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instrument measures both forward and reflected power, and it can be calibrated to measure VSWR The Bird model 43 Thruline RF wattmeter shown in Fig 27-18 has for years been one of the industry standards in communications service work Although it is slightly more expensive than lesser instruments, it is also versatile, and it is accurate and rugged The Thruline meter can be inserted into the transmission line of an
536 Measurements and adjustment techniques
27-18 Bird model 43 RF wattmeter
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 The heart of the Thruline meter is the directional coupler transmission line assembly shown in Fig 27-19A; it is connected in series with the antenna, or dummy load, transmission line The plug-in directional element can be rotated 180 to measure both forward and reverse power levels A sampling loop and diode detector are contained within each plug-in element The main RF barrel is actually a special coaxial line segment with a 50- characteristic impedance The Thruline sensor works due to the mutual inductance between the sample loop and center conductor of the coaxial element Figure 27-19B shows an equivalent circuit The output voltage from the sampler (e) is the sum of two voltages, er and em Voltage er is created by the voltage divider action of R and C on transmission line voltage E If R is much less then XC, then we may write the expression for er as er = RE = RE( j C) XC [2714]
Voltage em, on the other hand, is due to mutual inductance, and is expressed by em = I( j ) M [2715]
We now have the expression for both factors that contribute to the total voltage e We know that: e = e r + em so, by substitution, e = j M E I Zo [2717] [2716]
Selecting and using RF wattmeters and antenna VSWR meters 537
ELEMENT CIRCUIT XMTR or load dc connector Load or XMTR Meter
Directional coupling Crystal diode Element
Bypass dc contact
Meter cable
27-19A Thruline sensor circuit
27-19B Circuit of pick-up element in a Model 43 wattmeter
At any given point in a transmission line, E is the sum of the forward (Ef ) and reflected (Er ) voltages, and the line current is equal to I= Ef Zo Er Zo [2718]
where Zo is the transmission line impedance We may specify e in the forms e= j M(2Ef ) Zo [2719]
538 Measurements and adjustment techniques and e= j M(2Er) Zo [2720]
The output voltage e of the coupler, then, is proportional to the mutual inductance and frequency (by virtue of j M) But the manufacturer terminates R in a capacitive reactance, so the frequency dependence is lessened (see Fig 27-19C) Each element is custom-calibrated, therefore, for a specific frequency and power range Beyond the specified range for any given element, however, performance is not guaranteed There are a large number of elements available that cover most commercial applications The Thruline meter is not a VSWR meter, but rather a power meter VSWR can be determined from the formula, or by using the nomographs in Fig 27-20 Some of the Thruline series intended for very high power (Fig 27-19D) applications use an in-line coaxial cable coupler (for broadcast-style hardline) and a remote indicator
Relative, response dB
0 5 10 0 5 10 0 5 10 20
10C element: 10 W, 100-250 MHz
100C element: 100 W, 100-250 MHz
500C element: 500 W, 100-250 MHz 40 60 80 100 200 400 600 800 1000
Frequency in MHz 27-19C Power-frequency calibration
27-19D High-power RF wattmeters
Selecting and using RF wattmeters and antenna VSWR meters 539
10 9 8 7 6 5 4 Reflected power (RF watts) 3 2 1 09 08 07 06 05 04 03 02 01 05 10 15 20 25 30 40 50 10 15 20 25 40 50 Forward power (RF watts) 100 150 200 300 400 500
20 18 16 1 14 12 VSWR 10 8 1 6 4 Reflected power Forward power Reflected power Forward power
Reflected power (RF watts)
20 18 16 14 12 10 8 6 4 2 05 10 15 20 25 30 40 50 10 15 20 25 40 50 100 150 200 300 400 500
Reflected power (RF watts)
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