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 589 We can also measure the forward and reflected power and calculate the VSWR from those readings: P REV / PFWD VSWR = 1+ ( ) 1– ( P / P ) REV FWD (27.3) where VSWR = voltage standing wave ratio P REV = reflected power = forward power P FWD If we can measure either the voltage maximum and minimum or the current maximum and minimum, we can calculate SWR: V VSWR = MAX V MIN (27.4) Finally, if the forward and reflected voltage components at any given point on the transmission line can be measured, then we can calculate the VSWR from V VSWR = V FWD FWD + V – V REV REV (27.5) where V FWD = forward voltage component V REV = reflected voltage The last equation, based on the forward and reflected voltages, is the basis for many modern VSWR bridges and RF power meters. If the feedpoint impedance of an antenna has both resistance and reactance, it is called complex. In mathematicians’ terms, it has real and imaginary parts, and the resulting voltages at the feedpoint and all along the transmission line are similarly complex. Summarizing some of the important findings of Chap. 4, we note: • Only the real part of the impedance of any part of an antenna system dissipates power. At MF and HF, these losses are most often found in the antenna radiation resistance (desirable) and in the I R losses (undesirable) of the transmission line(s) feeding the antenna, other nearby conductors, and (especially if the radiator is a grounded monopole) the earth beneath the antenna. • In an ideal system the antenna feedpoint resistance, the transmission line characteristic impedance, and the transmitter output impedance are identical and there are no reactive loads and no resistive losses, so SWR = 1.0:1 everywhere throughout the system. However, that is not always possible, so we often have to employ impedance-Âmatching devices, such as ATUs, baluns, transmission line transformers, and the like, to minimize SWR and undesirable losses. • Even if the real, or resistive, part of the antenna impedance perfectly matches the characteristic impedance of the transmission line at their attachment point,
590 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 any reactive component of the antenna feedpoint impedance will cause the SWR to deviate from 1.0:1 just as though R L ≠ Z 0 , and even the resistive part of Z IN will vary along the line. • All “real” antennas exhibit SWR bandwidth. Bandwidth can be too narrow (typical for antennas that are small with respect to a wavelength) or too wide (typical for antennas having resistive or dissipative losses comparable to the radiation resistance of the antenna itself). • High SWR on a transmission line causes greater losses along the line than when the line is matched to the load impedance. A large part of this is because the high SWR causes high currents near current maxima along the line, increasing the line’s I 2 R dissipation. In the following sections of this chapter, we look at a variety of useful instruments for measuring the RF characteristics of antenna systems and for diagnosing problems in those systems: • Impedance bridges • RF noise bridges • Dip oscillators • Field strength meters • SWR bridges • SWR analyzers • RF wattmeters • Vector network analyzers • Dummy loads Impedance Bridges We can make antenna impedance measurements using a variant of the old-Âfashioned Wheatstone bridge. Figure 27.2A shows the basic configuration of the bridge in its most generalized form. The current flowing in the meter will be zero when (Z 1 /Z 2 ) = (Z 3 /Z 4 ). If one arm of the bridge is the antenna impedance and a second arm is the reference (R 0 ), we ineratively adjust the other two arms until we obtain a null at the measurement frequency. A typical example is shown in Fig. 27.2B. An antenna connected at J 2 constitutes one arm of the bridge, while R 2 is a second. The value of R 2 should be 50 W or 75 W, depending upon the value of the expected antenna impedance, but 68 W is an acceptable compromise value for simple meters that must be used with both system impedance levels. The other two arms of the bridge are the reactances of C 1A and C 1B , which are two halves of a single differential capacitor. In operation, a very low level RF signal is applied at J 1 and C 1 is adjusted for minimum meter reading. The antenna resistance at that frequency is then read from the previously calibrated dial. Calibrating the instrument is simple. Noninductive resistors having standard values from 10 to 1000 W are connected across J 2 . For each resistor the meter is nulled and
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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 589<br />
We can also measure the forward and reflected power and calculate the VSWR from<br />
those readings:<br />
P<br />
REV<br />
/ PFWD<br />
VSWR = 1+ ( )<br />
1– ( P / P )<br />
REV<br />
FWD<br />
(27.3)<br />
where VSWR = voltage standing wave ratio<br />
P REV = reflected power<br />
= forward power<br />
P FWD<br />
If we can measure either the voltage maximum and minimum or the current maximum<br />
and minimum, we can calculate SWR:<br />
V<br />
VSWR = MAX<br />
V<br />
MIN<br />
(27.4)<br />
Finally, if the forward and reflected voltage components at any given point on the<br />
transmission line can be measured, then we can calculate the VSWR from<br />
V<br />
VSWR =<br />
V<br />
FWD<br />
FWD<br />
+ V<br />
– V<br />
REV<br />
REV<br />
(27.5)<br />
where V FWD = forward voltage component<br />
V REV = reflected voltage<br />
The last equation, based on the forward and reflected voltages, is the basis for many<br />
modern VSWR bridges and RF power meters.<br />
If the feedpoint impedance of an antenna has both resistance and reactance, it is<br />
called complex. In mathematicians’ terms, it has real and imaginary parts, and the resulting<br />
voltages at the feedpoint and all along the transmission line are similarly complex.<br />
Summarizing some of the important findings of Chap. 4, we note:<br />
• Only the real part of the impedance of any part of an antenna system dissipates<br />
power. At MF and HF, these losses are most often found in the antenna radiation<br />
resistance (desirable) and in the I R losses (undesirable) of the transmission<br />
line(s) feeding the antenna, other nearby conductors, and (especially if the<br />
radiator is a grounded monopole) the earth beneath the antenna.<br />
• In an ideal system the antenna feedpoint resistance, the transmission line<br />
characteristic impedance, and the transmitter output impedance are identical<br />
and there are no reactive loads and no resistive losses, so SWR = 1.0:1 everywhere<br />
throughout the system. However, that is not always possible, so we often have<br />
to employ impedance-Âmatching devices, such as ATUs, baluns, transmission line<br />
transformers, and the like, to minimize SWR and undesirable losses.<br />
• Even if the real, or resistive, part of the antenna impedance perfectly matches<br />
the characteristic impedance of the transmission line at their attachment point,
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