Practical_Antenna_Handbook_0071639586

598 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
type, the link must go through the hole in the doughnut-Âshaped core and then connect
to the Antenna terminals on the bridge. After this, proceed exactly as you would for the
series-Âtuned tank measurement.
Capacitance and Inductance Measurements
With just two additional components—a 100-ÂpF silver mica test capacitor and a 4.7-ÂmH
test inductor—the noise bridge is capable of measuring inductance and capacitance,
respectively, over a wide range of frequencies. The idea is to use one or the other of the
test components to form a series-Âtuned resonant circuit with an unknown component.
If you find a resonant frequency, then you can calculate the value of the unknown component.
In both cases, the series-Âtuned circuit is connected across the unknown terminals
of the dip meter, and the series tuned procedure is followed.
To measure inductance, connect the 100-ÂpF capacitor in series with the unknown
coil across the Antenna terminals of the dip meter. When the null frequency is found,
find the inductance from
L= 253
(27.7)
2
f
where L = inductance, in microhenries
f = frequency, in megahertz
Similarly, to find the value of an unknown capacitor:
1. Connect the test inductor across the Antenna terminals in series with the
unknown capacitance.
2. Set the resistance control to zero, tune the receiver to 2 MHz, and readjust the
reactance control for null.
3. Without readjusting the noise bridge control, connect the test inductor in series
with the unknown capacitance and retune the receiver for a null.
4. Capacitance can now be calculated from:
C = 5389
2
f
(27.8)
where C is in picofarads
f is in megahertz
Dip Oscillators
One of the most common instruments for determining the resonant frequency of an
antenna is the so-Âcalled dip oscillator or dip meter. Originally called the grid dip meter or
grid dip oscillator (GDO) because early circuits were based on the use of a vacuum tube,
the principle behind this instrument is that its output energy can be absorbed by a
nearby resonant circuit (or antenna, which is an electrically resonant LC circuit). When
the inductor of the dip oscillator (see Fig. 27.7) is brought into close proximity to a resonant
circuit, a small amount of energy is transferred if the oscillator is tuned to the tank