MFJ-269D Instruction Manual LF/HF/VHF/UHF SWR Analyzer
17
When a reactance is present, voltage and current are no longer in phase (or exactly out-of-phase) and so
the phase angle increases from 0 degrees to a maximum angle of 90 degrees. The angle becomes 90
degrees when the load is a pure reactance, and zero degrees when the load is a pure resistance.
This analyzer will determine the angle in degrees, but it will
not
describe the load reactance specifically
as either capacitive or inductive. It is a simple matter to determine the direction by adding a small
amount of reactance in series with the load and watching the angle change. If the angle decreases, the
load reactance is opposite to the sign or type of test reactance. If the angle increases, the load reactance
is the same sign as the added reactance.
5.4.1.2
Series Equivalent Impedance
The
Advanced-1
display sub-mode is reached by pressing the
Gate
button once while in the
Magnitude
and Phase of Load Impedance
mode. This mode displays the series-equivalent impedance of the load.
It is the most common form used to describe antenna system impedance. In this mode, the load
impedance is described as a
resistance in series with a reactance
. In order to cancel the reactance
without changing the resistance, a reactance of the opposite type and value must be connected in
series
with the load at the point of measurement.
The digital display shows SWR, resistive part of load impedance (
Rs
), and reactive part of load
impedance (
Xs
). The
Impedance
meter displays the
Z
in ohms while the
SWR
meter displays 50-ohm
referenced SWR.
Series Equivalent Impedance
display examples:
7.1598 MHz 3.2
Rs=50 Xs= 62
14.095 MHz >31
Rs(Z>1500)
s
W
R
s
W
R
In the left-hand display (above), resistance would remain at 50 ohms, reactance would go to zero, and
SWR would become 1:1 if an opposite-sign reactance of
62 ohms
was connected in
series
with the
feedline at the point where the measurement is made. The screen on the right illustrates a reactance value
out of measurement range.
Note:
Every series impedance has a parallel equivalent counterpart. A
Series Impedance
of Rs=50,
Xs=62 is equal to the
Parallel Equivalent Impedance
of Rp=126, Xp=102 ohms. You can make the
conversion in this mode by pressing the
Gate
button (see section 5.4.1.3 below).
5.4.1.3
Parallel Equivalent Impedance
Pressing the
Gate
button twice from the
Magnitude and Phase of Load Impedance
mode toggles the
analyzer into a parallel equivalent impedance sub-mode.
Parallel equivalent display examples:
7.1598 MHz 3.2
Rs=126 Xs=102
14.095 MHz >31
Rs(Z>1500)
s
W
R
s
W
R
In the left hand display, the
Equivalent Parallel Resistance
is Rs=126 ohms. That resistance appears to
be in
parallel
with Xs=102 ohms. If we
parallel
connect an opposite-sign reactance of 102 ohms at the
measurement point, the
parallel
equivalent reactance is canceled, leaving only the Rs=126-ohm (pure)
resistance.
This is a powerful tool used in matching antennas, and the MFJ-269D places it at your fingertips. By
checking a load for both
Rp
and
Rs
, you can see if either one is close to the desired resistance. If one
resistance value is close to the desired value, adding only one component will match the load by
canceling reactance.