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.
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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<br />
Small<br />
whip<br />
antenna<br />
Tune<br />
C 3<br />
100 pF<br />
C 1<br />
D 1<br />
L 1<br />
C 2<br />
D 2<br />
C 4<br />
0.01<br />
F<br />
R 1<br />
12 k<br />
D 1 , D 2 : 1N34, 1N60, etc<br />
M 1<br />
50 A<br />
R 2<br />
25-k<br />
sensitivity<br />
Figure 27.9B Tuned field strength meter.<br />
Band<br />
(meters)<br />
L 1<br />
(H)<br />
C 1<br />
(pF)<br />
C 2<br />
(pF)<br />
75/80<br />
40<br />
30<br />
20<br />
17<br />
15<br />
13<br />
10<br />
27<br />
10<br />
4.4<br />
2.25<br />
1.7<br />
1.3<br />
0.8<br />
0.5<br />
25<br />
15<br />
15<br />
10<br />
10<br />
10<br />
10<br />
10<br />
100<br />
68<br />
68<br />
68<br />
56<br />
47<br />
39<br />
39<br />
Figure 27.9C Component values for field strength meters.<br />
RF Wattmeters and VSWR Meters<br />
Perhaps the single most important instrument to have for day-Âto-Âday operation of a<br />
transmitter is the RF power meter (or wattmeter) capable of reading both forward (incident)<br />
and reflected power. A closely related instrument is the antenna VSWR meter,<br />
which gives only a relative indication of forward and reflected power. Its meter is calibrated<br />
to display the dimensionless units of voltage standing wave ratio (VSWR or SWR).<br />
Many modern instruments combine both RF power and VSWR measurement capabilities.<br />
We will look at two of these as examples in this chapter.
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