Palstar ZM-30, Owner'S Manual

Page: 16 / 37

16
Mode pushbutton menu. You can accurately
measure inductor values as long as the
reactance at the measurement frequency is
within the impedance measurement
specifications of the analyzer (about 10-to-
1000 ohms).
Yet another method is to do the measurement
in a series resonant circuit as shown below in
Figure 1.
51
Ω Ω Ω Ω
L C
TO
J3
Figure 1
This requires a capacitor of known value in
addition to a 51-ohm carbon composition or
film resistor. The capacitor should have a
tolerance no wider than 10% and have a low
loss dielectric composition such as NP0
ceramic or mica. A capacitance value of
about 100 pf is appropriate for many RF
measurements.
You can make your own precision capacitor
from a piece of coaxial cable. Common RG-
58 type 50 ohm coax has a capacitance of
about 29 to 30 pf (detailed data is available at
http://thewireman.com/coaxdata.pdf ). For
example RG58/U is specified at 28.8 pf per
foot so a length of about 3.5 ft – including a
1” pigtail for attachment will serve as a fairly
accurate 100 pf capacitor.
To measure inductance by the second method,
connect the components as shown in as Figure
1. See also Figure 6. Adjust the operating
frequency for lowest SWR and record the
frequency. Now you can calculate the
capacitance using the formula:
L = 25330/(F*F*C)
where L is the inductance in microhenries, F is
the frequency in MHz and C is the capacitance
in picofarads.
Measure 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 below 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 composition 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). Record this resistance value.
Now connect the 51-ohm resistor directly
across the analyzer’s RF output connector and
measure its exact value at the resonance
frequency and record it. Next subtract the
exact 51-ohm resistor value from the
measured R value and use this new resistance
in the above formula to calculate the Q value.
Transmission Line Characteristic
Impedance
The characteristic impedance of coaxial,
twisted pair, open wire or ribbon type
feedlines can be estimated using the ZM-30.
Q = XL
R