxial
cable connections.
THE OUTPUT COMBINER
The operation of the output combiner is reversed from
that of the input power divider. In this application we have
four – 50
Ω inputs and one 12.5 Ω output, which is
transformed to 50 Ω by a 1:4 impedance ratio transformer.
An arrangement similar to the input power divider is
employed in the combiner. The baluns consist of straight
pieces of coaxial cable loaded by a sleeve of magnetic
material (ferrite). The line length is determined by the
physical dimensions of the ferrite sleeves. The
µr versus
cross sectional area should be calculated or measured to
give sufficient loading inductance.
Straight line baluns as these have the advantage over
multiturn toroidal types in introducing a smaller possibility
for phase errors, due to the smaller length of the line. The
largest possible phase errors occur in the input and output
connecting cables, whose lengths are 18
″ and 10″
respectively. All four input and output cables must be of equal
length within approximately 1/4
″, and the excess in some,
caused by the asymmetrical system layout, can be coiled
or formed into loops.
The output connecting cables between the power
amplifier outputs and the combiner are made of low loss
RG-142B/U coaxial cable, that can adequately handle the
300 W power with the average current of 2.45 A.
The balun transmission lines are also made of
RG-142B/U coaxial cable, with an outer diameter of 0.20 ″.
The line length is not critical as it is well below the maximum
length permitted for 30 MHz
(7) . The minimum inductance,
as in the input divider, is 16
µH per line. Measurements were
made between two port combiners, one having the line
inductance of 17
µH (7 Ferroxcube 768 series 3E2A toroids)
and the other 4.2 µH (one Stackpole 57-0572-27A ferrite
sleeve). The results are shown in Table 3.
Table 3.
f
MHz
Isolation dB
(Line Inductance
17 µH)
Isolation dB
(Line Inductance
4.2 µH)
2.0 40.2 29.1
4.0 40.0 38.3
7.5 39.6 39.1
15 37.5 37.8
20 35.8 36.2
30 33.4 33.5