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6.0 Advanced Usage
The ZM-30 antenna analyzer is an instrument that can be used for a number of uses well beyond simple SWR and
impedance measurements. The following list presents some of these advanced uses.
Tuned Circuit Resonance
Resonance of a tuned circuit can be measured in several ways.
The most accurate method is to connect it as series resonant circuit by
putting it in series with a non-inductive (carbon composition or film)
¼ or ½ watt 51 ohm resistor as shown in Figure 1.
Now connect this to J3 on the Analyzer and tune for lowest SWR.
The frequency at which this is seen is the tuned circuit resonant frequency.
2. If the individual L and C are connected as a parallel resonant circuit
and cannot be connected is series as above (for example an antenna trap)
the parallel resonance can be estimated. Connect the parallel resonant
circuit as shown in Figure 2.
Connect the circuit to J3 on the Analyzer and again tune for lowest SWR.
The measured frequency is slightly lower than the tuned circuit resonant
frequency due to stray capacitance of the test setup.
Inductor Q
The Q of an RF inductor can be measured with a very simple setup.
First measure the inductive reactance XL of the inductor and record this
value. Now connect it to the Analyzer as shown in Figure 3. Capacitor C
must be chosen to resonate with L at the frequency where you want to
measure the inductor’s Q. The Inductor and Capacitor Measurement section
of this manual shows how this capacitor value can be determined.
Now tune the Analyzer for the lowest R (resistance) value with a reading of
zero X (reactance). If R is above 10 ohms you can now calculate inductor Q
using the formula:
If R is less than 10 ohms a slightly different method needs to be used. In this
case use the test setup shown in Figure 1. Adding the non-inductive (carbon
compostion or film) ¼ or ½ watt 51ohm resistor allows more accurate measurement
of the series resistance of the inductor.
Again tune the analyzer for lowest R (resistance) value with a reading of zero
X (reactance). Next subtract 51 ohms from the measured R value and use
This new resistance in the above formula to calculate the Q value.
Transmission line Stub Lengths
Measurement of quarter and half wave transmission line stubs can be performed regardless of the transmission line
characteristic impedance. The method relies on the fact that an open-circuited quarter wavelength line or a short-
circuited line acts like a precise short circuit at the chosen frequency of operation.
With either type of feedline first cut the feedline to about 10% longer than the desired length, taking the appropriate
velocity factor into account. The velocity factor of common feedlines is available from manufacturer’s literature or
51
Ω
Ω
Ω
Ω
L
C
TO
J3
Figure 1
51
Ω
Ω
Ω
Ω
L
C
TO
J3
Figure 2
L
C
TO
J3
Figure 3
Q = XL
R
