Practical_Antenna_Handbook_0071639586
C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 609 Figure 27.16 Amateur radio RF wattmeter. The heart of the Thruline meter is the directional coupler transmission line assembly shown in Fig. 27.18A; it is connected in series with the transmission line to the antenna or dummy load. The plug-Âin directional element can be rotated 180 degrees to measure either forward or reverse power levels. (Some radio repair shops use two meters in series, for simultaneous viewing of the power in both directions.) Each plug-Âin element contains a sampling loop and diode detector designed to cover a specific range of fre- Figure 27.17 Bird model 43 RF wattmeter.
610 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 ELEMENT CIRCUIT XMTR or load Load or XMTR Directional coupling Crystal diode Element dc connector Bypass dc contact Meter cable Meter Figure 27.18A Thruline sensor circuit. quencies. The main RF barrel is actually a special coaxial line segment with a 50-ÂW or 75-ÂW characteristic impedance. The Thruline sensor depends for its operation on the mutual inductance between the sample loop and the center conductor of the coaxial element. Figure 27.18B shows an equivalent circuit. The output voltage from the sampler (e) is the sum of two voltages, e r and e m . Voltage e r is created by the voltage divider action of R and C on transmission line voltage V. If R is much less than X C , we may write the expression for e r as RV RV er e= r = X X C C = RV = RV ( jw( jC w) C) (27.12) e m Voltage e m , on the other hand, is due to mutual inductance, and is expressed by e m = ± M ( jw I) (27.13) e r We now have the expression for both contributors to the total voltage e. We know that e = er + e (27.14) m Figure 27.18B Circuit of pick-Âup element in a Model 43 wattmeter.
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C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 609<br />
Figure 27.16 Amateur radio RF wattmeter.<br />
The heart of the Thruline meter is the directional coupler transmission line assembly<br />
shown in Fig. 27.18A; it is connected in series with the transmission line to the antenna<br />
or dummy load. The plug-Âin directional element can be rotated 180 degrees to measure<br />
either forward or reverse power levels. (Some radio repair shops use two meters in series,<br />
for simultaneous viewing of the power in both directions.) Each plug-Âin element<br />
contains a sampling loop and diode detector designed to cover a specific range of fre-<br />
Figure 27.17 Bird model 43 RF wattmeter.
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