Practical_Antenna_Handbook_0071639586

608 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
One of the latest designs in VSWR meter sensors is the current transformer assembly
shown in Fig. 27.15. In this instrument, a single-Âturn ferrite toroid transformer is used
as the directional sensor. The transmission line passing through the hole in the toroid
forms the primary winding of a broadband RF transformer. The secondary, which consists
of 10 to 40 turns of small enamel wire, is connected to a measurement bridge circuit
(C 1 + C 2 + load) with a rectified dc output.
Figures 27.16 and 27.17 show instruments that can be left in the transmission line for
all amateur power levels. The upper unit in Fig. 27.16, from Diamond Antennas, houses
the transformer with the metering and control functions. The Kenwood wattmeter below
it allows the user to Âfront-Âpanel select any of up to three separate companion transformer
units, each capable of being located as much as 18 in from the meter enclosure.
The Bird Model 43 Thruline RF wattmeter shown in Fig. 27.17 has for years been one
of the industry standards in communications service work. Although it is slightly more
expensive than other instruments, it is versatile and has a reputation for being accurate
and rugged. The Thruline meter can be inserted into the transmission line of an antenna
system with so little loss that it may be left permanently in the line during normal operations.
The Model 43 Thruline is popular with land mobile and marine radio technicians.
T 1
J 1
XMTR
C 1
10 pF
Conductor
D 1
IN60
D 2
IN60
C 2
10 pF
J 2
ant
C 3
330 pF
R 1
68 k
R 2
68 k
C 4
330 pF
L 1
C 5
0.001 F
1 mH
10 k
cal
S 1
REV FWD
10 k
cal
L2 1 mH
C 6
0.001 F
S 2
VSWR PWR
C 7
0.01 F
M 1
0–100 A
Figure 27.15 Current transformer wattmeter.