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 601 two simple passive (which means no dc power is required) field strength meters useable for adjusting HF radio antennas. Two circuits are shown in Fig. 27.9; both are basically variations on the old-Âfashioned “crystal set” theme. Figure 27.9A shows the simplest form of untuned FSM. In this circuit a small whip antenna used for signal pickup is connected to one end of a grounded RF choke (RFC). The RF voltage developed across the RFC by the signal being measured is applied to a germanium diode detector; either a 1N34 or an 1N60 can be used. Silicon diodes are normally preferred in signal applications, but in this case we need the lower contact potential of germanium diodes in order to improve sensitivity (V g is 0.2 to 0.3 V for Ge and 0.6 to 0.7 V for Si). A potentiometer is used both as the load for the diode and as a sensitivity control to set the meter reading to a convenient level. The untuned version of the FSM suffers from a lack of sensitivity, which may limit its use, depending on the distances and power levels involved in your tests. Only a certain amount of signal can be developed across the RFC, so this limits the instrument. Also, the RFC does not make a good impedance match to the detector diode D 1 . An improvement is possible by adding a tuned circuit and an impedance-Âmatching scheme, as shown in Fig. 27.9B. In this case, a variable capacitor C 3 is used as a tuning control in parallel resonance with inductor L 1 . A tapped capacitive voltage divider (C 1 /C 2 ) is used to provide impedance matching to the diode. Figure 27.9C gives values of capacitance and inductance for various bands. Operation of the tunable FSM is simple and straightforward. Set the sensitivity control to approximately half-Âscale, and then key the transmitter. Adjust the tuning control (C 3 ) for maximum deflection of the meter pointer (readjustment of the sensitivity control may be needed). After these adjustments are made, the tunable FSM works just like any other FSM. As a general rule, the FSM is best utilized at least a few wavelengths from the transmitting antenna. Small whip antenna D 1 IN34 or IN60 RFC 1 1 mH 0.01 F M 1 50 A 50 k Figure 27.9A Simple field strength meter.
602 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 Small whip antenna Tune C 3 100 pF C 1 D 1 L 1 C 2 D 2 C 4 0.01 F R 1 12 k D 1 , D 2 : 1N34, 1N60, etc M 1 50 A R 2 25-k sensitivity Figure 27.9B Tuned field strength meter. Band (meters) L 1 (H) C 1 (pF) C 2 (pF) 75/80 40 30 20 17 15 13 10 27 10 4.4 2.25 1.7 1.3 0.8 0.5 25 15 15 10 10 10 10 10 100 68 68 68 56 47 39 39 Figure 27.9C Component values for field strength meters. RF Wattmeters and VSWR Meters Perhaps the single most important instrument to have for day-Âto-Âday operation of a transmitter is the RF power meter (or wattmeter) capable of reading both forward (incident) and reflected power. A closely related instrument is the antenna VSWR meter, which gives only a relative indication of forward and reflected power. Its meter is calibrated to display the dimensionless units of voltage standing wave ratio (VSWR or SWR). Many modern instruments combine both RF power and VSWR measurement capabilities. We will look at two of these as examples in this chapter.
- Page 571: Figure 25.3B Bent dipole wire and s
- Page 574 and 575: C h a p t e r 2 5 : A n t e n n a M
- Page 576 and 577: C h a p t e r 2 5 : A n t e n n a M
- Page 578 and 579: C h a p t e r 2 5 : A n t e n n a M
- Page 580 and 581: CHAPTER 26 The Smith Chart The math
- Page 582 and 583: Figure 26.2 Normalized impedance li
- Page 584 and 585: A Figure 26.3 Constant resistance c
- Page 586 and 587: A Figure 26.4A Constant inductive r
- Page 588 and 589: C h a p t e r 2 6 : T h e S m i t h
- Page 590 and 591: C h a p t e r 2 6 : T h e S m i t h
- Page 592 and 593: D C B A Figure 26.5 Radially scaled
- Page 594 and 595: C h a p t e r 2 6 : T h e S m i t h
- Page 596 and 597: C h a p t e r 2 6 : T h e S m i t h
- Page 598 and 599: C h a p t e r 2 6 : T h e S m i t h
- Page 600 and 601: C h a p t e r 2 6 : T h e S m i t h
- Page 602 and 603: 0.165 Y ' 1.0 j1.1 G' 1.0 Y ' 0.2
- Page 604 and 605: C h a p t e r 2 6 : T h e S m i t h
- Page 606 and 607: C h a p t e r 2 6 : T h e S m i t h
- Page 608 and 609: CHAPTER 27 Testing and Troubleshoot
- Page 610 and 611: C h a p t e r 2 7 : T e s t i n g a
- Page 612 and 613: C h a p t e r 2 7 : T e s t i n g a
- Page 614 and 615: C h a p t e r 2 7 : T e s t i n g a
- Page 616 and 617: C h a p t e r 2 7 : T e s t i n g a
- Page 618 and 619: C h a p t e r 2 7 : T e s t i n g a
- Page 620 and 621: C h a p t e r 2 7 : T e s t i n g a
- Page 624 and 625: C h a p t e r 2 7 : T e s t i n g a
- Page 626 and 627: C h a p t e r 2 7 : T e s t i n g a
- Page 628 and 629: C h a p t e r 2 7 : T e s t i n g a
- Page 630 and 631: C h a p t e r 2 7 : T e s t i n g a
- Page 632 and 633: C h a p t e r 2 7 : T e s t i n g a
- Page 634 and 635: C h a p t e r 2 7 : T e s t i n g a
- Page 636 and 637: C h a p t e r 2 7 : T e s t i n g a
- Page 638 and 639: C h a p t e r 2 7 : T e s t i n g a
- Page 640 and 641: C h a p t e r 2 7 : T e s t i n g a
- Page 642 and 643: C h a p t e r 2 7 : T e s t i n g a
- Page 644 and 645: Mechanical Construction and Install
- Page 646 and 647: CHAPTER 28 Supports for Wires and V
- Page 648 and 649: C h a p t e r 2 8 : S u p p o r t s
- Page 650 and 651: C h a p t e r 2 8 : S u p p o r t s
- Page 652 and 653: C h a p t e r 2 8 : S u p p o r t s
- Page 654 and 655: C h a p t e r 2 8 : S u p p o r t s
- Page 656 and 657: C h a p t e r 2 8 : S u p p o r t s
- Page 658 and 659: C h a p t e r 2 8 : S u p p o r t s
- Page 660 and 661: C h a p t e r 2 8 : S u p p o r t s
- Page 662 and 663: C h a p t e r 2 8 : S u p p o r t s
- Page 664 and 665: C h a p t e r 2 8 : S u p p o r t s
- Page 666 and 667: C h a p t e r 2 8 : S u p p o r t s
- Page 668 and 669: C h a p t e r 2 8 : S u p p o r t s
- Page 670 and 671: C h a p t e r 2 8 : S u p p o r t s
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 601<br />
two simple passive (which means no dc power is required) field strength meters useable<br />
for adjusting HF radio antennas.<br />
Two circuits are shown in Fig. 27.9; both are basically variations on the old-Âfashioned<br />
“crystal set” theme. Figure 27.9A shows the simplest form of untuned FSM. In this circuit<br />
a small whip antenna used for signal pickup is connected to one end of a grounded<br />
RF choke (RFC). The RF voltage developed across the RFC by the signal being measured<br />
is applied to a germanium diode detector; either a 1N34 or an 1N60 can be used.<br />
Silicon diodes are normally preferred in signal applications, but in this case we need the<br />
lower contact potential of germanium diodes in order to improve sensitivity (V g is 0.2<br />
to 0.3 V for Ge and 0.6 to 0.7 V for Si). A potentiometer is used both as the load for the<br />
diode and as a sensitivity control to set the meter reading to a convenient level.<br />
The untuned version of the FSM suffers from a lack of sensitivity, which may limit<br />
its use, depending on the distances and power levels involved in your tests. Only a<br />
certain amount of signal can be developed across the RFC, so this limits the instrument.<br />
Also, the RFC does not make a good impedance match to the detector diode D 1 . An<br />
improvement is possible by adding a tuned circuit and an impedance-Âmatching scheme,<br />
as shown in Fig. 27.9B. In this case, a variable capacitor C 3 is used as a tuning control in<br />
parallel resonance with inductor L 1 . A tapped capacitive voltage divider (C 1 /C 2 ) is used<br />
to provide impedance matching to the diode. Figure 27.9C gives values of capacitance<br />
and inductance for various bands.<br />
Operation of the tunable FSM is simple and straightforward. Set the sensitivity control<br />
to approximately half-Âscale, and then key the transmitter. Adjust the tuning control<br />
(C 3 ) for maximum deflection of the meter pointer (readjustment of the sensitivity control<br />
may be needed). After these adjustments are made, the tunable FSM works just like<br />
any other FSM.<br />
As a general rule, the FSM is best utilized at least a few wavelengths from the transmitting<br />
antenna.<br />
Small<br />
whip<br />
antenna<br />
D 1<br />
IN34<br />
or<br />
IN60<br />
RFC 1<br />
1 mH 0.01 F<br />
M 1<br />
50 A<br />
50 k<br />
Figure 27.9A Simple field strength meter.